# Python – Substitute K for first occurrence of elements

• Last Updated : 03 Jul, 2020

Sometimes, while working with Python data, we can have a problem in which, we need to perform substitution to the first occurrence of each element in list. This type of problem can have application in various domains such as web development. Let’s discuss certain ways in which this task can be performed.

Input : test_list = [4, 3, 3], K = 10
Output : [10, 10, 3]

Input : test_list = [4, 3, 7], K = 8
Output : [8, 8, 8]

Method #1 : Using loop
This is brute way to solve this problem. In this, we run a loop for each element in list and store already occurred element for lookup, and accordingly assign K.

 `# Python3 code to demonstrate working of ``# Substitute K for first occurrence of elements``# Using loop`` ` `# initializing list``test_list ``=` `[``4``, ``3``, ``3``, ``7``, ``8``, ``7``, ``4``, ``6``, ``3``] `` ` `# printing original list``print``(``"The original list is : "` `+` `str``(test_list))`` ` `# initializing K``K ``=` `10`` ` `# Substitute K for first occurrence of elements``# Using loop``lookp ``=` `set``()``res ``=` `[]``for` `ele ``in` `test_list:``    ``if` `ele ``not` `in` `lookp:``        ``lookp.add(ele)``        ``res.append(K)``    ``else``:``        ``res.append(ele)`` ` `# printing result ``print``(``"List after Substitution : "` `+` `str``(res))`
Output :
```The original list is : [4, 3, 3, 7, 8, 7, 4, 6, 3]
List after Substitution : [10, 10, 3, 10, 10, 7, 4, 10, 3]
```

Method #2 : Using `defaultdict() + next()` + count + list comprehension
The combination of above functions provide shorthand to solve this problem. In this, we perform the task of checking for 1st occurrence using count and next() returns if element its 1st occurrence and creates boolean list comprising of 1s for 1st occurrence and 0 for duplicates. These are converted to desired result using list comprehension.

 `# Python3 code to demonstrate working of ``# Substitute K for first occurrence of elements``# Using defaultdict() + next() + count + list comprehension``from` `itertools ``import` `count``from` `collections ``import` `defaultdict`` ` `# initializing list``test_list ``=` `[``4``, ``3``, ``3``, ``7``, ``8``, ``7``, ``4``, ``6``, ``3``] `` ` `# printing original list``print``(``"The original list is : "` `+` `str``(test_list))`` ` `# initializing K``K ``=` `10`` ` `# Substitute K for first occurrence of elements``# Using defaultdict() + next() + count + list comprehension``freq ``=` `defaultdict(count)``temp ``=` `[``int``(``next``(freq[val]) ``=``=` `0``) ``for` `val ``in` `test_list]``res ``=` `[K ``if` `ele ``else` `test_list[idx] ``for` `idx, ele ``in` `enumerate``(temp)]`` ` `# printing result ``print``(``"List after Substitution : "` `+` `str``(res))`
Output :
```The original list is : [4, 3, 3, 7, 8, 7, 4, 6, 3]
List after Substitution : [10, 10, 3, 10, 10, 7, 4, 10, 3]
```

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