Input and Output
****************

There are two distinct classes of input and output functions.  The first set are modeled after the functions available in MATLAB.  The second set are modeled after the standard I/O library used by the C programming language.  The C-style I/O functions offer more flexibility and control over the output, but are not quite as easy to use as the simpler MATLAB-style I/O functions.

When running interactively, Octave normally sends any output intended for your terminal that is more than one screen long to a paging program, such as less or more.  This avoids the problem of having a large volume of output stream by before you can read it.  With less (and some versions of more) it also allows you to scan forward and backward, and search for specific items.

No output is displayed by the pager until just before Octave is ready to print the top level prompt, or read from the standard input using the fscanf or scanf functions.  This means that there may be some delay before any output appears on your screen if you have asked Octave to perform a significant amount of work with a single command statement.  The function fflush may be used to force output to be sent to the pager immediately.  See C-Style I/O Functions.

You can select the program to run as the pager by setting the variable PAGER, and you can turn paging off by setting the value of the variable page_screen_output to the string "false". See User Preferences.

Basic Input and Output
======================

Since Octave normally prints the value of an expression as soon as it has been evaluated, the simplest of all I/O functions is a simple expression.  For example, the following expression will display the value of pi

     octave:13> pi
     pi = 3.1416

This works well as long as it is acceptable to have the name of the variable (or ans) printed along with the value.  To print the value of a variable without printing its name, use the function disp.  For example, the following expression

     disp ("The value of pi is:"), disp (pi)

will print

     The value of pi is:
     3.1416

Note that the output from disp always ends with a newline.

A simple way to control the output format is with the format statement.  For example, to print more digits for pi you can use the command

     format long

Then the expression above will print

     The value of pi is:
     3.14159265358979

Here is a summary of the options for format:

short
     This is the default format.  Octave will try to print numbers with at least 5 significant figures within a field that is a maximum of 10 characters wide.

     If Octave is unable to format a matrix so that columns line up on the decimal point and all the numbers fit within the maximum field width, it switches to an e format.

long
     Octave will try to print numbers with at least 15 significant figures within a field that is a maximum of 24 characters wide.

     As will the short format, Octave will switch to an e format if it is unable to format a matrix so that columns line up on the decimal point and all the numbers fit within the maximum field width.

long e
short e
     The same as format long or format short but always display output with an e format.  For example, with the short e format, pi is displayed as

           3.14e+00

long E
short E
     The same as format long e or format short e but always display output with an uppercase E format.  For example, with the long E format, pi is displayed as

           3.14159265358979E+00

free
none
     Print output in free format, without trying to line up columns of matrices on the decimal point.  This also causes complex numbers to be formatted like this (0.604194, 0.607088) instead of like this 0.60419 + 0.60709i.

bank
     Print in a fixed format with two places to the right of the decimal point.

+
     Print a + symbol for nonzero matrix elements and a space for zero matrix elements.  This format can be very useful for examining the structure of a large matrix.

The input function may be used for prompting the user for a value and storing the result in a variable.  For example,

     input ("Pick a number, any number! ")

prints the prompt

     Pick a number, any number!

and waits for the user to enter a value.  The string entered by the user is evaluated as an expression, so it may be a literal constant, a variable name, or any other valid expression.

Currently, input only returns one value, regardless of the number of values produced by the evaluation of the expression.

If you are only interested in getting a literal string value, you can call input with the character string s as the second argument.  This tells Octave to return the string entered by the user directly, without evaluating it first.

Because there may be output waiting to be displayed by the pager, it is a good idea to always call fflush (stdout) before calling input.  This will ensure that all pending output is written to the screen before your prompt.  See C-Style I/O Functions.

The second input function, keyboard, is normally used for simple debugging.  Using keyboard, it is possible to examine the values of variables within a function, and to assign newassign new variables Like input, it prompts the user for input, but no value is returned, and it continues to prompt for input until the user types quit, or exit.

If keyboard is invoked without any arguments, a default prompt of debug>  is used.

For both of these functions, the normal command line history and editing functions are available at the prompt.

To save variables in a file, use the save command.  For example, the command

     save data a b c

saves the variables a, b, and c in the file data.

The SAVE command can read files in Octave's text and binary formats as well as MATLAB's binary format.  You can specify the default format with the built-in variable DEFAULT_SAVE_FORMAT using one of the following values: "binary" or "mat-binary".  The initial default save format is Octave's text format.

You can use the built-in variable save_precision to specify the number of digits to keep when saving data in text format.

