7.10) Repeating Actions With The 'While' Loop

If you want to repeat a task several times in a program, you can do this with loops. The while loop will repeat a set of actions until a test is no longer True. Here’s the syntax:

while <Test>:
    <statement>

For example:

counter=3
print 'Let’s square some numbers'
print '(start of loop)'
while (counter <= 6):
    print counter, 'squared is', counter**2
    counter+=1   # equivalent to counter=counter+1
print '(end of the loop)'

Notice the colon (:) and the indentation (usually four spaces) which indicates the group of statements to be repeated. The typical way to use this loop is to have a counter that is changed within the loop (here it increases by 1).
Visualise this loop in action with the pythontutor.com visualisation tool.
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If you don’t modify the counter within the loop, the test is always True and so the loop will be repeated infinitely. To stop an infinite loop type CTRL-c.  This will escape from the loop and return an error. Try it out yourself in IDLE:

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<div class="syntaxhighlighter nogutter  python">
<div class="line number1 index0 alt2"><code class="python plain">counter</code><code class="python keyword">=</code><code class="python value">3</code></div>
<div class="line number2 index1 alt1"><code class="python keyword">while</code> <code class="python plain">counter <</code><code class="python keyword">=</code> <code class="python value">8</code><code class="python plain">:</code></div>
<div class="line number3 index2 alt2"><code class="python spaces">    </code><code class="python functions">print</code> <code class="python string">'execute the loop body'</code></div>
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