Tcl Language Reference -- scan tcl

scan tcl

Overview

Parse string using conversion specifiers in the style of sscanf

Syntax

scan string format varName ?varName ...?

Introduction

This command parses fields from an input string in the same fashion as the ANSI C sscanf procedure and returns a count of the number of fields sucessfully parsed. String gives the input to be parsed and format indicates how to parse it, using % conversion specifiers as in sscanf. Each varName gives the name of a variable; when a field is scanned from string the result is converted back into a string and assigned to the corresponding variable.

Details on Scanning

Scan operates by scanning string and formatString together. If the next character in formatString is a blank or tab then it is ignored. Otherwise, if it isn't a % character then it must match the next non-white-space character of string. When a % is encountered in formatString, it indicates the start of a conversion specifier. A conversion specifier contains three fields after the %: a *, which indicates that the converted value is to be discarded instead of assigned to a variable; a number indicating a maximum field width; and a conversion character. All of these fields are optional except for the conversion character.

When scan finds a conversion specifier in formatString, it first skips any white-space characters in string. Then it converts the next input characters according to the conversion specifier and stores the result in the variable given by the next argument to scan. The following conversion characters are supported:

d

The input field must be a decimal integer. It is read in and the value is stored in the variable as a decimal string.

o

The input field must be an octal integer. It is read in and the value is stored in the variable as a decimal string.

x

The input field must be a hexadecimal integer. It is read in and the value is stored in the variable as a decimal string.

c

A single character is read in and its binary value is stored in the variable as a decimal string. Initial white space is not skipped in this case, so the input field may be a white-space character. This conversion is different from the ANSI standard in that the input field always consists of a single character and no field width may be specified.

s

The input field consists of all the characters up to the next white-space character; the characters are copied to the variable.

e or f or g

The input field must be a floating-point nu

mber consisting of an optional sign, a string of decimal digits possibly con taining a decimal point, and an optional exponent consisting of an e or E followed by an optional sign and a string of decimal digits. It is read in and stored in the variable as a floating-point string.

[chars]

The input field consists of any number of characters in chars. The matching string is stored in the variable. If the first character between the brackets is a ] then it is treated as part of chars rather than the closing bracket for the set.

[^chars]

The input field consists of any number of characters not in chars. The matching string is stored in the variable. If the character immediately following the ^ is a ] then it is treated as part of the set rather than the closing bracket for the set.

The number of characters read from the input for a conversion is the largest number that makes sense for that particular conversion (e.g. as many decimal digits as possible for %d, as many octal digits as possible for %o, and so on). The input field for a given conversion terminates either when a white-space character is encountered or when the maximum field width has been reached, whichever comes first. If a * is present in the conversion specifier then no variable is assigned and the next scan argument is not consumed.

Differences from ANSI sscanf

The behavior of the scan command is the same as the behavior of the ANSI C sscanf procedure except for the following differences:

[1] %p and %n conversion specifiers are not currently supported.

[2] For %c conversions a single character value is converted to a decimal string, which is then assigned to the corresponding varName; no field width may be specified for this conversion.

[3] The l, h, and L modifiers are ignored; integer values are always converted as if there were no modifier present and real values are always converted as if the l modifier were present (i.e. type double is used for the internal representation).

Keywords

conversion specifier, parse, scan

Top of Page
Copyright © 1996 America Online, Inc.

d	The input field must be a decimal integer. It is read in and the value is stored in the variable as a decimal string.
o	The input field must be an octal integer. It is read in and the value is stored in the variable as a decimal string.
x	The input field must be a hexadecimal integer. It is read in and the value is stored in the variable as a decimal string.
c	A single character is read in and its binary value is stored in the variable as a decimal string. Initial white space is not skipped in this case, so the input field may be a white-space character. This conversion is different from the ANSI standard in that the input field always consists of a single character and no field width may be specified.
s	The input field consists of all the characters up to the next white-space character; the characters are copied to the variable.
e or f or g	The input field must be a floating-point nu mber consisting of an optional sign, a string of decimal digits possibly con taining a decimal point, and an optional exponent consisting of an e or E followed by an optional sign and a string of decimal digits. It is read in and stored in the variable as a floating-point string.
[chars]	The input field consists of any number of characters in chars. The matching string is stored in the variable. If the first character between the brackets is a ] then it is treated as part of chars rather than the closing bracket for the set.
[^chars]	The input field consists of any number of characters not in chars. The matching string is stored in the variable. If the character immediately following the ^ is a ] then it is treated as part of the set rather than the closing bracket for the set.