The list of variables to save may include wildcard patterns containing the following special characters:

?
     Match any single character.

*
     Match zero or more characters.

[ LIST ]
     Match the list of characters specified by LIST.  If the first character is ! or ^, match all characters except those specified by LIST.  For example, the pattern [a-zA-Z] will match all lower and upper case alphabetic characters.

The following options may be specified for save.

-ascii
     Save the data in Octave's text data format.  Using this flag overrides the value of the built-in variable default_save_format.

-binary
     Save the data in Octave's binary data format.  Using this flag overrides the value of the built-in variable default_save_format.

-float-binary
     Save the data in Octave's binary data format but only using single precision.  Using this flag overrides the value of the built-in variable default_save_precision.  You should use this format only if you know that all the values to be saved can be represented in single precision.

-mat-binary
     Save the data in MATLAB's binary data format.  Using this flag overrides the value of the built-in variable default_save_format.

-save-builtins
     Force Octave to save the values of built-in variables too.  By default, Octave does not save built-in variables.

Saving global variables also saves the global status of the variable, so that if it is restored at a later time using load, it will be restored as a global variable.

To restore the values from a file, use the load command.  For example, to restore the variables saved in the file data, use the command

     load data

Octave will refuse to overwrite existing variables unless you use the option -force.

If a variable that is not marked as global is loaded from a file when a global symbol with the same name already exists, it is loaded in the global symbol table.  Also, if a variable is marked as global in a file and a local symbol exists, the local symbol is moved to the global symbol table and given the value from the file.  Since it seems that both of these cases are likely to be the result of some sort of error, they will generate warnings.

As with save, you may specify a list of variables and load will only extract those variables with names that match.

The load command can read data stored in Octave's text and binary formats, and MATLAB's binary format.  It will automatically detect the type of file and do conversion from different floating point formats (currently only IEEE big and little endian, though other formats may added in the future).

The following options may be specified for save.

-force
     Force variables currently in memory to be overwritten by variables with the same name found in the file.

-ascii
     Force Octave to assume the file is in Octave's text format.

-binary
     Force Octave to assume the file is in Octave's binary format.

-mat-binary
     Force Octave to assume the file is in MATLAB's binary format.

C-Style I/O Functions
=====================

The C-style input and output functions provide most of the functionality of the C programming language's standard I/O library.  The argument lists for some of the input functions are slightly different, however, because Octave has no way of passing arguments by reference.

In the following, FILE refers either to an integer file number (as returned by fopen) or a file name.

There are three files that are always available:

stdin
     The standard input stream (file number 0).  When Octave is used interactively, this is filtered through the command line editing functions.

stdout
     The standard output stream (file number 1).  Data written to the standard output is normally filtered through the pager.

stderr
     The standard error stream (file number 2).  Even if paging is turned on, the standard error is not sent to the pager.  It is useful for error messages and prompts.

You should always use the symbolic names given in the table above, rather than referring to these files by number, since it will make your programs clearer.

Opening and Closing Files
-------------------------

To open a file, use the function fopen (name, mode).  It returns an integer value that may be used to refer to the file later.  The second argument is a one or two character string that specifies whether the file is to be opened for reading, writing, or both.

For example,

     myfile = fopen ("splat.dat", "r");

opens the file splat.dat for reading.  Opening a file that is already open has no effect.

The possible values mode may have are

r
     Open a file for reading.

w
     Open a file for writing.  The previous contents are discared.

a
     Open or create a file for writing at the end of the file.

r+
     Open an existing file for reading and writing.

w+
     Open a file for reading or writing.  The previous contents are discared.

a+
     Open or create a file for reading or writing at the end of the file.

To close a file once you are finished with it, use the function fclose (FILE).  If an error is encountered while trying to close the file, an error message is printed and fclose returns 0. Otherwise, it returns 1.

Formatted Output
----------------

This section describes how to call printf and related functions.

The following functions are available for formatted output.  They are modelled after the C language functions of the same name.

printf (TEMPLATE, ...)
     The printf function prints the optional arguments under the control of the template string TEMPLATE to the stream stdout.

fprintf (FILE, TEMPLATE, ...)
     This function is just like printf, except that the output is written to the stream FILE instead of stdout.

sprintf (TEMPLATE, ...)
     This is like printf, except that the output is written to a string.  Unlike the C library function, which requires you to provide a suitably sized string as an argument, Octave's sprintf function returns the string, automatically sized to hold all of the items converted.

