5.4.2. Traditional for Statement

The syntactic form of the for statement is:

for (init-statement condition; expression)
    statement

The for and the part inside the parentheses is often referred to as the for header.

init-statement must be a declaration statement, an expression statement, or a null statement. Each of these statements ends with a semicolon, so the syntactic form can also be thought of as

for (initializer; condition; expression)
      statement

In general, init-statement is used to initialize or assign a starting value that is modified over the course of the loop. condition serves as the loop control. As long as condition evaluates as true, statement is executed. If the first evaluation of condition yields false, statement is not executed. expression usually modifies the variable(s) initialized in init-statement and tested in condition. expression is evaluated after each iteration of the loop. As usual, statement can be either a single or a compound statement.

Execution Flow in a Traditional for Loop

Given the following for loop from § 3.2.3 (p. 94):

// process characters in s until we run out of characters or we hit a whitespace
for (decltype(s.size()) index = 0;
     index != s.size() && !isspace(s[index]); ++index)
        s[index] = toupper(s[index]); // capitalize the current character

the order of evaluation is as follows:

1. init-statement is executed once at the start of the loop. In this example, index is defined and initialized to zero.

2. Next, condition is evaluated. If index is not equal to s.size() and the character at s[index] is not whitespace, the for body is executed. Otherwise, the loop terminates. If the condition is false on the first iteration, then the for body is not executed at all.

3. If the condition is true, the for body executes. In this case, the for body makes the character at s[index] uppercase.

4. Finally, expression is evaluated. In this example, index is incremented by 1.

These four steps represent the first iteration of the for loop. Step 1 is executed only once on entry to the loop. Steps 2, 3, and 4 are repeated until the condition evaluates as false—that is, when we encounter a whitespace character in s, or index is greater than s.size().

Multiple Definitions in the for Header

As in any other declaration, init-statement can define several objects. However, init-statement may be only a single declaration statement. Therefore, all the variables must have the same base type (§ 2.3, p. 50). As one example, we might write a loop to duplicate the elements of a vector on the end as follows:

// remember the size of v and stop when we get to the original last element
for (decltype(v.size()) i = 0, sz = v.size(); i != sz; ++i)
    v.push_back(v[i]);

In this loop we define both the index, i, and the loop control, sz, in init-statement.

Omitting Parts of the for Header

A for header can omit any (or all) of init-statement, condition, or expression.

We can use a null statement for init-statement when an initialization is unnecessary. For example, we might rewrite the loop that looked for the first negative number in a vector so that it uses a for:

auto beg = v.begin();
for ( /* null */; beg != v.end() && *beg >= 0; ++beg)
    ; // no work to do

Note that the semicolon is necessary to indicate the absence of init-statement—more precisely, the semicolon represents a null init-statement. In this loop, the for body is also empty because all the work of the loop is done inside the for condition and expression. The condition decides when it’s time to stop looking and the expression increments the iterator.

Omitting condition is equivalent to writing true as the condition. Because the condition always evaluates as true, the for body must contain a statement that exits the loop. Otherwise the loop will execute indefinitely:

for (int i = 0; /* no condition */ ; ++i) {
    // process i; code inside the loop must stop the iteration!
}

We can also omit expression from the for header. In such loops, either the condition or the body must do something to advance the iteration. As an example, we’ll rewrite the while loop that read input into a vector of ints:

vector<int> v;
for (int i; cin >> i; /* no expression */ )
   v.push_back(i);

In this loop there is no need for an expression because the condition changes the value of i. The condition tests the input stream so that the loop ends when we’ve read all the input or encounter an input error.