Accessing the index in 'for' loops?
How do I access the index itself for a list like the following?
ints = [8, 23, 45, 12, 78]
When I loop through it using a for loop, how do I access the loop index, from 1 to 5 in this case?
Using an additional state variable, such as an index variable (which you would normally use in languages such as C or PHP), is considered non-pythonic.
The better option is to use the built-in function enumerate() , available in both Python 2 and 3:
for idx, val in enumerate(ints):
print(idx, val)
Check out PEP 279 for more.
Using a for loop, how do I access the loop index, from 1 to 5 in this case?
Use enumerate to get the index with the element as you iterate:
for index, item in enumerate(items):
print(index, item)
And note that Python's indexes start at zero, so you would get 0 to 4 with the above. If you want the count, 1 to 5, do this:
for count, item in enumerate(items, start=1):
print(count, item)
Unidiomatic control flow
What you are asking for is the Pythonic equivalent of the following, which is the algorithm most programmers of lower-level languages would use:
index = 0 # Python's indexing starts at zero
for item in items: # Python's for loops are a "for each" loop
print(index, item)
index += 1
Or in languages that do not have a for-each loop:
index = 0
while index < len(items):
print(index, items[index])
index += 1
or sometimes more commonly (but unidiomatically) found in Python:
for index in range(len(items)):
print(index, items[index])
Use the Enumerate Function
Python's enumerate function reduces the visual clutter by hiding the accounting for the indexes, and encapsulating the iterable into another iterable (an enumerate object) that yields a two-item tuple of the index and the item that the original iterable would provide. That looks like this:
for index, item in enumerate(items, start=0): # default is zero
print(index, item)
This code sample is fairly well the canonical example of the difference between code that is idiomatic of Python and code that is not. Idiomatic code is sophisticated (but not complicated) Python, written in the way that it was intended to be used. Idiomatic code is expected by the designers of the language, which means that usually this code is not just more readable, but also more efficient.
Getting a count
Even if you don't need indexes as you go, but you need a count of the iterations (sometimes desirable) you can start with 1 and the final number will be your count.
for count, item in enumerate(items, start=1): # default is zero
print(item)
print('there were {0} items printed'.format(count))
The count seems to be more what you intend to ask for (as opposed to index) when you said you wanted from 1 to 5.
Breaking it down - a step by step explanation
To break these examples down, say we have a list of items that we want to iterate over with an index:
items = ['a', 'b', 'c', 'd', 'e']
Now we pass this iterable to enumerate, creating an enumerate object:
enumerate_object = enumerate(items) # the enumerate object
We can pull the first item out of this iterable that we would get in a loop with the next function:
iteration = next(enumerate_object) # first iteration from enumerate
print(iteration)
And we see we get a tuple of 0 , the first index, and 'a' , the first item:
(0, 'a')
we can use what is referred to as "sequence unpacking" to extract the elements from this two-tuple:
index, item = iteration
# 0, 'a' = (0, 'a') # essentially this.
and when we inspect index , we find it refers to the first index, 0, and item refers to the first item, 'a' .
>>> print(index)
0
>>> print(item)
a
Conclusion
So do this:
for index, item in enumerate(items, start=0): # Python indexes start at zero
print(index, item)
It's pretty simple to start it from 1 other than 0 :
for index in enumerate(iterable, start=1):
print index
Note
Important hint, though a little misleading, since index will be a tuple (idx, item) here. Good to go.
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