Mysterious IndexError - very basic issue (Python)

Question

I'm having a problem with a bit of code, I feel like I must be missing something fundamental here. A simple example that gives the same error as I am having is as follows:

from numpy import array,zeros
x = array([1,2,3])
f = zeros(len(x))
for i in x:
 f[i] = x[i] + 1

And the traceback reads as follows:

---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
C:\WINDOWS\system32\<ipython-input-5-6b6b88f30156> in <module>()
 1 for i in x:
----> 2 f[i] = x[i] + 1
 3 
IndexError: index out of bounds

This has been puzzling me for far too long, but I just can't seem to see what the problem is here? Could someone lend a hand?

• Jon Clements: I'm impressed you didn't get an error on array[(1, 2, 3)] or the lack of zeros being imported... - are you copying/pasting code, or typing it in?
• user3008862: Typing it in, it was done correctly in the console, sorry about that!

Answer 1

In this loop:

for i in x:
 f[i] = x[i] + 1

i takes the values 1, 2 and then 3. x[i] is not what you think it is. i already contains the content of a cell of the array x. Since the indexes of the array start from 0, you do an IndexError when trying to get the element of index 3 (which would be the 4th element).

You probably wanted something such as:

for i in range(len(x)):
 f[i] = x[i] + 1

That could also be written:

for i, v in enumerate(x):
 f[i] = v + 1

Answer 2

When you do for i in some_list, i refers to elements of that list, rather than their indexes. For example:

In [1]: for i in [3, 2, 1]:
 ...: print i
 ...: 
3
2
1

However, you then use i as an index.

You iterate with i over x, and thus i takes values of 1, 2 and 3. But 3 is a too big index for an array of length 3. The last index is 2, as in Python indexes start with 0.

• user3008862: I think I see, so even though Python indexes start with 0 the for loop starts at i=1? In that case, adding an extra element to f should fix my problem, but that doesn't seem like a particularly elegant solution?
• Lev Levitsky: @user3008862 It starts with i=1 because 1 is the 0-th element of the array. I expanded the answer a bit to make it clearer.