The printf function can be used to print any number of arguments. The template string argument you supply in a call provides information not only about the number of additional arguments, but also about their types and what style should be used for printing them.

Ordinary characters in the template string are simply written to the output stream as-is, while conversion specifications introduced by a % character in the template cause subsequent arguments to be formatted and written to the output stream.  For example,

     pct = 37;
     filename = "foo.txt";
     printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n",
             filename, pct);

produces output like

     Processing of `foo.txt' is 37% finished.
     Please be patient.

This example shows the use of the %d conversion to specify that a scalar argument should be printed in decimal notation, the %s conversion to specify printing of a string argument, and the %% conversion to print a literal % character.

There are also conversions for printing an integer argument as an unsigned value in octal, decimal, or hexadecimal radix (%o, %u, or %x, respectively); or as a character value (%c).

Floating-point numbers can be printed in normal, fixed-point notation using the %f conversion or in exponential notation using the %e conversion.  The %g conversion uses either %e or %f format, depending on what is more appropriate for the magnitude of the particular number.

You can control formatting more precisely by writing modifiers between the % and the character that indicates which conversion to apply.  These slightly alter the ordinary behavior of the conversion. For example, most conversion specifications permit you to specify a minimum field width and a flag indicating whether you want the result left- or right-justified within the field.

The specific flags and modifiers that are permitted and their interpretation vary depending on the particular conversion.  They're all described in more detail in the following sections.

Output Conversion Syntax
------------------------

This section provides details about the precise syntax of conversion specifications that can appear in a printf template string.

Characters in the template string that are not part of a conversion specification are printed as-is to the output stream.

The conversion specifications in a printf template string have the general form:

     % FLAGS WIDTH [ . PRECISION ] TYPE CONVERSION

For example, in the conversion specifier %-10.8ld, the - is a flag, 10 specifies the field width, the precision is 8, the letter l is a type modifier, and d specifies the conversion style.  (This particular type specifier says to print a numeric argument in decimal notation, with a minimum of 8 digits left-justified in a field at least 10 characters wide.)

In more detail, output conversion specifications consist of an initial % character followed in sequence by:

  * Zero or more flag characters that modify the normal behavior of the conversion specification.

  * An optional decimal integer specifying the minimum field width. If the normal conversion produces fewer characters than this, the field is padded with spaces to the specified width.  This is a minimum value; if the normal conversion produces more characters than this, the field is not truncated.  Normally, the output is right-justified within the field.

     You can also specify a field width of *.  This means that the next argument in the argument list (before the actual value to be printed) is used as the field width.  The value is rounded to the nearest integer.  If the value is negative, this means to set the - flag (see below) and to use the absolute value as the field width.

  * An optional precision to specify the number of digits to be written for the numeric conversions.  If the precision is specified, it consists of a period (.) followed optionally by a decimal integer (which defaults to zero if omitted).

     You can also specify a precision of *.  This means that the next argument in the argument list (before the actual value to be printed) is used as the precision.  The value must be an integer, and is ignored if it is negative.

  * An optional type modifier character.  This character is ignored by Octave's printf function, but is recognized to provide compatibility with the C language printf.

  * A character that specifies the conversion to be applied.

The exact options that are permitted and how they are interpreted vary between the different conversion specifiers.  See the descriptions of the individual conversions for information about the particular options that they use.

Table of Output Conversions
---------------------------

Here is a table summarizing what all the different conversions do:

%d, %i
     Print an integer as a signed decimal number.  See Integer Conversions, for details.  %d and %i are synonymous for output, but are different when used with scanf for input (see Table of Input Conversions).

%o
     Print an integer as an unsigned octal number.  See Integer Conversions, for details.

%u
     Print an integer as an unsigned decimal number.  See Integer Conversions, for details.

%x, %X
     Print an integer as an unsigned hexadecimal number.  %x uses lower-case letters and %X uses upper-case.  See Integer Conversions, for details.

%f
     Print a floating-point number in normal (fixed-point) notation. See Floating-Point Conversions, for details.

%e, %E
     Print a floating-point number in exponential notation.  %e uses lower-case letters and %E uses upper-case.  See Floating-Point Conversions, for details.

%g, %G
     Print a floating-point number in either normal or exponential notation, whichever is more appropriate for its magnitude.  %g uses lower-case letters and %G uses upper-case.  See Floating-Point Conversions, for details.

%c
     Print a single character.  See Other Output Conversions.

%s
     Print a string.  See Other Output Conversions.

%%
     Print a literal % character.  See Other Output Conversions.

