In lesson 6.3 -- Arrays and loops, we showed examples where we used a for loop to iterate through each element of an array.
For example:
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#include <iostream> int main() { const int numStudents = 5; int scores[numStudents] = { 84, 92, 76, 81, 56 }; int maxScore = 0; // keep track of index of our largest score for (int student = 0; student < numStudents; ++student) if (scores[student] > maxScore) maxScore = scores[student]; std::cout << "The best score was " << maxScore << '\n'; return 0; } |
While for loops provide a convenient and flexible way to iterate through an array, they are also easy to mess up and prone to off-by-one errors.
C++11 introduces a new type of loop called a for each loop (also called a range-based for loop) that provides a simpler and safer method for cases where we want to iterate through every element in an array (or other list-type structure).
For each loops
The for each statement has a syntax that looks like this:
for (element_declaration : array) statement;
When this statement is encountered, the loop will iterate through each element in array, assigning the value of the current array element to the variable declared in element_declaration. For best results, element_declaration should have the same type as the array elements, otherwise type conversion will occur.
Let’s take a look at a simple example that uses a for each loop to print all of the elements in an array named fibonacci:
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#include <iostream> int main() { int fibonacci[] = { 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89 }; for (int number : fibonacci) // iterate over array fibonacci std::cout << number << ' '; // we access the array element for this iteration through variable number return 0; } |
This prints:
0 1 1 2 3 5 8 13 21 34 55 89
Let’s take a closer look at how this works. First, the for loop executes, and variable number is set to the value of the first element, which has value 0. The program executes the statement, which prints 0. Then the for loop executes again, and number is set to the value of the second element, which has value 1. The statement executes again, which prints 1. The for loop continues to iterate through each of the numbers in turn, executing the statement for each one, until there are no elements left in the array to iterate over. At that point, the loop terminates, and the program continues execution (returning 0 to the operating system).
Note that variable number is not an array index. It’s assigned the value of the the array element for the current loop iteration.
For each loops and the auto keyword
Because element_declaration should have the same type as the array elements, this is an ideal case in which to use the auto keyword, and let C++ deduce the type of the array elements for us.
Here’s the above example, using auto:
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#include <iostream> int main() { int fibonacci[] = { 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89 }; for (auto number : fibonacci) // type is auto, so number has its type deduced from the fibonacci array std::cout << number << ' '; return 0; } |
Rewriting the max scores example using a for each loop
Here’s the example at the top of the lesson rewritten using a for each loop:
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#include <iostream> int main() { const int numStudents = 5; int scores[numStudents] = { 84, 92, 76, 81, 56 }; int maxScore = 0; // keep track of index of our largest score for (auto score: scores) // iterate over array scores, assigning each value in turn to variable score if (score > maxScore) maxScore = score; std::cout << "The best score was " << maxScore << '\n'; return 0; } |
Note that in this example, we no longer have to manually subscript the array. We can access the array element directly through variable score.
A few forward-looking notes
For each loops don’t only work with fixed arrays, they work with many kinds of list-like structures, such as vectors (e.g. std::vector), linked lists, trees, and maps. We haven’t covered any of these yet, so don’t worry if you don’t know what these are. Just remember that for each loops provide a flexible and generic way to iterate through more than just arrays.
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#include <vector> int main() { std::vector<int> fibonacci = { 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89 }; // note use of std::vector here rather than a fixed array for (auto number : fibonacci) std::cout << number << ' '; return 0; } |
Note that for each loops do not provide a way to get the array index of the current element. This is because many of the structures that for each loops can be used with (such as linked lists) are not directly indexable!
Second, in all of our above examples, our element declarations are declared by value:
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int array[5] = { 9, 7, 5, 3, 1 }; for (auto element: array) // element will be a copy of the current array element std::cout << element << ' '; |
Copying array elements can be expensive, and most of the time we really just to reference the original element. Fortunately, C++ provides a syntax for doing this:
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int array[5] = { 9, 7, 5, 3, 1 }; for (auto &element: array) // The ampersand makes element a reference to the actual array element, preventing a copy from being made std::cout << element << ' '; |
Since we haven’t covered references yet, you can treat this as a bit of magic for now. We’ll revisit this topic again when we get to the lesson on references.
Conclusion
For each loops provide a superior syntax for iterating through an array when we need to access all of the array elements in forwards sequential order. It should be preferred over the standard for loop in the cases where it can be used. To prevent making copies of each element, the element declaration should ideally be a reference.
Note that because for each was added in C++11, it won’t work with older compilers.
Quiz
This one should be easy.
1) Declare a fixed array with the following names: Alex, Betty, Caroline, Dave, Emily, Fred, Greg, and Holly. Ask the user to enter a name. Use a for each loop to see if the name the user entered is in the array.
Sample output:
Enter a name: Betty Betty was found.
Enter a name: Megatron Megatron was not found.
Hint: Use std::string as your array type.
Quiz answers
 6.4 -- Sorting an array using selection sort
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 Index
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 6.3 -- Arrays and loops
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