syntax - python: how do i know when i am on the last for cycle

Question

for i in range(len(results_histogram)):
    if i!=len(results_histogram)-1:
      url+=str(results_histogram[i])+','

my if statement is checking whether i am on the last loop, but it is not working. what am i doing wrong?

Answer 1

To avoid the question slightly, you seem to have rewritten str.join:

','.join(results_histogram)

If you get an error like TypeError: sequence item 0: expected string, int found, then you can convert the intermediate results to a string with

','.join(map(str, results_histogram))

---

str.join is undoubtedly more efficient than concatenating multiple strings in a loop, because in Python, strings are immutable, so every concatenation results in the creation of a new string, which then has to be garbage collected later.

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Specifically, your example is "not working" because you skip the last element entirely, when you only want to skip adding the comma. This is clear and obvious with a small example:

>>> x = [1,2,3]
>>> for i in range(len(x)):
...   if i != len(x) - 1:
...     print str(x[i]) + ',',
... 
1, 2,

So you could rewrite your example as

for i in range(len(results_histogram)):
    url += str(results_histogram[i])
    if i!=len(results_histogram)-1:
      url += ','

But you should still stick with str.join.