If the syntax of a conversion specification is invalid, unpredictable things will happen, so don't do this.  If there aren't enough function arguments provided to supply values for all the conversion specifications in the template string, or if the arguments are not of the correct types, the results are unpredictable.  If you supply more arguments than conversion specifications, the extra argument values are simply ignored; this is sometimes useful.

Integer Conversions
-------------------

This section describes the options for the %d, %i, %o, %u, %x, and %X conversion specifications.  These conversions print integers in various formats.

The %d and %i conversion specifications both print an numeric argument as a signed decimal number; while %o, %u, and %x print the argument as an unsigned octal, decimal, or hexadecimal number (respectively).  The %X conversion specification is just like %x except that it uses the characters ABCDEF as digits instead of abcdef.

The following flags are meaningful:

-
     Left-justify the result in the field (instead of the normal right-justification).

+
     For the signed %d and %i conversions, print a plus sign if the value is positive.

     For the signed %d and %i conversions, if the result doesn't start with a plus or minus sign, prefix it with a space character instead.  Since the + flag ensures that the result includes a sign, this flag is ignored if you supply both of them.

#
     For the %o conversion, this forces the leading digit to be 0, as if by increasing the precision.  For %x or %X, this prefixes a leading 0x or 0X (respectively) to the result.  This doesn't do anything useful for the %d, %i, or %u conversions.

0
     Pad the field with zeros instead of spaces.  The zeros are placed after any indication of sign or base.  This flag is ignored if the - flag is also specified, or if a precision is specified.

If a precision is supplied, it specifies the minimum number of digits to appear; leading zeros are produced if necessary.  If you don't specify a precision, the number is printed with as many digits as it needs.  If you convert a value of zero with an explicit precision of zero, then no characters at all are produced.

Floating-Point Conversions
--------------------------

This section discusses the conversion specifications for floating-point numbers: the %f, %e, %E, %g, and %G conversions.

The %f conversion prints its argument in fixed-point notation, producing output of the form [-]DDD.DDD, where the number of digits following the decimal point is controlled by the precision you specify.

The %e conversion prints its argument in exponential notation, producing output of the form [-]D.DDDe[+|-]DD. Again, the number of digits following the decimal point is controlled by the precision.  The exponent always contains at least two digits.  The %E conversion is similar but the exponent is marked with the letter E instead of e.

The %g and %G conversions print the argument in the style of %e or %E (respectively) if the exponent would be less than -4 or greater than or equal to the precision; otherwise they use the %f style.  Trailing zeros are removed from the fractional portion of the result and a decimal-point character appears only if it is followed by a digit.

The following flags can be used to modify the behavior:

-
     Left-justify the result in the field.  Normally the result is right-justified.

+
     Always include a plus or minus sign in the result.

     If the result doesn't start with a plus or minus sign, prefix it with a space instead.  Since the + flag ensures that the result includes a sign, this flag is ignored if you supply both of them.

#
     Specifies that the result should always include a decimal point, even if no digits follow it.  For the %g and %G conversions, this also forces trailing zeros after the decimal point to be left in place where they would otherwise be removed.

0
     Pad the field with zeros instead of spaces; the zeros are placed after any sign.  This flag is ignored if the - flag is also specified.

The precision specifies how many digits follow the decimal-point character for the %f, %e, and %E conversions.  For these conversions, the default precision is 6.  If the precision is explicitly 0, this suppresses the decimal point character entirely.  For the %g and %G conversions, the precision specifies how many significant digits to print.  Significant digits are the first digit before the decimal point, and all the digits after it. If the precision is 0 or not specified for %g or %G, it is treated like a value of 1.  If the value being printed cannot be expressed precisely in the specified number of digits, the value is rounded to the nearest number that fits.

Other Output Conversions
------------------------

This section describes miscellaneous conversions for printf.

The %c conversion prints a single character.  The - flag can be used to specify left-justification in the field, but no other flags are defined, and no precision or type modifier can be given. For example:

     printf ("%c%c%c%c%c", "h", "e", "l", "l", "o");

prints hello.

The %s conversion prints a string.  The corresponding argument must be a string.  A precision can be specified to indicate the maximum number of characters to write; otherwise characters in the string up to but not including the terminating null character are written to the output stream.  The - flag can be used to specify left-justification in the field, but no other flags or type modifiers are defined for this conversion.  For example:

     printf ("%3s%-6s", "no", "where");

prints  nowhere .

