Nested WHILE loops in Python

Question

I am a beginner with Python and trying few programs. I have something like the following WHILE loop construct in Python (not exact).

IDLE 2.6.4      
>>> a=0
>>> b=0
>>> while a < 4:
      a=a+1
      while b < 4:
         b=b+1
         print a, b

  
1 1
1 2
1 3
1 4

I am expecting the outer loop to loop through 1,2,3 and 4. And I know I can do this with FOR loop like this

>>> for a in range(1,5):
       for b in range(1,5):
           print a,b

  
1 1
1 2
.. ..
.. .. // Other lines omitted for brevity
4 4

But, what is wrong with WHILE loop? I guess I am missing some thing obvious, but could not make out.

Answer: The corrected WHILE loop..

>>> a=0
>>> while a < 4:
    a=a+1
    b=0
    while b<4:
        b=b+1
        print a,b

        
1 1
.. ..
.. .. // Other lines omitted for brevity
4 4

P.S: Searched out SO, found few questions but none as close to this. Don't know whether this could classified as homework, the actual program was different, the problem is what puzzles me.

Alex Martelli · Accepted Answer · 2010-05-15 19:40:13Z

7

You're not resetting b to 0 right inside your outer loop, so b stays at the value it had after the first leg of the outer loop -- 4 -- and the inner loop never executes again.

The for loops work fine because they do reset their loop control variables correctly; with the less-structured while loops, such resetting is in your hands, and you're not doing it.

answered May 15, 2010 at 19:40

Alex Martelli

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