Loops in C++

Loops

In computer programming, loops are used to repeat a block of code.

For example, let's say we want to show a message 100 times. Then instead of writing the print statement 100 times, we can use a loop.

That was just a simple example; we can achieve much more efficiency and sophistication in our programs by making effective use of loops.

There are 3 types of loops in C++ :

• for loop
• while loop
• do...while loop

While Loops

Syntax of While Loop

while(condition) {
    // do something
}

• A while loop evaluates the condition
• If the condition evaluates to true, the code inside the while loop is executed.
• The condition is evaluated again.
• This process continues until the condition is false.
• When the condition evaluates to false, the loop terminates.

Flow Chart of While Loop:

Example:

Print numbers 1 to n on the stdout

#include<iostream>
using namespace std;

int main() {
    int n;
    cin>>n;
    int num = 1;
    while( num <= n ) {
        cout << num << endl; // Printing the number
        num  = num + 1 // Increasing number from 1 to 2 then 2 to 3 then 3 to 4 until it is equal to n

        /* Can also be written like this:
         * num += 1;
         * num++;
         * ++num;
         */
    }
}

Example:

Print the sum of positive numbers entered by the user.

#include<iostream>
using namespace std;

int main() {
    int number;
    int sum = 0;

    // take input from the user
    cout << "Enter a number: ";
    cin >> number;

    while (number >= 0) {
        // add all positive numbers
        sum += number;

        // take input again if the number is positive
        cout << "Enter a number: ";
        cin >> number;
    }

    // display the sum
    cout << endl << "The sum is " << sum << endl;
}

Patterns:

Square:

Input: 4
Output:
****
****
****
****

Input: 5
Output:
*****
*****
*****
*****
*****

Solution:

• Check for rows and columns
• Here, 4 rows for 4 columns
• rows == columns

#include<iostream>
using namespace std;

int main() {
    int n;
    cin>>n;

    int i = 1; // Starting position

    while(i <= n) {
        int j = 1;
        while(j <= n) {
            cout<<'*';
            j++;
        }
        cout<<endl;
        i++;
    }
}

Numbered Square:

Input: 4
Output:
1111 -> 1st row
2222 -> 2nd row
3333 -> 3rd row
4444 -> 4th row

Solution:

• Check for rows and columns
• Here, 4 rows for 4 columns
• rows == columns
• And, instead of * we need to print the row number

#include<iostream>
using namespace std;

int main() {
    int n;
    cin>>n;

    int row = 1; // Starting position

    while(row <= n) {
        int col = 1;
        while(col <= n) {
            cout<<row; // Notice what we're printing
            col++;
        }
        cout<<endl;
        row++;
    }
}