Formatted Input
---------------

Here are the descriptions of the functions for performing formatted input.

scanf (TEMPLATE)
     The scanf function reads formatted input from the stream stdin under the control of the template string TEMPLATE. The resulting values are returned.

fscanf (FILE, TEMPLATE)
     This function is just like scanf, except that the input is read from the stream FILE instead of stdin.

sscanf (STRING, TEMPLATE)
     This is like scanf, except that the characters are taken from the string STRING instead of from a stream.  Reaching the end of the string is treated as an end-of-file condition.

Calls to scanf are superficially similar to calls to printf in that arbitrary arguments are read under the control of a template string.  While the syntax of the conversion specifications in the template is very similar to that for printf, the interpretation of the template is oriented more towards free-format input and simple pattern matching, rather than fixed-field formatting. For example, most scanf conversions skip over any amount of ``white space'' (including spaces, tabs, and newlines) in the input file, and there is no concept of precision for the numeric input conversions as there is for the corresponding output conversions. Ordinarily, non-whitespace characters in the template are expected to match characters in the input stream exactly.

When a matching failure occurs, scanf returns immediately, leaving the first non-matching character as the next character to be read from the stream, and scanf returns all the items that were successfully converted.

The formatted input functions are not used as frequently as the formatted output functions.  Partly, this is because it takes some care to use them properly.  Another reason is that it is difficult to recover from a matching error.

Input Conversion Syntax
-----------------------

A scanf template string is a string that contains ordinary multibyte characters interspersed with conversion specifications that start with %.

Any whitespace character in the template causes any number of whitespace characters in the input stream to be read and discarded.  The whitespace characters that are matched need not be exactly the same whitespace characters that appear in the template string.  For example, write ,  in the template to recognize a comma with optional whitespace before and after.

Other characters in the template string that are not part of conversion specifications must match characters in the input stream exactly; if this is not the case, a matching failure occurs.

The conversion specifications in a scanf template string have the general form:

     % FLAGS WIDTH TYPE CONVERSION

In more detail, an input conversion specification consists of an initial % character followed in sequence by:

  * An optional flag character *, which says to ignore the text read for this specification.  When scanf finds a conversion specification that uses this flag, it reads input as directed by the rest of the conversion specification, but it discards this input, does not use a pointer argument, and does not increment the count of successful assignments.

  * An optional decimal integer that specifies the maximum field width.  Reading of characters from the input stream stops either when this maximum is reached or when a non-matching character is found, whichever happens first.  Most conversions discard initial whitespace characters (those that don't are explicitly documented), and these discarded characters don't count towards the maximum field width.

  * An optional type modifier character.  This character is ignored by Octave's scanf function, but is recognized to provide compatibility with the C language scanf.

  * A character that specifies the conversion to be applied.

The exact options that are permitted and how they are interpreted vary between the different conversion specifiers.  See the descriptions of the individual conversions for information about the particular options that they allow.

Table of Input Conversions
--------------------------

Here is a table that summarizes the various conversion specifications:

%d
     Matches an optionally signed integer written in decimal.  See Numeric Input Conversions.

%i
     Matches an optionally signed integer in any of the formats that the C language defines for specifying an integer constant.  See Numeric Input Conversions.

%o
     Matches an unsigned integer written in octal radix. See Numeric Input Conversions.

%u
     Matches an unsigned integer written in decimal radix. See Numeric Input Conversions.

%x, %X
     Matches an unsigned integer written in hexadecimal radix. See Numeric Input Conversions.

%e, %f, %g, %E, %G
     Matches an optionally signed floating-point number.  See Numeric Input Conversions.

%s
     Matches a string containing only non-whitespace characters. See String Input Conversions.

%c
     Matches a string of one or more characters; the number of characters read is controlled by the maximum field width given for the conversion. See String Input Conversions.

%%
     This matches a literal % character in the input stream.  No corresponding argument is used.

If the syntax of a conversion specification is invalid, the behavior is undefined.  If there aren't enough function arguments provided to supply addresses for all the conversion specifications in the template strings that perform assignments, or if the arguments are not of the correct types, the behavior is also undefined.  On the other hand, extra arguments are simply ignored.

Numeric Input Conversions
-------------------------

This section describes the scanf conversions for reading numeric values.

The %d conversion matches an optionally signed integer in decimal radix.

The %i conversion matches an optionally signed integer in any of the formats that the C language defines for specifying an integer constant.

For example, any of the strings 10, 0xa, or 012 could be read in as integers under the %i conversion.  Each of these specifies a number with decimal value 10.

