7.10. Increment and decrement operators

Incrementing and decrementing are such common operations that C++ provides special operators for them. The ++ operator adds one to the current value of an int, char or double, and – subtracts one. Neither operator works on strings, and neither should be used on bools.

Technically, it is legal to increment a variable and use it in an expression at the same time. For example, you might see something like:

cout << i++ << '\n';

Looking at this, it is not clear whether the increment will take effect before or after the value is displayed. Because expressions like this tend to be confusing, I would discourage you from using them. In fact, to discourage you even more, I’m not going to tell you what the result is. If you really want to know, you can try it.

The active code demonstrates how using increment operators with cout statements can be confusing.

Looping and counting
 1#include <cstddef>
 2#include <iostream>
 3using namespace std;
 4
 5using std::size_t;
 6
 7int main() {
 8   int x = 0;
 9   // We incremented x, so it should print out 1 now right?
10   cout << x++ << '\n'; // Weird, x is still 0?
11}

If you're curious about this, feel free to search up about prefix and postfix increment operators. But for now, just avoid incrementing a variable and using it in an expression at the same time.

Using the increment operators, we can rewrite the letter-counter:

std::size_t index = 0;
while (index < fruit.size()) {
  if (fruit[index] == 'a') {
    count++;
  }
  index++;
}

The active code below adds increment operators to our old letter-counter.

Looping and counting
 1#include <cstddef>
 2#include <iostream>
 3#include <string>
 4
 5using std::size_t;
 6
 7int main() {
 8   std::string fruit = "banana";
 9   std::size_t count = 0;
10
11   std::size_t index = 0;
12   while (index < fruit.size()) {
13       if (fruit[index] == 'a') {
14           count++;
15       }
16       index++;
17   }
18   std::cout << count;
19}

It is a common error to write something like

index = index++;             // WRONG!!

Unfortunately, this is syntactically legal, so the compiler will not warn you. The effect of this statement is to leave the value of index unchanged. This is often a difficult bug to track down.

Warning

Remember, you can write index = index +1;, or you can write index++;, but you shouldn’t mix them.

Q1

Click on the incorrect or not suggested increment statements.


    
    
    
    
    
}

Q2

What does the following code print?

1int x = -5;
2while (x < 0) {
3  x++;
4  cout << x << ' ';
5}

Q3

Print every number from 1-10 in this format: "Number 1". Each number should be on its own line.

  1.     x++;
}
  2. --x; #distractor
  3. cout << "Number " << x << '\n';
  4. cout << "Number " << x; #distractor
  5. int x = 1;
  6. while (x <= 10) {
  7. x = 1; #distractor