Accessing the index in 'for' loops

Question

How do I access the index while iterating over a sequence with a for loop?

xs = [8, 23, 45]

for x in xs:
    print("item #{} = {}".format(index, x))

Desired output:

item #1 = 8
item #2 = 23
item #3 = 45

Answer 1

Use the built-in function enumerate():

for idx, x in enumerate(xs):
    print(idx, x)

It is non-pythonic to manually index via for i in range(len(xs)): x = xs[i] or manually manage an additional state variable.

Check out PEP 279 for more.

Answer 2

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:

count = 0 # in case items is empty and you need it after the loop
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.

count = 0 # in case items is empty
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

• Python indexes start at zero
• To get these indexes from an iterable as you iterate over it, use the enumerate function
• Using enumerate in the idiomatic way (along with tuple unpacking) creates code that is more readable and maintainable:

So do this:

for index, item in enumerate(items, start=0):   # Python indexes start at zero
    print(index, item)

Answer 3

It's pretty simple to start it from 1 other than 0:

for index, item in enumerate(iterable, start=1):
   print index, item  # Used to print in python<3.x
   print(index, item) # Migrate to print() after 3.x+
   

Answer 4

for i in range(len(ints)):
   print(i, ints[i]) # print updated to print() in Python 3.x+ 

Answer 5

Tested on Python 3.12

Here are twelve examples of how you can access the indices with their corresponding array's elements using for loops, while loops and some looping functions. Note that array indices always start from zero by default (see example 4 to change this).

1. Looping elements with counter and += operator.

items = [8, 23, 45, 12, 78]
counter = 0

for value in items:
    print(counter, value)
    counter += 1

---

Result:

#    0 8
#    1 23
#    2 45
#    3 12
#    4 78

2. Iterating elements using enumerate() built-in function.

items = [8, 23, 45, 12, 78]

for i in enumerate(items):
    print("index/value", i)

---

Result:

#    index/value (0, 8)
#    index/value (1, 23)
#    index/value (2, 45)
#    index/value (3, 12)
#    index/value (4, 78)

3. Getting list's element and its index separately.

items = [8, 23, 45, 12, 78]

for index, value in enumerate(items):
    print("index", index, "for value", value)

---

Result:

#    index 0 for value 8
#    index 1 for value 23
#    index 2 for value 45
#    index 3 for value 12
#    index 4 for value 78

4. You can change the index value to any increment.

items = [8, 23, 45, 12, 78]

for i, item in enumerate(items, start=100):
    print(i, item)

---

Result:

#    100 8
#    101 23
#    102 45
#    103 12
#    104 78

5. Automatic counter incrementation with range(len(...)) methods.

items = [8, 23, 45, 12, 78]

for i in range(len(items)):
    print("Index:", i, "Value:", items[i])

---

Result:

#    ('Index:', 0, 'Value:', 8)
#    ('Index:', 1, 'Value:', 23)
#    ('Index:', 2, 'Value:', 45)
#    ('Index:', 3, 'Value:', 12)
#    ('Index:', 4, 'Value:', 78)

6. Using for loop inside function.

items = [8, 23, 45, 12, 78]

def enum(items, start=0):
    counter = start

    for value in items:
        print(counter, value)
        counter += 1
    
enum(items)

---

Result:

#    0 8
#    1 23
#    2 45
#    3 12
#    4 78

7. Of course, we can't forget about while loop.

items = [8, 23, 45, 12, 78]
counter = 0

while counter < len(items):
    print(counter, items[counter])
    counter += 1

---

Result:

#    0 8
#    1 23
#    2 45
#    3 12
#    4 78

8. yield statement returning a generator object.

def createGenerator():        
    items = [8, 23, 45, 12, 78]

    for (j, k) in enumerate(items):
        yield (j, k)
        

generator = createGenerator()

for i in generator:
    print(i)

---

Result:

#    (0, 8)
#    (1, 23)
#    (2, 45)
#    (3, 12)
#    (4, 78)

9. Inline expression with for loop and lambda.

items = [8, 23, 45, 12, 78]

xerox = lambda upperBound: [(i, items[i]) for i in range(0, upperBound)]
print(xerox(5))

---

Result:

