5.4.3. Range for Statement
The new standard introduced a simpler for statement that can be used to iterate through the elements of a container or other sequence. The syntactic form of the range for statement is:
for (declaration : expression)
statement
expression must represent a sequence, such as a braced initializer list (§ 3.3.1, p. 98), an array (§ 3.5, p. 113), or an object of a type such as vector or string that has begin and end members that return iterators (§ 3.4, p. 106).
declaration defines a variable. It must be possible to convert each element of the sequence to the variable’s type (§ 4.11, p. 159). The easiest way to ensure that the
Exercises Section 5.4.2
Exercise 5.15: Explain each of the following loops. Correct any problems you detect.
(a) for (int ix = 0; ix != sz; ++ix) { /* . . . */ }
if (ix != sz)
// . . .
(b) int ix;
for (ix != sz; ++ix) { /* . . . */ }
(c) for (int ix = 0; ix != sz; ++ix, ++ sz) { /* . . . */ }
Exercise 5.16: The while loop is particularly good at executing while some condition holds; for example, when we need to read values until end-of-file. The for loop is generally thought of as a step loop: An index steps through a range of values in a collection. Write an idiomatic use of each loop and then rewrite each using the other loop construct. If you could use only one loop, which would you choose? Why?
Exercise 5.17: Given two vectors of ints, write a program to determine whether one vector is a prefix of the other. For vectors of unequal length, compare the number of elements of the smaller vector. For example, given the vectors containing 0, 1, 1, and 2 and 0, 1, 1, 2, 3, 5, 8, respectively your program should return true.
types match is to use the auto type specifier (§ 2.5.2, p. 68). That way the compiler will deduce the type for us. If we want to write to the elements in the sequence, the loop variable must be a reference type.
On each iteration, the control variable is defined and initialized by the next value in the sequence, after which statement is executed. As usual, statement can be a single statement or a block. Execution ends once all the elements have been processed.
We have already seen several such loops, but for completeness, here is one that doubles the value of each element in a vector:
vector<int> v = {0,1,2,3,4,5,6,7,8,9};
// range variable must be a reference so we can write to the elements
for (auto &r : v) // for each element in v
r *= 2; // double the value of each element in v
The for header declares the loop control variable, r, and associates it with v. We use auto to let the compiler infer the correct type for r. Because we want to change the value of the elements in v, we declare r as a reference. When we assign to r inside the loop, that assignment changes the element to which r is bound.
A range for is defined in terms of the equivalent traditional for:
for (auto beg = v.begin(), end = v.end(); beg != end; ++beg) {
auto &r = *beg; // r must be a reference so we can change the element
r *= 2; // double the value of each element in v
}
Now that we know how a range for works, we can understand why we said in § 3.3.2 (p. 101) that we cannot use a range for to add elements to a vector (or other container). In a range for, the value of end() is cached. If we add elements to (or remove them from) the sequence, the value of end might be invalidated (§ 3.4.1, p. 110). We’ll have more to say about these matters in § 9.3.6 (p. 353).