Question

Does anybody know why

  vector<int> test(10);   int a=0;    for_each(test.begin(),test.end(),(_1+=var(a),++var(a)));    for_each(test.begin(),test.end(),(cout << _1 << ' '));   cout << '\n' 

Gives : ‘0 1 2 3 4 5 6 7 8 9’

but

  transform(test.begin(),test.end(),test.begin(), (_1+=var(a),++var(a)));   ...(as before) 

Gives : ‘1 2 3 4 5 6 7 8 9 10’

?

Answer 1

Comma operator evaluates left to right, so the result of the

_1+=var(a), ++var(a) 

is ++var(a), which you’ll store using the transform version.

• for_each:

  _1 += var(a) is evaluated, updating your sequence (via the lambda _1), then ++var(a) is evaluated, but this has no effect on your sequence.

• transform:

  _1+=var(a) is evaluated, updating your sequence (just like before), then ++var(a) is evaluated, this also gives the result of the whole expression, then that is used to update your sequence again (via the transform)