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Message: <blockquote data-quote="gayan kalhara" data-source="post: 3322065" data-attributes="member: 60168">part 4  Count to 104.1 While loops Presenting our first control structure. Ordinarily the computer starts with the first line and then goes down from there. Control structures change the order that statements are executed or decide if a certain statement will be run. Here’s thesource for a program that uses the while control structure:a = 0while a < 10:a = a + 1print aAnd here is the extremely exciting output:12345678910(And you thought it couldn’t get any worse after turning your computer into a five dollar calculator?) So what doesthe program do? First it sees the line a = 0 and makes a zero. Then it sees while a < 10: and so the computerchecks to see if a < 10. The first time the computer sees this statement a is zero so it is less than 10. In other wordswhile a is less than ten the computer will run the tabbed in statements.Here is another example of the use of while:13a = 1s = 0print ’Enter Numbers to add to the sum.’print ’Enter 0 to quit.’while a != 0 :print ’Current Sum:’,sa = input(’Number? ’)s = s + aprint ’Total Sum =’,sThe first time I ran this program Python printed out:File "sum.py", line 3while a != 0ˆSyntaxError: invalid syntaxI had forgotten to put the : after the while. The error message complained about that problem and pointed out whereit thought the problem was with the ˆ . After the problem was fixed here was what I did with the program:Enter Numbers to add to the sum.Enter 0 to quit.Current Sum: 0Number? 200Current Sum: 200Number? -15.25Current Sum: 184.75Number? -151.85Current Sum: 32.9Number? 10.00Current Sum: 42.9Number? 0Total Sum = 42.9Notice how print ’Total Sum =’,s is only run at the end. The while statement only affects the line that are tabbed in (a.k.a. indented). The != means does not equal so while a != 0 : means until a is zero run the tabbed in statements that are afterwards. Now that we have while loops, it is possible to have programs that run forever. An easy way to do this is to write a program like this:while 1 == 1:print "Help, I’m stuck in a loop."This program will output Help, I’m stuck in a loop. until the heat death of the universe or you stop it. Tip The way to stop it is to hit the Control (or Ctrl) button and ‘c’ (the letter) at the same time. This will kill the program. (Note: sometimes you will have to hit enter after the Control C.)</blockquote>

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