Comma-separated lines of code inside braceless loop [duplicate]

manpreet Best Answer 3 years ago

int n, num;
scanf("%d", &n);
vector<int> arr;
arr.reserve(n);

while (n--)
        scanf("%d", &num),
        arr.push_back(num);

for (int i : arr) printf("%d ", i);
printf("\n");

manpreet 3 years ago

This is the comma operator at work. It evaluates the left hand side of the operator (scanf("%d", &num)), then the right hand side (arr.push_back(num)) and returns the result of this expression.

I don't think the code snippet is a good usage scenario for the comma operator. It seems to be employed for not typing {} to create a body for the while loop, which is lazy and unnecessarily "smart". But it works.

If you're interested in legitimate use cases for the comma operator, have a look at this article.