#    [(0, 8), (1, 23), (2, 45), (3, 12), (4, 78)]

10. Iterate over two lists at once using Python's zip() function.

items = [8, 23, 45, 12, 78]
indices = []

for index in range(len(items)):
    indices.append(index)

for item, index in zip(items, indices):
    print("{}: {}".format(index, item))

---

Result:

#    0: 8
#    1: 23
#    2: 45
#    3: 12
#    4: 78

11. Loop over 2 lists with a while loop and iter() & next() methods.

items = [8, 23, 45, 12, 78]
indices = range(len(items))

iterator1 = iter(indices)
iterator2 = iter(items)
  
try:
    while True:
        i = next(iterator1)
        element = next(iterator2)
        print(i, element)
except StopIteration:
    pass

---

Result:

#    0  8
#    1  23
#    2  45
#    3  12
#    4  78

12. Also, it's nice to iterate list's elements inside class' Static Method.

items = [8, 23, 45, 12, 78]

class ElementPlus:
    @staticmethod                            # decorator
    def indexForEachOfMy(iterable):
        for pair in enumerate(iterable):
            print pair

ElementPlus.indexForEachOfMy(items)

---

Result:

#    (0, 8)
#    (1, 23)
#    (2, 45)
#    (3, 12)
#    (4, 78)

Answer 6

As is the norm in Python, there are several ways to do this. In all examples assume: lst = [1, 2, 3, 4, 5]

1. Using enumerate (considered most idiomatic)

---

for index, element in enumerate(lst):
    # Do the things that need doing here

This is also the safest option in my opinion because the chance of going into infinite recursion has been eliminated. Both the item and its index are held in variables and there is no need to write any further code to access the item.

2. Creating a variable to hold the index (using for)

---

for index in range(len(lst)):   # or xrange
    # you will have to write extra code to get the element

3. Creating a variable to hold the index (using while)

---

index = 0
while index < len(lst):
    # You will have to write extra code to get the element
    index += 1  # escape infinite recursion

4. There is always another way

---

As explained before, there are other ways to do this that have not been explained here and they may even apply more in other situations. For example, using itertools.chain with for. It handles nested loops better than the other examples.

Answer 7

Accessing indexes & Performance Benchmarking of approaches

The fastest way to access indexes of list within loop in Python 3.7 is to use the enumerate method for small, medium and huge lists.

Please see different approaches which can be used to iterate over list and access index value and their performance metrics (which I suppose would be useful for you) in code samples below:

# Using range
def range_loop(iterable):
    for i in range(len(iterable)):
        1 + iterable[i]

# Using enumerate
def enumerate_loop(iterable):
    for i, val in enumerate(iterable):
        1 + val

# Manual indexing
def manual_indexing_loop(iterable):
    index = 0
    for item in iterable:
        1 + item
        index += 1

See performance metrics for each method below:

from timeit import timeit

def measure(l, number=10000):
    print("Measure speed for list with %d items" % len(l))
    print("range: ", timeit(lambda :range_loop(l), number=number))
    print("enumerate: ", timeit(lambda :enumerate_loop(l), number=number))
    print("manual_indexing: ", timeit(lambda :manual_indexing_loop(l), number=number))

# Measure speed for list with 1000 items
measure(range(1000))
# range:  1.161622366
# enumerate:  0.5661940879999996
# manual_indexing:  0.610455682

# Measure speed for list with 100000 items
measure(range(10000))
# range:  11.794482958
# enumerate:  6.197628574000001
# manual_indexing:  6.935181098000001

# Measure speed for list with 10000000 items
measure(range(10000000), number=100)
# range:  121.416859069
# enumerate:  62.718909123
# manual_indexing:  69.59575057400002

As the result, using enumerate method is the fastest method for iteration when the index needed.

Adding some useful links below:

• What is the difference between range and xrange functions in Python 2.X?

• What is faster for loop using enumerate or for loop using xrange in Python?

• range(len(list)) or enumerate(list)?

Answer 8

Old fashioned way:

for ix in range(len(ints)):
    print(ints[ix])

List comprehension:

[ (ix, ints[ix]) for ix in range(len(ints))]

>>> ints
[1, 2, 3, 4, 5]
>>> for ix in range(len(ints)): print ints[ix]
... 
1
2
3
4
5
>>> [ (ix, ints[ix]) for ix in range(len(ints))]
[(0, 1), (1, 2), (2, 3), (3, 4), (4, 5)]
>>> lc = [ (ix, ints[ix]) for ix in range(len(ints))]
>>> for tup in lc:
...     print(tup)
... 
(0, 1)
(1, 2)
(2, 3)
(3, 4)
(4, 5)
>>> 

Answer 9

You can use enumerate and embed expressions inside string literals to obtain the solution.

