"'generator' object is not subscriptable" error

Question

Why am I getting this error, from line 5 of my code, when attempting to solve Project Euler Problem 11?

for x in matrix:
    p = 0
    for y in x:
        if p < 17:
            currentProduct = int(y) * int(x[p + 1]) * int(x[p + 2]) * int(x[p + 3])
            if currentProduct > highestProduct:
                print(currentProduct)
                highestProduct = currentProduct
        else:
                break
            p += 1

'generator' object is not subscriptable

Asked By: Matthew Hannah

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Source

Answer 1

Your x value is is a generator object, which is an Iterator: it generates values in order, as they are requested by a for loop or by calling next(x).

You are trying to access it as though it were a list or other Sequence type, which let you access arbitrary elements by index as x[p + 1].

If you want to look up values from your generator’s output by index, you may want to convert it to a list:

x = list(x)

This solves your problem, and is suitable in most cases. However, this requires generating and saving all of the values at once, so it can fail if you’re dealing with an extremely long or infinite list of values, or the values are extremely large.

If you just needed a single value from the generator, you could instead use itertools.islice(x, p) to discard the first p values, then next(...) to take the one you need. This eliminate the need to hold multiple items in memory or compute values beyond the one you’re looking for.

import itertools

result = next(itertools.islice(x, p))

Answered By: Jeremy

Answer 2

As an extension to Jeremy’s answer some thoughts about the design of your code:

Looking at your algorithm, it appears that you do not actually need truly random access to the values produced by the generator: At any point in time you only need to keep four consecutive values (three, with an extra bit of optimization). This is a bit obscured in your code because you mix indexing and iteration: If indexing would work (which it doesn’t), your y could be written as x[p + 0].

For such algorithms, you can apply kind of a "sliding window" technique, demonstrated below in a stripped-down version of your code:

import itertools, functools, operator
vs = [int(v) for v in itertools.islice(x, 3)]
for v in x:
    vs.append(int(v))
    currentProduct = functools.reduce(operator.mul, vs, 1)
    print(currentProduct)
    vs = vs[1:]

Answered By: Dirk Herrmann

"'generator' object is not subscriptable" error

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