The %o, %u, and %x conversions match unsigned integers in octal, decimal, and hexadecimal radices, respectively.

The %X conversion is identical to the %x conversion.  They both permit either uppercase or lowercase letters to be used as digits.

Unlike the C language scanf, Octave ignores the h, l, and L modifiers.

String Input Conversions
------------------------

This section describes the scanf input conversions for reading string and character values: %s and %c.

The %c conversion is the simplest: it matches a fixed number of characters, always.  The maximum field with says how many characters to read; if you don't specify the maximum, the default is 1.  This conversion does not skip over initial whitespace characters.  It reads precisely the next N characters, and fails if it cannot get that many.

The %s conversion matches a string of non-whitespace characters. It skips and discards initial whitespace, but stops when it encounters more whitespace after having read something.

For example, reading the input:

      hello, world

with the conversion %10c produces " hello, wo", but reading the same input with the conversion %10s produces "hello,".

Binary I/O
----------

Octave has to C-style functions for reading and writing binary data. They are fread and fwrite and are patterned after the standard C functions with the same names.

fread (FILE, SIZE, PRECISION)
     This function reads data in binary form of type PRECISION from the specified FILE, which may be either a file name, or a file number as returned from fopen.

     The argument SIZE specifies the size of the matrix to return.  It may be a scalar or a two-element vector.  If it is a scalar, fread returns a column vector of the specified length.  If it is a two-element vector, it specifies the number of rows and columns of the result matrix, and fread fills the elements of the matrix in column-major order.

     The argument PRECISION is a string specifying the type of data to read and may be one of "char", "schar", "short", "int", "long", "float", "double", "uchar", "ushort", "uint", or "ulong".  The default precision is "uchar".

     The fread function returns two values, data, which is the data read from the file, and count, which is the number of elements read.

fwrite (FILE, DATA, PRECISION)
     This function writes data in binary form of type PRECISION to the specified FILE, which may be either a file name, or a file number as returned from fopen.

     The argument DATA is a matrix of values that are to be written to the file.  The values are extracted in column-major order.

     The argument PRECISION is a string specifying the type of data to read and may be one of "char", "schar", "short", "int", "long", "float", "double", "uchar", "ushort", "uint", or "ulong".  The default precision is "uchar".

     The fwrite function returns the number of elements written.

     The behavior of fwrite is undefined if the values in DATA are too large to fit in the specified precision.

Other I/O Functions
-------------------

     fgets (FILE, len)

Read len characters from a file.

To flush output to a stream, use the function fflush (FILE). This is useful for ensuring that all pending output makes it to the screen before some other event occurs.  For example, it is always a good idea to flush the standard output stream before calling input.

Three functions are available for setting and determining the position of the file pointer for a given file.

The position of the file pointer (as the number of characters from the beginning of the file) can be obtained using the the function ftell (FILE).

To set the file pointer to any location within the file, use the function fseek (FILE, offset, origin).  The pointer is placed offset characters from the origin, which may be one of the predefined variables SEEK_CUR (current position), SEEK_SET (beginning), or SEEK_END (end of file). If origin is omitted, SEEK_SET is assumed.  The offset must be zero, or a value returned by ftell (in which case origin must be SEEK_SET.  See Predefined Constants.

The function frewind (FILE) moves the file pointer to the beginning of a file, returning 1 for success, and 0 if an error was encountered.  It is equivalent to fseek (FILE, 0, SEEK_SET).

The following example stores the current file position in the variable marker, moves the pointer to the beginning of the file, reads four characters, and then returns to the original position.

     marker = ftell (myfile);
     frewind (myfile);
     fourch = fgets (myfile, 4);
     fseek (myfile, marker, SEEK_SET);

The function feof (FILE) allows you to find out if an end-of-file condition has been encountered for a given file.  Note that it will only return 1 if the end of the file has already been encountered, not if the next read operation will result in an end-of-file condition.

Similarly, the function ferror (FILE) allows you to find out if an error condition has been encountered for a given file.  Note that it will only return 1 if an error has already been encountered, not if the next operation will result in an error condition.

The function kbhit may be usd to read a single keystroke from the keyboard.  For example,

     x = kbhit ();

will set X to the next character typed at the keyboard, without requiring a carriage return to be typed.

Finally, it is often useful to know exactly which files have been opened, and whether they are open for reading, writing, or both.  The command freport prints this information for all open files.  For example,

     octave:13> freport

      number  mode  name

           0     r  stdin
           1     w  stdout
           2     w  stderr
           3     r  myfile