# Python – Dictionary construction from front-rear key values

• Last Updated : 29 Aug, 2020

Given a list, construct dictionary using key as first half values and values starting from last.

Input : test_list = [4, 10, 5, 3]
Output : {4: 3, 10: 5}
Explanation : First (4) is paired with rear (3) and so on.

Input : test_list = [5, 3]
Output : {5: 3}
Explanation : First (5) is paired with rear (3).

Input : test_dict = {“Apple” : 2, “Mango” : 2, “Grapes” : 2}, {“Apple” : 2, “Mango” : 2, “Grapes” : 2}
Output : 12
Explanation : (2*2) + (2*2) + (2*2) = 12.

Input : test_dict = {“Apple” : 3, “Mango” : 2, “Grapes” : 3}, {“Apple” : 2, “Mango” : 2, “Grapes” : 2}
Output : 16
Explanation : The summation of product leads to 16 as above.

Method #1 : Using loop

This is brute way in which this task can be performed. In this, we run a loop from beginning and extract values from beg-end, and construct key value mapping accordingly.

## Python3

 `# Python3 code to demonstrate working of ``# Dictionary construction from front-rear key values``# Using loop`` ` `# initializing list``test_list ``=` `[``4``, ``6``, ``3``, ``10``, ``5``, ``3``]`` ` `# printing original list``print``(``"The original list : "` `+` `str``(test_list))`` ` ` ` `# initializing size and empty Dictionary``n ``=` `len``(test_list)``res ``=` `dict``()`` ` `# running loop till mid``for` `idx ``in` `range``(n ``/``/` `2``):`` ` `    ``# mapping as required``    ``res.__setitem__(test_list[idx], test_list[n ``-` `idx ``-` `1``]) `` ` `# printing result ``print``(``"The mapped dictionary : "` `+` `str``(res))`
Output
```The original list : [4, 6, 3, 10, 5, 3]
The mapped dictionary : {4: 3, 6: 5, 3: 10}
```

Method #2 : Using zip() + dict()

This is yet another way in which this task can be performed. In this, we perform the task of zipping key and value elements using zip() and dict() is used to convert result to dictionary.

## Python3

 `# Python3 code to demonstrate working of ``# Dictionary construction from front-rear key values``# Using zip() + dict()`` ` `# initializing list``test_list ``=` `[``4``, ``6``, ``3``, ``10``, ``5``, ``3``]`` ` `# printing original list``print``(``"The original list : "` `+` `str``(test_list))`` ` `# using zip to cut first and second half ``n ``=` `len``(test_list)``res ``=` `dict``(``zip``(test_list[:n ``/``/` `2``], test_list[n ``/``/` `2``:][::``-``1``]))`` ` `# printing result ``print``(``"The mapped dictionary : "` `+` `str``(res))`
Output
```The original list : [4, 6, 3, 10, 5, 3]
The mapped dictionary : {4: 3, 6: 5, 3: 10}
```

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