For loops

We use loops in programming to repeat execution of a block of code instead of repeating the entire code. In C++ we have 3 types of loops: - for-loops - while loops - do-while loops.

The execution of a loop can be controlled using the following keywords - break - terminates the loop (or switch) -statement and transfers execution to the statement directly after the loop - continue - will cause the loop to process to the next element skipping the current one - goto LABEL - transfers control to the specified label. IT IS NOT ADVISED TO USE THIS STATEMENT!

"for" loop

if we know the exact number of times to repeat the loop for, we use a "for" loop. The syntax has 4 parts, the initialization, test expression, modifying expression and the code block to be executed in loop.

// syntax of for loop
for (initialization; test condition; update) {
    // body of the loop
}

Here's an example

// prints Hello C++ for 10 times
for(int i = 0; i < 10; i++)
{
    cout << "Hello C++" << endl;
}

In the above example, we are declaring a loop variable i, which is initialised to 0.

Then if the test condition turns out to be true, we continue with the loop and execute the body of the loop. After every iteration of the loop, the modifying expression is run ( Here we are using i++ which increments the value of i by 1 in every step ). We continue this process till the test condition turns out to be false i.e. when i becomes 10 in the above loop.

The loop can be read as "As long as i is less than 10, run the below Code".

We can also write the initialisation part before and modifying expression in the body of the loop

// print numbers 0 - 9
int i = 0;
for( ; i < 10 ; )
{
    cout << i << endl;
    i++;
}

The for loop in C++ (since C++11) can also be used to iterate over an array. Some other modern programming languages reference to the technique in the example below as a "foreach-loop":

// fetch each array-element and print it out
int arr[] = {1,2,3,4,5,6};

for(int n : arr)
{
    cout << n << endl;
}

/*
  Warning: the example above will reference the original memory of arr[] and has write-access!

  As you often don't need to write to that adress-space, you should consider to access it read-only for safety reasons.
  To avoid write-access, you might consider using a const-reference like shown below,
  which will create a constant -and therefore unchangeable- reference named "n" to each existing value of "arr",
  effectively referncing the values read-only.

  You'll learn more about reference's and pointer's in the next chapters.
*/

// fetch each array-element and print it out (readonly)
int arr[] = {1,2,3,4,5,6};

for(const int& n : arr)
{
    cout << n << endl;
}

Exercise

Try to print all the even numbers below 20 using a for loop in ascending order.