Using nested for loop with a single list

[mikeavison]

I am trying to examine every sum of three numbers from a single list

So for example if the list was 1,2 I want to examine 1=1+1+1 and 1+1+2 and 1+2+2 and 2+2+2

In the code below:

The variable I am examining is threesum

The list is three_digit_squares[]

Whenever I run the code it gives me "index out of range" error on the line starting "threesum ="

three_digit_squares = []

for i in range (1,4):
    three_digit_squares.extend([i])
    
for i in three_digit_squares:
    for j in three_digit_squares:
        for k in three_digit_squares:
            threesum = three_digit_squares[i] + three_digit_squares[j] + three_digit_squares[k]
            if threesum == 4:
            
                print "yes"

[ichabod801]

three_digit_squares is [1, 2, 3]. When it gets to three it gives an index error, because x[3] is the fourth item in the list (Python is 0 indexed).

[mikeavison]

Thanks Craig

I realised what I'd done wrong halfway round a bike ride, you'd already replied by then.
It was rookie error from someone more used to basic and fortran, I forgot that i is the value not the index in the form of code I gave.

So changing my code as little as possible, this works OK

three_digit_squares = []

for i in range (1,4):
three_digit_squares.extend([i])

a=len(three_digit_squares)-1
for i in range(a):
for j in range(a):
for k in range(a):
threesum = three_digit_squares[i] + three_digit_squares[j] + three_digit_squares[k]
if threesum == 4:

print "yes"

I did initially try putting the len(three_digit_squares)-1 inside the range statement, but that did not work. So I had to assign it to a and use a in the range statements. It seems Python is not as infinitely substitutional as say C or C-shell? Or have I misunderstood that too!

[ichabod801]

Please use python tags to preserve the indentation of your code. See the BBCode link in my signature for details.

That's a bad way to do the loop, I should have mentioned that in my first response. In Python you don't want to be looping over the indexes, you want to loop over the list:

for i in three_digit_squares:
    for j in three_digit_squares:
        for k in three_digit_squares:
            if i + j + k == 4:
                print 'yes'

Furthermore, check out the itertools module. It has several combinatoric generators that simplify things like this:

from itertools import product
...
for i, j, k in product(three_digit_squares, repeat = 3):
    if i + k + k == 4:
        print 'yes'

Also, if you are just learning Python, I would suggest learning Python 3.x instead of Python 2.x.