Tcl - Strings

The primitive data-type of Tcl is string and often we can find quotes on Tcl as string only language. These strings can contain alphanumeric character, just numbers, Boolean, or even binary data. Tcl uses 16 bit unicode characters and alphanumeric characters can contain letters including non-Latin characters, number or punctuation.

Boolean value can be represented as 1, yes or true for true and 0, no, or false for false.

String Representations

Unlike other languages, in Tcl, you need not include double quotes when it's only a single word. An example can be −

#!/usr/bin/tclsh

set myVariable hello
puts $myVariable

When the above code is executed, it produces the following result −

hello

When we want to represent multiple strings, we can use either double quotes or curly braces. It is shown below −

#!/usr/bin/tclsh

set myVariable "hello world"
puts $myVariable
set myVariable {hello world}
puts $myVariable

When the above code is executed, it produces the following result −

hello world
hello world

String Escape Sequence

A character literal can be a plain character (e.g., 'x'), an escape sequence (e.g., '\t'), or a universal character (e.g., '\u02C0').

There are certain characters in Tcl when they are preceded by a backslash they will have special meaning and they are used to represent like newline (\n) or tab (\t). Here, you have a list of some of such escape sequence codes −

Following is the example to show a few escape sequence characters −

#!/usr/bin/tclsh

puts "Hello\tWorld\n\nHowcodex";

When the above code is compiled and executed, it produces the following result −

Hello   World

Howcodex

String Command

The list of subcommands for string command is listed in the following table −

Examples of some commonly used Tcl string sub commands are given below.

String Comparison

#!/usr/bin/tclsh

set s1 "Hello"
set s2 "World"
set s3 "World"
puts [string compare $s1 $s2]
if {[string compare $s2 $s3] == 0} {
   puts "String \'s1\' and \'s2\' are same.";
}

if {[string compare $s1 $s2] == -1} {
   puts "String \'s1\' comes before \'s2\'.";
}

if {[string compare $s2 $s1] == 1} {
   puts "String \'s2\' comes after \'s1\'.";
}

When the above code is compiled and executed, it produces the following result −

-1
String 's1' and 's2' are same.
String 's1' comes before 's2'.
String 's2' comes after 's1'.

Index of String

#!/usr/bin/tclsh

set s1 "Hello World"
set s2 "o"
puts "First occurrence of $s2 in s1"
puts [string first $s2 $s1]
puts "Character at index 0 in s1"
puts [string index $s1 0]
puts "Last occurrence of $s2 in s1"
puts [string last $s2 $s1]
puts "Word end index in s1"
puts [string wordend $s1 20]
puts "Word start index in s1"
puts [string wordstart $s1 20]

When the above code is compiled and executed, it produces the following result −

First occurrence of o in s1
4
Character at index 0 in s1
H
Last occurrence of o in s1
7
Word end index in s1
11
Word start index in s1
6

Length of String

#!/usr/bin/tclsh

set s1 "Hello World"
puts "Length of string s1"
puts [string length $s1]

When the above code is compiled and executed, it produces the following result −

Length of string s1
11

Handling Cases

#!/usr/bin/tclsh

set s1 "Hello World"
puts "Uppercase string of s1"
puts [string toupper $s1]
puts "Lowercase string of s1"
puts [string tolower $s1]

When the above code is compiled and executed, it produces the following result −

Uppercase string of s1
HELLO WORLD
Lowercase string of s1
hello world

Trimming Characters

#!/usr/bin/tclsh

set s1 "Hello World"
set s2 "World"
puts "Trim right $s2 in $s1"
puts [string trimright $s1 $s2]

set s2 "Hello"
puts "Trim left $s2 in $s1"
puts [string trimleft $s1 $s2]

set s1 " Hello World "
set s2 " "
puts "Trim characters s1 on both sides of s2"
puts [string trim $s1 $s2]

When the above code is compiled and executed, it produces the following result −

Trim right World in Hello World
Hello 
Trim left Hello in Hello World
 World
Trim characters s1 on both sides of s2
Hello World

Matching Strings

#!/usr/bin/tclsh

set s1 "test@test.com" 
set s2 "*@*.com"
puts "Matching pattern s2 in s1"
puts [string match "*@*.com" $s1 ]
puts "Matching pattern tcl in s1"
puts [string match {tcl} $s1]

When the above code is compiled and executed, it produces the following result −

Matching pattern s2 in s1
1
Matching pattern tcl in s1
0

Append Command

#!/usr/bin/tclsh

set s1 "Hello" 
append s1 " World"
puts $s1

When the above code is compiled and executed, it produces the following result −

Hello World

Format command

The following table shows the list of format specifiers available in Tcl −

Some simple examples are given below −

#!/usr/bin/tclsh

puts [format "%f" 43.5]
puts [format "%e" 43.5]
puts [format "%d %s" 4 tuts]
puts [format "%s" "Tcl Language"]
puts [format "%x" 40]

When the above code is compiled and executed, it produces the following result −

43.500000
4.350000e+01
4 tuts
Tcl Language
28

Scan command

Scan command is used for parsing a string based to the format specifier. Some examples are shown below.

#!/usr/bin/tclsh

puts [scan "90" {%[0-9]} m]
puts [scan "abc" {%[a-z]} m]
puts [scan "abc" {%[A-Z]} m]
puts [scan "ABC" {%[A-Z]} m]

When the above code is compiled and executed, it produces the following result −

1
1
0
1