C++ For Loop and Range-based For

The for loop puts the three parts of a counting loop, start, condition and step, on one line. C++ also has a range-based for that visits every element of a collection without an index.

The counting for loop

#include <iostream>

int main() {
    for (int i = 1; i <= 5; i++) {
        std::cout << i * i << " ";
    }
    std::cout << "\n";
    for (int i = 10; i >= 0; i -= 5) {
        std::cout << i << " ";
    }
    std::cout << "\n";
    return 0;
}

1 4 9 16 25 
10 5 0 

The start runs once; the condition is checked before every round; the step runs after every round. The variable i exists only inside the loop.

Nested loops

#include <iostream>

int main() {
    for (int row = 1; row <= 3; row++) {
        for (int col = 1; col <= 4; col++) {
            std::cout << row * col << "\t";
        }
        std::cout << "\n";
    }
    return 0;
}

1	2	3	4	
2	4	6	8	
3	6	9	12	

The range-based for loop

for (element : collection) gives each element in turn. It works with strings, arrays, vectors and all standard containers, and it cannot go past the end.

#include <iostream>
#include <string>
#include <vector>

int main() {
    std::vector<std::string> fruits = {"apple", "mango", "banana"};
    for (const std::string& f : fruits) {
        std::cout << f << " has " << f.size() << " letters\n";
    }
    int total = 0;
    for (int n : {3, 5, 9}) {
        total += n;
    }
    std::cout << "total " << total << "\n";
    for (char c : std::string("abc")) {
        std::cout << c << "-";
    }
    std::cout << "\n";
    return 0;
}

apple has 5 letters
mango has 5 letters
banana has 6 letters
total 17
a-b-c-

const std::string& f reads each element through a reference, without copying it, and const promises not to change it. To change the elements, leave out const: for (int& n : numbers) n *= 2;.

Choosing the loop

Use the range-based for whenever you visit every element; it is shorter and cannot make index mistakes. Use the counting for when you need the index, a different step, or only part of the collection. Remember that i < size (not <=) is the right condition for indexes from 0.

Common mistakes

• Off by one: for (int i = 0; i <= v.size(); i++) visits one element too many. Indexes go from 0 to size − 1, so the condition is i < v.size().
• Signed and unsigned: size() returns an unsigned type; comparing it with an int index gives a warning, and counting down with an unsigned index never goes below zero. Use std::size_t for indexes, or a range-based for.
• Changing a vector while looping over it: adding or removing elements inside a range-based for over the same vector is not allowed and can crash. Collect the changes and apply them after the loop.

Practice exercises

1. Print the multiplication table of a number the user types.
2. Add up the numbers from 1 to 100.
3. Print a right-angled triangle of * with nested loops.
4. Double every element of a vector with a range-based for and a reference.