Assignment and retrieval along with Post increment in single line in c-CodePudding

This is the code which I ran.

int minimumCardPickup(vector<int>& cards) {
        int N = cards.size();
        int l = 0, r = 0, res = INT_MAX;
        unordered_map<int, int> pos;
        
        while (r < N) {
            if (pos.find(cards[r]) != pos.end()) {
                l = pos[cards[r]];
                res = min(res, r - l   1);
            }
            pos[cards[r]] = r  ; //Here
        }
        
        return res == INT_MAX ? -1 : res;
    }

let's take an example: cards = [1, 2, 3, 2, 4] The line with the comment should update the map to the following after 3 iterations:

pos = {[1, 0], [2, 1], [3, 2]}

But instead it updates the map like this:

pos = {[2, 0], [3, 1]}

It should have done something like this: get value of cards[r] and then assign pos[cards[r]] with the value of r and then do r for next iteration but instead, it did r then get value of cards[r] and then assign pos[cards[r]] with value of r before increment.

If I do this, it works fine.

pos[cards[r]] = r;
r  ;

Can someone explain why it works like that?

CodePudding user response：

As you correctly identified, the problem is here:

pos[cards[r]] = r  ; //Here

In earlier standards:

If you post-increment or post-decrement a variable, you should not read the value of it again before a sequence point (in this case, ;). This is because post-increment might occur any time during the statement (as long as the expression evaluates to the old value).

From C 17, for any operation l = r, the evaluation of r is sequenced-before the evaluation of l (including side effects), so it’s a valid expression, but still not what’s expected in the code. Either do the post-increment on the left-hand side of the = (thereby restricting to c 17 and newer), or simply do the increment as a distinct statement (as you correctly specified).