Teaching Kids Programming – Final Value of Variable After Performing Operations (via Reduce Function)

Teaching Kids Programming: Videos on Data Structures and Algorithms

There is a programming language with only four operations and one variable X:

++X and X++ increments the value of the variable X by 1.
–X and X– decrements the value of the variable X by 1.
Initially, the value of X is 0.

Given an array of strings operations containing a list of operations, return the final value of X after performing all the operations.

Example 1:
Input: operations = [“–X”,”X++”,”X++”]
Output: 1
Explanation: The operations are performed as follows:
Initially, X = 0.
–X: X is decremented by 1, X = 0 – 1 = -1.
X++: X is incremented by 1, X = -1 + 1 = 0.
X++: X is incremented by 1, X = 0 + 1 = 1.
Example 2:

Input: operations = [“++X”,”++X”,”X++”]
Output: 3
Explanation: The operations are performed as follows:
Initially, X = 0.
++X: X is incremented by 1, X = 0 + 1 = 1.
++X: X is incremented by 1, X = 1 + 1 = 2.
X++: X is incremented by 1, X = 2 + 1 = 3.
Example 3:

Input: operations = [“X++”,”++X”,”–X”,”X–“]
Output: 0
Explanation: The operations are performed as follows:
Initially, X = 0.
X++: X is incremented by 1, X = 0 + 1 = 1.
++X: X is incremented by 1, X = 1 + 1 = 2.
–X: X is decremented by 1, X = 2 – 1 = 1.
X–: X is decremented by 1, X = 1 – 1 = 0.

Constraints:
1 <= operations.length <= 100
operations[i] will be either “++X”, “X++”, “–X”, or “X–“.

Final Value of Variable After Performing Operations

The evaluation process is simple: check each operation and update the value correspondingly. The second character ‘+’ means increment or ‘-‘ means decrement.

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class Solution:
    def finalValueAfterOperations(self, operations: List[str]) -> int:
        ans = 0
        for i in operations:
            if i[1] == '+':
                ans += 1
            else:
                ans -= 1
        return ans

class Solution:
    def finalValueAfterOperations(self, operations: List[str]) -> int:
        ans = 0
        for i in operations:
            if i[1] == '+':
                ans += 1
            else:
                ans -= 1
        return ans

Time complexity is O(N) – space O(1). We can use Pythonic one-liner via Reduce function:

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class Solution:
    def finalValueAfterOperations(self, operations: List[str]) -> int:
        return reduce(lambda a, b: a + 1 if b[1] == '+' else a - 1, operations, 0)

class Solution:
    def finalValueAfterOperations(self, operations: List[str]) -> int:
        return reduce(lambda a, b: a + 1 if b[1] == '+' else a - 1, operations, 0)

The first parameter to the reduce function (from functools import reduce) is a function where we can pass a lambda function.

–EOF (The Ultimate Computing & Technology Blog) —