TCL String

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,

set myVariable hello

puts $myVariable

When above code is executed, it produces following result.

hello

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

set myVariable "hello world"

puts $myVariable

set myVariable {hello world}

puts $myVariable

When above code is executed, it produces 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:

Escape sequence	Meaning
\\	\ character
\'	' character
\"	" character
\?	? character
\a	Alert or bell
\b	Backspace
\f	Form feed
\n	Newline
\r	Carriage return
\t	Horizontal tab
\v	Vertical tab

Following is the example to show few escape sequence characters:

puts("Hello\tWorld\n\n");

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

Hello   World

String command

The list of subcommands for string command are listed in the following table.

SN	Methods with Description
1	compare string1 string2 Compares string1 and string2 lexographically. Returns 0 if equal, -1 if string1 comes before string2, else 1.
2	<bfirst< b="" style="box-sizing: border-box;">string1 string2</bfirst<> Returns the index first occurrence of string1 in string2. If not found, returns -1.
3	index string index Returns the character at index.
4	last string1 string2 Returns the index last occurrence of string1 in string2. If not found, returns -1.
5	length string Returns the length of string.
6	match pattern string Returns 1 if the string matches the pattern.
7	range string index1 index2 Return the range of characters in string from index1 to index2.
8	tolower string Returns the lowercase string.
9	toupper string Returns the uppercase string.
10	trim string ?trimcharacters? Removes trimcharacters in both ends of string. The default trimcharacters is whitespace.
11	trimleft string ?trimcharacters? Removes trimcharacters in left beginning of string. The default trimcharacters is whitespace.
12	trimright string ?trimcharacters? Removes trimcharacters in left end of string. The default trimcharacters is whitespace.
13	wordend findstring index Return the index in findstring of the character after the word containing the character at index.
14	wordstart findstring index Return the index in findstring of the first character in the word containing the character at index.

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

String comparison

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 before \'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 before 's1'.

Index of string

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

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

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

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

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

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.

Specifier	Use
%s	String representation
%d	Integer representation
%f	Floating point representation
%e	Floating point representation with mantissa-exponent form
%x	Hexa decimal representation

Some simple examples are given below.

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.

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