This is a simple way:

a=[4,5,6,8]
for b, val in enumerate(a):
    print('item #{} = {}'.format(b+1, val))

Answer 10

First of all, the indexes will be from 0 to 4. Programming languages start counting from 0; don't forget that or you will come across an index-out-of-bounds exception. All you need in the for loop is a variable counting from 0 to 4 like so:

for x in range(0, 5):

Keep in mind that I wrote 0 to 5 because the loop stops one number before the maximum. :)

To get the value of an index, use

list[index]

Answer 11

According to this discussion: object's list index

Loop counter iteration

The current idiom for looping over the indices makes use of the built-in range function:

for i in range(len(sequence)):
    # Work with index i

Looping over both elements and indices can be achieved either by the old idiom or by using the new zip built-in function:

for i in range(len(sequence)):
    e = sequence[i]
    # Work with index i and element e

or

for i, e in zip(range(len(sequence)), sequence):
    # Work with index i and element e

via PEP 212 – Loop Counter Iteration.

Answer 12

You can do it with this code:

ints = [8, 23, 45, 12, 78]
index = 0

for value in (ints):
    index +=1
    print index, value

Use this code if you need to reset the index value at the end of the loop:

ints = [8, 23, 45, 12, 78]
index = 0

for value in (ints):
    index +=1
    print index, value
    if index >= len(ints)-1:
        index = 0

Answer 13

If I were to iterate nums = [1, 2, 3, 4, 5] I would do

for i, num in enumerate(nums, start=1):
    print(i, num)

Or get the length as l = len(nums)

for i in range(l):
    print(i+1, nums[i])

Answer 14

In your question, you write "how do I access the loop index, from 1 to 5 in this case?"

However, the index for a list runs from zero. So, then we need to know if what you actually want is the index and item for each item in a list, or whether you really want numbers starting from 1. Fortunately, in Python, it is easy to do either or both.

First, to clarify, the enumerate function iteratively returns the index and corresponding item for each item in a list.

alist = [1, 2, 3, 4, 5]

for n, a in enumerate(alist):
    print("%d %d" % (n, a))

The output for the above is then,

0 1
1 2
2 3
3 4
4 5

Notice that the index runs from 0. This kind of indexing is common among modern programming languages including Python and C.

If you want your loop to span a part of the list, you can use the standard Python syntax for a part of the list. For example, to loop from the second item in a list up to but not including the last item, you could use

for n, a in enumerate(alist[1:-1]):
    print("%d %d" % (n, a))

Note that once again, the output index runs from 0,

0 2
1 3
2 4

That brings us to the start=n switch for enumerate(). This simply offsets the index, you can equivalently simply add a number to the index inside the loop.

for n, a in enumerate(alist, start=1):
    print("%d %d" % (n, a))

for which the output is

1 1
2 2
3 3
4 4
5 5

Answer 15

If there is no duplicate value in the list:

for i in ints:
    indx = ints.index(i)
    print(i, indx)

Answer 16

You can also try this:

data = ['itemA.ABC', 'itemB.defg', 'itemC.drug', 'itemD.ashok']
x = []
for (i, item) in enumerate(data):
      a = (i, str(item).split('.'))
      x.append(a)
for index, value in x:
     print(index, value)

The output is

0 ['itemA', 'ABC']
1 ['itemB', 'defg']
2 ['itemC', 'drug']
3 ['itemD', 'ashok']

Answer 17

You can use the index method:

ints = [8, 23, 45, 12, 78]
inds = [ints.index(i) for i in ints]

It is highlighted in a comment that this method doesn’t work if there are duplicates in ints. The method below should work for any values in ints:

ints = [8, 8, 8, 23, 45, 12, 78]
inds = [tup[0] for tup in enumerate(ints)]

Or alternatively

ints = [8, 8, 8, 23, 45, 12, 78]
inds = [tup for tup in enumerate(ints)]

if you want to get both the index and the value in ints as a list of tuples.

