**Looping 102 - For and incr**


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Tcl supports a loop construct similar to the `for` loop in C.  The `for` command in Tcl takes four arguments; an initialization, a test, an increment, and the body of code to evaluate on each pass through the loop.  The syntax for the `for` command is:

======
   for start test next body
======



During evaluation of the `for` command, the `start` code is evaluated once, before any other arguments are evaluated.  After the start code has been evaluated, the `test` is evaluated.  If the `test` evaluates to true, then the `body` is evaluated, and finally, the `next` argument is evaluated.  After evaluating the `next` argument, the interpreter loops back to the `test`, and repeats the process.  If the `test` evaluates as false, then the loop will exit immediately.

`start` is the initialization portion of the command.  It is usually used to initialize the iteration variable, but can contain any code that you wish to execute before the loop starts.

The `test` argument is evaluated as an expression, just as with the `expr` `while` and `if` commands.

`next` is commonly an incrementing command, but may contain any command which the Tcl interpreter can evaluate.

`body` is the body of code to execute.

When braces are used for grouping, the newline is not treated as the end of a Tcl command.  This makes it simpler to write multiple line commands.  However, the opening brace '''must''' be on the line with the `for` command, or the Tcl interpreter will treat the close of the `next` brace as the end of the command, and you will get an error.  This is different than other languages like C or Perl, where it doesn't matter where you place your braces.

Within the `body` code, the commands `break` and `continue` may be used just as they are used with the `while` command.  When a `break` is encountered, the loop exits immediately.  When a `continue` is encountered, evaluation of the `body` ceases, and the next iteration is started (if there is one left).

Because incrementing the iteration variable is so common, Tcl has a special command for this:

======
   incr varName ?increment?
======


This command adds the value in the second argument to the variable named in the first argument.  If no value is given for the second argument, it defaults to 1.

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***Example***

======
for {set i 0} {$i < 10} {incr i} {
    puts "I inside first loop: $i"
}

for {set i 3} {$i < 2} {incr i} {
    puts "I inside second loop: $i"
}

puts "Start"
set i 0
while {$i < 10} {
    puts "I inside third loop: $i"
    incr i
    puts "I after incr: $i"
}

set i 0
incr i
# This is equivalent to:
set i [expr {$i + 1}]
======

<<discussion>>Resulting output
======none
I inside first loop: 0
I inside first loop: 1
I inside first loop: 2
I inside first loop: 3
I inside first loop: 4
I inside first loop: 5
I inside first loop: 6
I inside first loop: 7
I inside first loop: 8
I inside first loop: 9
Start
I inside third loop: 0
I after incr: 1
I inside third loop: 1
I after incr: 2
I inside third loop: 2
I after incr: 3
I inside third loop: 3
I after incr: 4
I inside third loop: 4
I after incr: 5
I inside third loop: 5
I after incr: 6
I inside third loop: 6
I after incr: 7
I inside third loop: 7
I after incr: 8
I inside third loop: 8
I after incr: 9
I inside third loop: 9
I after incr: 10
======
<<enddiscussion>>

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