Range-based for loop in C++
Range-based for loop in C++ has been added since C++ 11. It executes a for loop over a range. Used as a more readable equivalent to the traditional for loop operating over a range of values, such as all elements in a container.
for ( range_declaration : range_expression )
loop_statement
Parameters :
range_declaration :
a declaration of a named variable, whose type is the
type of the element of the sequence represented by
range_expression, or a reference to that type.
Often uses the auto specifier for automatic type
deduction.
range_expression :
any expression that represents a suitable sequence
or a braced-init-list.
loop_statement :
any statement, typically a compound statement, which
is the body of the loop.
C++ implementation:
// Illustration of range-for loop
// using CPP code
#include <iostream>
#include <map>
#include <string>
#include <vector>
using namespace std;
// Driver
int main()
{
// Iterating over whole array
vector<int> v = { 0, 1, 2, 3, 4, 5 };
for (auto i : v)
cout << i << ' ';
cout << '\n';
// the initializer may be a braced-init-list
for (int n : { 0, 1, 2, 3, 4, 5 })
cout << n << ' ';
cout << '\n';
// Iterating over array
int a[] = { 0, 1, 2, 3, 4, 5 };
for (int n : a)
cout << n << ' ';
cout << '\n';
// Just running a loop for every array
// element
for (int n : a)
cout << "In loop" << ' ';
cout << '\n';
// Printing string characters
string str = "Beginner";
for (char c : str)
cout << c << ' ';
cout << '\n';
// Printing keys and values of a map
map<int, int> MAP({ { 1, 1 }, { 2, 2 }, { 3, 3 } });
for (auto i : MAP)
cout << '{' << i.first << ", " << i.second << "}\n";
}
Output
0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 In loop In loop In loop In loop In loop In loop G e e k s {1, 1} {2, 2} {3, 3}
C++ 17 or higher: Range-based loops can also be used with maps like this:
for (auto& [key, value]: myMap) {
cout << key << " has value " << value << std::endl;
}
Here [key, value] works like elements of pair which can be directly accessed without specifying first or second keyword.