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Catalog / Tcl Programming Cheatsheet

Tcl Programming Cheatsheet

A quick reference guide to the Tcl scripting language, covering syntax, commands, and common usage patterns.

Tcl Basics

Syntax Fundamentals

Command Structure

 
commandName arg1 arg2 ... argN

Commands are space-separated.

Variable Assignment

 
set variableName value

Assigns a value to a variable.

Variable Substitution

 
$variableName

Substitutes the value of a variable.

Command Substitution

 
[commandName arg1 arg2]

Executes a command and substitutes the result.

Comments

 
# This is a comment

Quoting

" - Prevents word splitting and allows variable substitution.
{} - Prevents word splitting and inhibits all substitutions.

Basic Commands

puts

Prints a string to the standard output.

puts "Hello, World!"

set

Assigns a value to a variable.

set name "John"
puts "Hello, $name!"

expr

Evaluates an expression.

set result [expr 2 + 2]
puts $result

if

Conditional execution.

if { $x > 10 } {
  puts "x is greater than 10"
}

for

Looping construct.

for {set i 0} {$i < 5} {incr i} {
  puts "Iteration $i"
}

proc

Defines a procedure.

proc greet {name} {
  puts "Hello, $name!"
}
greet "Alice"

Control Flow and Procedures

Conditional Statements

if-elseif-else

if {condition1} {
  # Code to execute if condition1 is true
} elseif {condition2} {
  # Code to execute if condition2 is true
} else {
  # Code to execute if all conditions are false
}

switch

switch $variable {
  value1 { code_block1 }
  value2 { code_block2 }
  default { default_code_block }
}

Looping Constructs

while

 
while {condition} {
  # Code to execute while the condition is true
}

foreach

 
foreach variable list {
  # Code to execute for each element in the list
}

break

Exits the current loop.

while {1} {
  if {condition} { break }
}

continue

Skips the current iteration and continues with the next.

foreach i {1 2 3} {
  if { $i == 2 } { continue }
  puts $i
}

Procedures (Functions)

Procedure Definition

proc procedureName {arg1 arg2 ...} {
  # Procedure body
  return value
}

Calling a Procedure

procedureName value1 value2

Example

proc add {a b} {
  return [expr $a + $b]
}

set sum [add 5 3]
puts $sum ;# Output: 8

Variable Scope - By default, variables are local to the procedure. Use global to access global variables.

set globalVar 10
proc modifyGlobal {} {
  global globalVar
  set globalVar [expr $globalVar + 5]
}
modifyGlobal
puts $globalVar ;# Output: 15

String Manipulation and Lists

String Operations

string length

Returns the length of a string.

string length "Hello"
;# Output: 5

string index

Returns the character at a specific index.

string index "Hello" 1
;# Output: e

string range

Extracts a substring.

string range "Hello" 1 3
;# Output: ell

string compare

Compares two strings.

string compare "apple" "banana"
;# Output: -1 (apple < banana)

string tolower

Converts a string to lowercase.

string tolower "HELLO"
;# Output: hello

string toupper

Converts a string to uppercase.

string toupper "hello"
;# Output: HELLO

List Manipulation

list

Creates a list.

list 1 2 3
;# Output: 1 2 3

lindex

Returns an element from a list by index.

lindex {1 2 3} 1
;# Output: 2

llength

Returns the length of a list.

llength {1 2 3}
;# Output: 3

lappend

Appends elements to a list.

set myList {1 2}
lappend myList 3 4
puts $myList ;# Output: 1 2 3 4

linsert

Inserts elements into a list at a given index.

linsert {1 2 3} 1 a b
;# Output: 1 a b 2 3

lreplace

Replaces elements in a list.

lreplace {1 2 3} 1 1 a b
;# Output: 1 a b 3

File I/O and Regular Expressions

File Input/Output

open

Opens a file.

set file [open "myfile.txt" r]

read

Reads data from a file.

set data [read $file]

puts (to file)

Writes data to a file.

puts $file "Hello, file!"

close

Closes a file.

close $file

gets

Reads a line from a file.

set line [gets $file lineVar]

Regular Expressions

regexp

Matches a regular expression against a string.

regexp {pattern} string [matchVar] [submatchVar1] [submatchVar2] ...

Example: Matching

 
if { [regexp {\d+} "abc123def" ] } {
  puts "Match found"
}

Example: Capturing

 
regexp {(\d+)} "abc123def" match number
puts "Match: $match, Number: $number"

regsub

Substitutes regular expression matches.

regsub {pattern} string replacement varName

Example: Substitution

 
regsub {\s+} "Hello  World" " " result
puts $result ;# Output: Hello World
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