[Tutor] nested "for" loops

[Bob Gailer]

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At 07:43 PM 5/2/2003 -0700, Peter Jakubowicz wrote:

>Hi,
>
>I've been slogging along learning Python for a while now. Nested "for" 
>loops confuse me (I have trouble trying to run through them in my head). 
>For example, does the following code generate (albeit redundantly) all 
>Pythagorean triples up to 20: i.e., all integers less than or equal to 20 
>for which i * i + j * j == k * k
>
>TIA,
>Peter
>
>
>
>for i in range(1, 21):
>     for j in range(1, 21):
>         for k in range(1, 21):
>             if (i * i) + (j * j) == (k * k):
>                 print "Pythagorean triple: %d, %d, %d" % (i, j, k)

Did you run the program? Did it deliver the desired results? There's your 
answer. Is that what you wanted to know? Or are you needing a way to 
comprehend nested loops?

BTW it's a good idea to anticipate performance and maintenance issues, 
especially if you wanted to expand this to a higher limit. Suggestion - 
move the calculation of i*i and j*j out of the inner loop. Right now these 
calculations are unnecessarily repeated. Also make the limit a variable 
whose value you set once. Then it's a lot easier to change the limit. Also 
to eliminate duplications start each inner loop with the current value of 
the next higher loop.

limit = 21
for i in range(1, limit ):
     iSq = i*i
     for j in range(i, limit ):
         jSq = j*j
         for k in range(j, limit ):
             if iSq+ jSq == (k * k):
                 print "Pythagorean triple: %d, %d, %d" % (i, j, k)


Bob Gailer
bgailer@alum.rpi.edu
303 442 2625

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