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Loops: while, for and foreach

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• WHILE-statement

  • The syntax of the WHILE-statement is:

    while TEST BODY

  • The meaning of the FOR-statement is:

    The while command evaluates TEST as an expression If the result is true, then BODY is executed (as a Tcl command) Otherwise, the while is terminated. After the code in BODY has been executed, TEST is evaluated again.

  • WARNING: use {   } quotes to group the TEST and BODY parts !!!

    (Because, you may inadvertantly write infinite loops !)

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• Examples of WHILE

  set x 0 while {$x < 10} { puts "x is $x" set x [expr {$x + 1}] } puts "exited loop with x equal to $x\n" Output: 0, 1, 2, ..., 9

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• WARNING
  • Consider this example:

    set x 0 while "$x < 10" { puts "x is $x" set x [expr {$x + 1}] } Output: 0, 1, 2, .... (infinite loop) !!! How is this expression executed: 1. TEST expression passed into WHILE statement is: "$x < 10" (" " allows substitution !) ==> "0 < 10" 2. The while command receives: TEST = (0 < 10) (0 < 10)will evaluate to TRUE every time !!! ====> infinite loop !!!

  • The WHILE statement will evaluate the expression TEST each time through the loop.

  • However, you must understand what TEST expression is passed into the WHILE statement to be evaluated !

    (See above for the actual TEST string that is passed to WHILE for testing)

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  • This example shows clearly why you must prevent substitution in the TEST expression using { }.

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• FOR-statement

  • The syntax of the FOR-statement is:

    for START TEST NEXT BODY

  • The meaning of the FOR-statement is:

    The START code is evaluated once (before any other arguments are evaluated.) After the START code has been evaluated, the TEST is evaluated. If TEST is true (i.e., not zero), the code in BODY is executed. After the code in BODY has been executed, the code in NEXT is evaluated. After evaluating the NEXT argument, the interpreter loops back to the TEST, and repeats the process. The loop will exit if the TEST evaluates as false (just like for-loop in Java or C++)

  • WARNING:

    Yes, you guessed it, you better use { ... } to prevent substitutions in all the parts

    (Because, you may inadvertantly write infinite loops !)

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• Simple Example
  • The following for loop runs the index from 0 to 9:

    Java equivalent: for (i = 0; i < 9; i = i + 1) for {set i 0} {$i < 10} {set i [expr $i + 1]} { puts "i = $i" }

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• The incr command
  • Because incrementing a variable is commonly used in for loops, Tcl provides an increment command for convenience:

    incr VARNAME [increment-amount] Examples: incr i i = i + 1 incr i 2 i = i + 2

  • The above simple for loops can be written as:

    Java equivalent: for (i = 0; i < 9; i = i + 1) for {set i 0} {$i < 10} {incr i} { puts "i = $i" }

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• Special statements

  • continue (just like Java, or C++)
    • When a continue statement within BODY is executed, the execution of the code in BODY terminates immediately

    • The TEST will be re-evaluated after the continue statement

  • break (just like Java, or C++)
    • When a break statement within BODY is executed, the execution of the code in BODY terminates immediately

    • The while is exited and execution continues with the statement following the while statement.

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• Further reading
  • While: click here
  • For: click here
  • Foreach on associative arrays: click here