14.2.11 Formatted Input

Octave provides the scanf, fscanf, and sscanf functions to read formatted input. There are two forms of each of these functions. One can be used to extract vectors of data from a file, and the other is more ‘C-like’.

: [val, count, errmsg] = fscanf (fid, template, size)

: [v1, v2, …, count, errmsg] = fscanf (fid, template, "C")

In the first form, read from fid according to template, returning the result in the matrix val.

The optional argument size specifies the amount of data to read and may be one of

Inf

Read as much as possible, returning a column vector.

nr

Read up to nr elements, returning a column vector.

[nr, Inf]

Read as much as possible, returning a matrix with nr rows. If the number of elements read is not an exact multiple of nr, the last column is padded with zeros.

[nr, nc]

Read up to nr * nc elements, returning a matrix with nr rows. If the number of elements read is not an exact multiple of nr, the last column is padded with zeros.

If size is omitted, a value of Inf is assumed.

A string is returned if template specifies only character conversions.

The number of items successfully read is returned in count.

If an error occurs, errmsg contains a system-dependent error message.

In the second form, read from fid according to template, with each conversion specifier in template corresponding to a single scalar return value. This form is more “C-like”, and also compatible with previous versions of Octave. The number of successful conversions is returned in count

See the Formatted Input section of the GNU Octave manual for a complete description of the syntax of the template string.

See also: fgets, fgetl, fread, scanf, sscanf, fopen.

: [val, count, errmsg] = scanf (template, size)

: [v1, v2, …, count, errmsg] = scanf (template, "C")

This is equivalent to calling fscanf with fid = stdin.

It is currently not useful to call scanf in interactive programs.

See also: fscanf, sscanf, printf.

: [val, count, errmsg, pos] = sscanf (string, template, size)

: [v1, v2, …, count, errmsg] = sscanf (string, template, "C")

This is like fscanf, 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. In addition to the values returned by fscanf, the index of the next character to be read is returned in pos.

See also: fscanf, scanf, sprintf.

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. For example, note that sscanf parses the string and whitespace differently when using mixed numeric and string output types:

teststr = "1 is a lonely number";
sscanf (teststr, "%s is a %s")
⇒ 1lonelynumber

sscanf (teststr, "%g is a %s")
⇒
     1
   108
   111
   110
   101
   108
   121

[a, b, c] = sscanf ("1 is a lonely number", "%g is a %s %s", "C")
⇒ a =  1
⇒ b = lonely
⇒ c = number

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.

The specific flags and modifiers that are permitted in the template string and their interpretation are all described in more detail in the following sections.