Nested loop for list in Python

List Comprehensions can use nested for loops. You can code any number of nested for loops within a list comprehension, and each for loop may have an optional associated if test. When doing so, the order of the for constructs is the same order as when writing a series of nested for statements. The general structure of list comprehensions looks like this:

[ expression for target1 in iterable1 [if condition1] for target2 in iterable2 [if condition2]... for targetN in iterableN [if conditionN] ]

For example, the following code flattening a list of lists using multiple for statements:

data = [[1, 2], [3, 4], [5, 6]] output = [] for each_list in data: for element in each_list: output.append(element) print(output) # Out: [1, 2, 3, 4, 5, 6]

can be equivalently written as a list comprehension with multiple for constructs:

data = [[1, 2], [3, 4], [5, 6]] output = [element for each_list in data for element in each_list] print(output) # Out: [1, 2, 3, 4, 5, 6]

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In both the expanded form and the list comprehension, the outer loop (first for statement) comes first.

In addition to being more compact, the nested comprehension is also significantly faster.

In [1]: data = [[1,2],[3,4],[5,6]] In [2]: def f(): ...: output=[] ...: for each_list in data: ...: for element in each_list: ...: output.append(element) ...: return output In [3]: timeit f() 1000000 loops, best of 3: 1.37 µs per loop In [4]: timeit [inner for outer in data for inner in outer] 1000000 loops, best of 3: 632 ns per loop

The overhead for the function call above is about 140ns.

Inline ifs are nested similarly, and may occur in any position after the first for:

data = [[1], [2, 3], [4, 5]] output = [element for each_list in data if len(each_list) == 2 for element in each_list if element != 5] print(output) # Out: [2, 3, 4]

Live Demo

For the sake of readability, however, you should consider using traditional for-loops. This is especially true when nesting is more than 2 levels deep, and/or the logic of the comprehension is too complex. multiple nested loop list comprehension could be error prone or it gives unexpected result.

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A nested loop is a loop inside a loop.

The "inner loop" will be executed one time for each iteration of the "outer loop":

Print each adjective for every fruit:

adj = ["red", "big", "tasty"]fruits = ["apple", "banana", "cherry"] for x in adj:  for y in fruits:

    print(x, y)

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A list can contain any sort object, even another list (sublist), which in turn can contain sublists themselves, and so on. This is known as nested list.

You can use them to arrange data into hierarchical structures.

Create a Nested List

A nested list is created by placing a comma-separated sequence of sublists.

L = ['a', ['bb', ['ccc', 'ddd'], 'ee', 'ff'], 'g', 'h']

Access Nested List Items by Index

You can access individual items in a nested list using multiple indexes.

The indexes for the items in a nested list are illustrated as below:

L = ['a', 'b', ['cc', 'dd', ['eee', 'fff']], 'g', 'h'] print(L[2]) # Prints ['cc', 'dd', ['eee', 'fff']] print(L[2][2]) # Prints ['eee', 'fff'] print(L[2][2][0]) # Prints eee

You can access a nested list by negative indexing as well.

Negative indexes count backward from the end of the list. So, L[-1] refers to the last item, L[-2] is the second-last, and so on.

The negative indexes for the items in a nested list are illustrated as below:

L = ['a', 'b', ['cc', 'dd', ['eee', 'fff']], 'g', 'h'] print(L[-3]) # Prints ['cc', 'dd', ['eee', 'fff']] print(L[-3][-1]) # Prints ['eee', 'fff'] print(L[-3][-1][-2]) # Prints eee

You can change the value of a specific item in a nested list by referring to its index number.

L = ['a', ['bb', 'cc'], 'd'] L[1][1] = 0 print(L) # Prints ['a', ['bb', 0], 'd']

Add items to a Nested list

To add new values to the end of the nested list, use append() method.

L = ['a', ['bb', 'cc'], 'd'] L[1].append('xx') print(L) # Prints ['a', ['bb', 'cc', 'xx'], 'd']

When you want to insert an item at a specific position in a nested list, use insert() method.

L = ['a', ['bb', 'cc'], 'd'] L[1].insert(0,'xx') print(L) # Prints ['a', ['xx', 'bb', 'cc'], 'd']

You can merge one list into another by using extend() method.

L = ['a', ['bb', 'cc'], 'd'] L[1].extend([1,2,3]) print(L) # Prints ['a', ['bb', 'cc', 1, 2, 3], 'd']

If you know the index of the item you want, you can use pop() method. It modifies the list and returns the removed item.

L = ['a', ['bb', 'cc', 'dd'], 'e'] x = L[1].pop(1) print(L) # Prints ['a', ['bb', 'dd'], 'e'] # removed item print(x) # Prints cc

If you don’t need the removed value, use the del statement.

L = ['a', ['bb', 'cc', 'dd'], 'e'] del L[1][1] print(L) # Prints ['a', ['bb', 'dd'], 'e']

If you’re not sure where the item is in the list, use remove() method to delete it by value.

L = ['a', ['bb', 'cc', 'dd'], 'e'] L[1].remove('cc') print(L) # Prints ['a', ['bb', 'dd'], 'e']

Find Nested List Length

You can use the built-in len() function to find how many items a nested sublist has.

L = ['a', ['bb', 'cc'], 'd'] print(len(L)) # Prints 3 print(len(L[1])) # Prints 2

Iterate through a Nested List

To iterate over the items of a nested list, use simple for loop.

L = [[1, 2, 3],[4, 5, 6],[7, 8, 9]] for list in L: for number in list: print(number, end=' ') # Prints 1 2 3 4 5 6 7 8 9