C ++用于循环后增量预增量差异

Question

i've been searching for an answer, but i get more and more confused.

i have these 2 for loops

for (int i = 1; (i < 5) && move.from[i - 1]; i++) {
    int const departurePoint = move.from[i - 1];
    int arrivalPoint = move.to[i - 1];

    if (arrivalPoint < 0) { // A blot was hit
        arrivalPoint = -arrivalPoint;
        board[1 - turn][BAR - arrivalPoint]--; // Remove the blot
        board[1 - turn][BAR]++; // and place it on the bar */
    }

    board[turn][departurePoint]--; // Move our own checker
    board[turn][arrivalPoint]++; // to it's landing spot.
}

and

for (int i = 1; (i < 5) && move.from[i - 1]; ++i) {
    int const departurePoint = move.from[i - 1];
    int arrivalPoint = move.to[i - 1];

    if (arrivalPoint < 0) { // We hit a blot
        arrivalPoint = -arrivalPoint;
        board[1 - turn][BAR - arrivalPoint]++; // Replace the blot
        board[1 - turn][BAR]--; // remove it from the bar
    }

    board[turn][departurePoint]++; // Replace our own checker
    board[turn][arrivalPoint]--; // to it's original spot.
}

my questions are:

1. In the for loop statement with pre-increment, has i been incremented when the "move.from[i - 1] is evaluated?
2. Has i been incremented in the body of the statement?

Answer 1

Your short question is What is the difference between i++ and ++i is the value of the expression?

The value i++ is the value of i before the increment . The value of ++i is the value of i after the increment .

Example:

int i = 2;
std::cout << i++ << std::cout; // shows 2
std::cout << i << std::cout; // shows 3

i = 2;
std::cout << ++i << std::cout; // shows 3
std::cout << i << std::cout; // shows 3

The i-- and --i operators works the same way.

Answer 2

for (int i = 1; (i < 5) && move.from[i - 1]; i++ /*i increments here and nowhere else*/)

and

for (int i = 1; (i < 5) && move.from[i - 1]; ++i /*i increments here and nowhere else*/)

Both codes are equivalent. The difference is very slight and it does not apply to this example.

when i==3 ,

++i means: 4=i+1=(++i) then i=4 .

i++ means: 3= i =(i++) then i=4 .

it does not make a difference until you assign it to another variable:

for(...; ...; k=i++)

or

for(...; ...; k=++i)

---

i+1 means:

store i to a temporary variable. Increase the temporary variable by one. It reads i but does not write to i . i will change only on ++ , -- or i= and a few other cases.

Answer 3

The for loop takes two statements and an expresion like this:

for(init_statement;condition_expresion;progress_statement){
 //...body...
}

The semantic of this instruction is basically the same as writing:

init_statement;
while(condition_expresion){
  //...body...
  progress_statement;
}

Notice the side effect of both i++ and ++i is the same, the variable i is incremented by one. Those two instructions are different when they are used as expresions, i++ evaluates to the value of i before the increment, and ++i evaluates to the value after the increment. This is all irrelevant for the for loop, as the value of the progression statement is discarded.