Loops

For loops

Syntax

The generic syntax for a for loop in C++ is

for (int i = 0; i < n; i++)
{
  // Some code here
}

where n is assumed to be some positive integer.

We can nest loops as follows:

for (int i = 0; i < n; i++)
{
  // Maybe some code here
  for (int j = 0; j < m; j++)
  {
    // More code here
    for (int k = 0; k < l; l++)
    {
      // More code here
    }
  }
}

Example 1

Let’s assume we want to sum up the first 100 integers. A for loop is suited for this and can be implemented as follows:

double s = 0;
for (int i = 0; i <= 100; i++)
{
  s += i;
}

Example 2

Let’s look at a nested for loop. We’ll implement matrix multiplication C = AB. Assume A is an (n x m) matrix, B is an (m x n) matrix. Then C is an (n x n) matrix. A for loop explicitly implementing this computation can look as follows (using Armadillo matrices – more about those later):

for (int j = 0; j < n; j++)
{
  for (int i = 0; i < n; i++)
  {
    for (int k = 0; k < m; k++)
    {
      C(i, j) += A(i, k)*B(k, j);
    }
  }
}

While loops

When we don’t know the precise number of iterations needed to perform a task, a while loop can be used. For these loops we specify a condition, and the loop terminates once the condition is false.

Syntax

The generic syntax for a while loop is

while (condition)
{
  // Code block to execute as long as condition is true.
}

Example 1

Let’s redo the summation of the 100 first integers, but this time use a while loop:

int i = 0;
double s = 0;
while (i <= 100)
{
  s += i;
  i++;
}