C++ For loop


Example

A for loop executes statements in the loop body, while the loop condition is true. Before the loop initialization statement is executed exactly once. After each cycle, the iteration execution part is executed.

A for loop is defined as follows:

for (/*initialization statement*/; /*condition*/; /*iteration execution*/)
{
    // body of the loop
}

Explanation of the placeholder statements:

  • initialization statement: This statement gets executed only once, at the beginning of the for loop. You can enter a declaration of multiple variables of one type, such as int i = 0, a = 2, b = 3. These variables are only valid in the scope of the loop. Variables defined before the loop with the same name are hidden during execution of the loop.
  • condition: This statement gets evaluated ahead of each loop body execution, and aborts the loop if it evaluates to false.
  • iteration execution: This statement gets executed after the loop body, ahead of the next condition evaluation, unless the for loop is aborted in the body (by break, goto, return or an exception being thrown). You can enter multiple statements in the iteration execution part, such as a++, b+=10, c=b+a.

The rough equivalent of a for loop, rewritten as a while loop is:

/*initialization*/
while (/*condition*/)
{
    // body of the loop; using 'continue' will skip to increment part below
    /*iteration execution*/
}

The most common case for using a for loop is to execute statements a specific number of times. For example, consider the following:

for(int i = 0; i < 10; i++) {
    std::cout << i << std::endl;
}

A valid loop is also:

for(int a = 0, b = 10, c = 20; (a+b+c < 100); c--, b++, a+=c) {
    std::cout << a << " " << b << " " << c << std::endl; 
}

An example of hiding declared variables before a loop is:

int i = 99; //i = 99
for(int i = 0; i < 10; i++) { //we declare a new variable i
    //some operations, the value of i ranges from 0 to 9 during loop execution
}
//after the loop is executed, we can access i with value of 99

But if you want to use the already declared variable and not hide it, then omit the declaration part:

int i = 99; //i = 99
for(i = 0; i < 10; i++) { //we are using already declared variable i
    //some operations, the value of i ranges from 0 to 9 during loop execution
}
//after the loop is executed, we can access i with value of 10

Notes:

  • The initialization and increment statements can perform operations unrelated to the condition statement, or nothing at all - if you wish to do so. But for readability reasons, it is best practice to only perform operations directly relevant to the loop.
  • A variable declared in the initialization statement is visible only inside the scope of the for loop and is released upon termination of the loop.
  • Don't forget that the variable which was declared in the initialization statement can be modified during the loop, as well as the variable checked in the condition.

Example of a loop which counts from 0 to 10:

for (int counter = 0; counter <= 10; ++counter)
{
    std::cout << counter << '\n';
}
// counter is not accessible here (had value 11 at the end)

Explanation of the code fragments:

  • int counter = 0 initializes the variable counter to 0. (This variable can only be used inside of the for loop.)
  • counter <= 10 is a Boolean condition that checks whether counter is less than or equal to 10. If it is true, the loop executes. If it is false, the loop ends.
  • ++counter is an increment operation that increments the value of counter by 1 ahead of the next condition check.

By leaving all statements empty, you can create an infinite loop:

// infinite loop
for (;;)
    std::cout << "Never ending!\n";

The while loop equivalent of the above is:

// infinite loop
while (true)
    std::cout << "Never ending!\n";

However, an infinite loop can still be left by using the statements break, goto, or return or by throwing an exception.

The next common example of iterating over all elements from an STL collection (e.g., a vector) without using the <algorithm> header is:

std::vector<std::string> names = {"Albert Einstein", "Stephen Hawking", "Michael Ellis"};
for(std::vector<std::string>::iterator it = names.begin(); it != names.end(); ++it) {
    std::cout << *it << std::endl;
}