# Python | Ways to sum list of lists and return sum list

• Difficulty Level : Easy
• Last Updated : 29 Jul, 2019

The list is an important container and used almost in every code of day-day programming as well as web-development, more it is used, more is the requirement to master it and hence knowledge of its operations is necessary. Given a list of lists, the program to suppose to return the sum as the final list.

Let’s see some of the methods to sum a list of list and return list.

Method # 1: Using Naive method

 `# Python code to demonstrate``# sum of list of list``# using naive method`` ` `# Declaring initial list of list``L ``=` `[[``1``, ``2``, ``3``],``     ``[``4``, ``5``, ``6``],``     ``[``7``, ``8``, ``9``]]``         ` `# Printing list of list``print``(``"Initial List - "``, ``str``(L))`` ` `# Using naive method``res ``=` `list``()``for` `j ``in` `range``(``0``, ``len``(L[``0``])):``    ``tmp ``=` `0``    ``for` `i ``in` `range``(``0``, ``len``(L)):``        ``tmp ``=` `tmp ``+` `L[i][j]``    ``res.append(tmp)``     ` `# printing result``print``(``"final list - "``, ``str``(res))`
Output:
```Initial List -  [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
final list -  [12, 15, 18]
```

Method #2: Using numpy array
A numpy is a general-purpose array-processing package. It provides a high-performance multidimensional array object, and tools for working with these arrays.

 `# Python code to demonstrate``# sum of list of list``# using numpy array functions``import` `numpy as np`` ` `# Declaring initial list of list``List` `=` `np.array([[``1``, ``2``, ``3``],``                 ``[``4``, ``5``, ``6``],``                 ``[``7``, ``8``, ``9``]])``         ` `# Printing list of list``print``(``"Initial List - "``, ``str``(``List``))`` ` `# Using numpy sum``res ``=` `np.``sum``(``List``, ``0``)``     ` `# printing result``print``(``"final list - "``, ``str``(res))`
Output:
```Initial List -  [[1 2 3]
[4 5 6]
[7 8 9]]
final list -  [12 15 18]
```

Method #3: Using `zip()` and list comprehension

 `# Python code to demonstrate``# sum of list of list using ``# zip and list comprehension`` ` `# Declaring initial list of list``List` `=` `[[``1``, ``2``, ``3``],``        ``[``4``, ``5``, ``6``],``        ``[``7``, ``8``, ``9``]]``         ` `# Printing list of list``print``(``"Initial List - "``, ``str``(``List``))`` ` `# Using list comprehension``res ``=` `[``sum``(i) ``for` `i ``in` `zip``(``*``List``)]``     ` `# printing result``print``(``"final list - "``, ``str``(res))`
Output:
```Initial List -  [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
final list -  [12, 15, 18]
```

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