Check if an element is in a hashMap works with 'in' keyword but doesn't work with if-CodePudding

I solved a simple Leetcode problem (that finds two numbers in a list that sum to a target number) in Python and found it curious why when I tried to change only one line to access the hashMap my code stopped functioning because of a KeyError. But I don't understand why the 'correct' way doesn't generate a keyError.

The way that works

  def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        hashMap = {}
        for ind, val in enumerate(nums):
            busq = target - val
            if busq in hashMap:  #This line is the only thing that changes
                return [hashMap[busq], ind]
            hashMap[val] = ind

This doesn't work

def twoSum(self, nums, target):
        hashMap = {}
        for ind, val in enumerate(nums):
            busq = target - val
                if(hashMap[busq]): #This doesn't work
                return [hashMap[busq], ind]
            hashMap[val] = ind

CodePudding user response：

You can't access a non existing key in a python dictionary by using square brackets notation like hashmap[busq], instead you should check for key existence before accessing it using in operator like in the first method.

You can also use if hashmap.has_key(busq): to check for key existence.

CodePudding user response：

Maybe use something like this instead:

try:
    return [hashMap[busq], ind]
except KeyError:
    hashMap[val] = ind

When the key is not in the dict it will not return “False” but throw an error. So “if” will not work.