It uses the method of enumerate in the selected answer to this question, but with list comprehension, making it faster with less code.

Answer 18

A simple answer using a while loop:

arr = [8, 23, 45, 12, 78]
i = 0
while i < len(arr):
    print("Item ", i + 1, " = ", arr[i])
    i += 1

Output:

Item  1  =  8
Item  2  =  23
Item  3  =  45
Item  4  =  12
Item  5  =  78

Answer 19

To print a tuple of (index, value) in a list comprehension using a for loop:

ints = [8, 23, 45, 12, 78]
print [(i,ints[i]) for i in range(len(ints))]

Output:

[(0, 8), (1, 23), (2, 45), (3, 12), (4, 78)]

Answer 20

You can simply use a variable such as count to count the number of elements in the list:

ints = [8, 23, 45, 12, 78]
count = 0
for i in ints:
    count = count + 1
    print('item #{} = {}'.format(count, i))

Answer 21

In addition to all the excellent answers above, here is a solution to this problem when working with pandas Series objects. In many cases, pandas Series have custom/unique indices (for example, unique identifier strings) that can't be accessed with the enumerate() function.

xs = pd.Series([8, 23, 45])

xs.index = ['G923002', 'G923004', 'G923005']

print(xs)

Output:

#    G923002     8
#    G923004    23
#    G923005    45
#    dtype: int64

We can see below that enumerate() doesn't give us the desired result:

for id, x in enumerate(xs):
    print("id #{} = {}".format(id, x))

Output:

#    id #0 = 8
#    id #1 = 23
#    id #2 = 45

We can access the indices of a pandas Series in a for loop using .items():

for id, x in xs.items():
    print("id #{} = {}".format(id, x))

Output:

#    id #G923002 = 8
#    id #G923004 = 23
#    id #G923005 = 45

Answer 22

One-liner lovers:

[index for index, datum in enumerate(data) if 'a' in datum]

Explaination:

>>> data = ['a','ab','bb','ba','alskdhkjl','hkjferht','lal']
>>> data
['a', 'ab', 'bb', 'ba', 'alskdhkjl', 'hkjferht', 'lal']
>>> [index for index, datum in enumerate(data) if 'a' in datum]
[0, 1, 3, 4, 6]
>>> [index for index, datum in enumerate(data) if 'b' in datum]
[1, 2, 3]
>>>

Points to take:

• Python list doesn't provide an index; if you are using for
• If you enumerate a list it will return you ANOTHER list
  • BUT that list will have a different type
  • it will wrap each and every element with an index as tuple
  • we can access tuples as variables, separated with comma(,)

Thanks. Keep me in your prayers.

Answer 23

You can use range(len(some_list)) and then lookup the index like this

xs = [8, 23, 45]
for i in range(len(xs)):
    print("item #{} = {}".format(i + 1, xs[i]))

Or use the Python’s built-in enumerate function which allows you to loop over a list and retrieve the index and the value of each item in the list

xs = [8, 23, 45]
for idx, val in enumerate(xs, start=1):
    print("item #{} = {}".format(idx, val))

Answer 24

Yet another way to run a counter in a for loop is to use itertools.count.

from itertools import count

my_list = ['a', 'b', 'a']
for i, item in zip(count(), my_list):
    print(i, item)

This is useful especially if you want the counters to be fractional numbers. In the following example, the "index" starts from 1.0 and is incremented by 0.5 in each iteration.

my_list = ['a', 'b', 'a']

for i, item in zip(count(start=1., step=0.5), my_list):
    print(f"loc={i}, item={item}")
    
# loc=1.0, item=a
# loc=1.5, item=b
# loc=2.0, item=a

Another method is to use list.index() inside a loop. However, in contrast to other answers on this page mentioning this method (1, 2, 3), the starting point of the index search (which is the second argument) must be passed to the list.index() method. This lets you achieve 2 things: (1) Doesn't expensively loop over the list from the beginning again; (2) Can find the index of all values, even duplicates.

my_list = ['a', 'b', 'a']
idx = -1
for item in my_list:
    idx = my_list.index(item, idx+1)
    #                         ^^^^^   <---- start the search from the next index
    print(f"index={idx}, item={item}")
    
# index=0, item=a
# index=1, item=b
# index=2, item=a

In terms of performance, if you want all/most of the indices, enumerate() is the fastest option. If you were looking for only specific indices, then list.index() may be more efficient. Below are two examples where list.index() is more efficient.

