How to get item's position in a list?

python list
Sean picture Sean · Dec 13, 2008 · Viewed 551.6k times · Source

I am iterating over a list and I want to print out the index of the item if it meets a certain condition. How would I do this?

Example: 

testlist = [1,2,3,5,3,1,2,1,6]
for item in testlist:
    if item == 1:
        print position

Answer

Charlie Martin picture Charlie Martin · Dec 13, 2008

Hmmm. There was an answer with a list comprehension here, but it's disappeared.

Here:

 [i for i,x in enumerate(testlist) if x == 1]

Example:

>>> testlist
[1, 2, 3, 5, 3, 1, 2, 1, 6]
>>> [i for i,x in enumerate(testlist) if x == 1]
[0, 5, 7]

Update:

Okay, you want a generator expression, we'll have a generator expression. Here's the list comprehension again, in a for loop:

>>> for i in [i for i,x in enumerate(testlist) if x == 1]:
...     print i
... 
0
5
7

Now we'll construct a generator...

>>> (i for i,x in enumerate(testlist) if x == 1)
<generator object at 0x6b508>
>>> for i in (i for i,x in enumerate(testlist) if x == 1):
...     print i
... 
0
5
7

and niftily enough, we can assign that to a variable, and use it from there...

>>> gen = (i for i,x in enumerate(testlist) if x == 1)
>>> for i in gen: print i
... 
0
5
7

And to think I used to write FORTRAN.

Related questions

Finding the index of an item in a list

Given a list ["foo", "bar", "baz"] and an item in the list "bar", how do I get its index (1) in Python?

Read More
How do I get the number of elements in a list?

Consider the following: items = [] items.append("apple") items.append("orange") items.append("banana") # FAKE METHOD: items.amount() # Should return 3 How do I get the number of elements in the list items?

Read More
How do I check if a list is empty?

For example, if passed the following: a = [] How do I check to see if a is empty?

Read More