Tcl - Loops

There may be a situation, where you need to execute a block of code several number of times. In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.

Programming languages provide various control structures that allow for more complicated execution paths.

A loop statement allows us to execute a statement or group of statements multiple times and following is the general form of a loop statement in most of the programming languages −

Loop Architecture

Tcl language provides the following types of loops to handle looping requirements.

Sr.No. Loop Type & Description
1 while loop

Repeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body.

2 for loop

Executes a sequence of statements multiple times and abbreviates the code that manages the loop variable.

3 nested loops

You can use one or more loop inside any another while, for or do..while loop.

Loop Control Statements

Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.

Tcl supports the following control statements.

Sr.No. Control Statement & Description
1 break statement

Terminates the loop or switch statement and transfers execution to the statement immediately following the loop or switch.

2 continue statement

Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.

The Infinite Loop

A loop becomes infinite loop if a condition never becomes false. The while loop is traditionally used for this purpose. You can make an endless loop by leaving the conditional expression as 1.

while {1} {
   puts "This loop will run forever."

When the conditional expression is absent, it is assumed to be true. Tcl programmers more commonly use the while {1} construct to signify an infinite loop.

NOTE − You can terminate an infinite loop by pressing Ctrl + C keys.

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