Example #1: Index of specific values

Suppose you want to find all indices where a specific value appears in a list (e.g. highest value). For example, in the following case, we want to find all indices that 2 appears in. This is a one-liner using enumerate(). However, we can also search for the indices of 2 by using the list.index() method in a while-loop; as mentioned before, in each iteration, we start the index search from where we left off in the previous iteration.

lst = [0, 2, 1, 2]
target = 2

result = []
pos = -1
while True:
    try:
        pos = lst.index(target, pos+1)
        result.append(pos)
    except ValueError:
        break
        
print(result)      # [1, 3]

In fact, in certain cases, it's much faster than enumerate() option that produces the same output especially if the list is long.

Example #2: Index of the first number that is smaller than a target

Another common exercise where index is often needed in a loop is to find the index of the first item in a list that satisfies some condition (e.g. greater/smaller than some target value). In the following example, we want to find the index of the first value that exceeds 2.5. This is a one-liner using enumerate() but using list.index() is more efficient because getting indices that won't be used as in enumerate() has a cost (which list.index() doesn't incur).

my_list = [1, 2, 3, 4]
target = 2.5

for item in my_list:
    if item > target:
        idx = my_list.index(item)
        break

or as a one-liner:

idx = next(my_list.index(item) for item in my_list if item > target)

---

Code used to produce the plot of runtime speed ratios:

import random
import matplotlib.pyplot as plt
import perfplot


def enumerate_1(lst, target=3):
    return [i for i, v in enumerate(lst) if v == target]


def list_index_1(lst, target=3):
    result = []
    pos = -1
    while True:
        try:
            pos = lst.index(target, pos+1)
            result.append(pos)
        except ValueError:
            break
    return result


def list_index_2(lst, target):
    for item in lst:
        if item > target:
            return lst.index(item)


def enumerate_2(lst, target):
    return next(i for i, item in enumerate(lst) if item > target)


setups = [lambda n: [random.randint(1, 10) for _ in range(n)], 
          lambda n: (list(range(n)), n-1.5)]
kernels_list = [[enumerate_1, list_index_1], [enumerate_2, list_index_2]]
titles = ['Get indices of a value', 'Get index that satisfies a condition']
n_range = [2**k for k in range(1,21)]
labels = ['enumerate', 'list.index']
xlabel = 'list length'

fig, axs = plt.subplots(1, 2, figsize=(10, 5), facecolor='white', dpi=60)
for i, (ax, su, ks, t) in enumerate(zip(axs, setups, kernels_list, titles)):
    plt.sca(ax)
    perfplot.plot(ks, n_range, su, None, labels, xlabel, t, relative_to=1)
    ax.xaxis.set_tick_params(labelsize=13)
plt.setp(axs, ylim=(0.7, 2.4), yticks=[i*0.25 + 0.75 for i in range(7)], 
         xlim=(1, 1100000), xscale='log', xticks=[1, 100, 10000, 1000000])
fig.tight_layout();

Answer 25

It can be achieved with the following code:

xs = [8, 23, 45]
for x, n in zip(xs, range(1, len(xs)+1)):
    print("item #{} = {}".format(n, x))

Here, range(1, len(xs)+1); If you expect the output to start from 1 instead of 0, you need to start the range from 1 and add 1 to the total length estimated since python starts indexing the number from 0 by default.

Final Output:
item #1 = 8
item #2 = 23
item #3 = 45

Answer 26

A loop with a "counter" variable set as an initialiser that will be a parameter, in formatting the string, as the item number.

The for loop accesses the "listos" variable which is the list. As we access the list by "i", "i" is formatted as the item price (or whatever it is).

listos = [8, 23, 45, 12, 78]
counter = 1
for i in listos:
    print('Item #{} = {}'.format(counter, i))
    counter += 1

Output:

Item #1 = 8
Item #2 = 23
Item #3 = 45
Item #4 = 12
Item #5 = 78

Answer 27

This serves the purpose well enough:

list1 = [10, 'sumit', 43.21, 'kumar', '43', 'test', 3]
for x in list1:
    print('index:', list1.index(x), 'value:', x)