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Introduction to Loops

Introduction to Loops

Learn the loops for a fixed number of iterations in C++.

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Repetition

The repetition of an instruction or a group of instructions is a commonly encountered phenomenon in programming. It is called a loop. As an example, let’s say we want our program to perform the following tasks:

  • To read a document or data file line by line repeatedly until the end of the file
  • To draw an image pixel by pixel, repeatedly using color values that show it on the screen
  • To iterate through a collection of variables one by one to process their values
  • To compute a result by applying a formula to several values

Let’s say we want to display integers from 00 to 44. We can simply display the numbers 00 to 44 one by one using the cout << statement. We can also use one variable and print its value repeatedly by changing the value of the variable, as shown below. For a better understanding, you can take the AI mentor’s help.

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#include <iostream>
using namespace std;
int main()
{
    int a;
    a = 0;
    cout << a << endl;
    a = 1;
    cout << a << endl;
    a = 2;
    cout << a << endl;
    a = 3;
    cout << a << endl;
    a = 4;
    cout << a << endl;
    return 0;
}

The code above clearly shows the repetitive use of cout << a << endl;. The benefit of using a single variable is that we can convert this repetition into a loop in C++.

The counter-controlled loop

The counter-controlled loop is used to generate a sequence of values. This loop has four parts after the for keyword as shown below:

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

The initialization value initializes the counter and is executed only once. The condition value checks the value of the counter at the start of each iteration before entering the body of the loop. If true, the body of the for loop is executed and if false, the for loop is terminated. The update value updates the value of the counter and again checks the condition. We use the semicolon (;) only at the end of the first two parts and enclose these parts in ( and ). We don’t use a semicolon after ). The body of the loop is a set of statements to be repeated.

Note: If there is only one statement in the body of the loop, the curly braces { and } are not needed.

The following code demonstrates the same output as the above program in a compact way using the counter-controlled loop.

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#include <iostream>
using namespace std;
int main()
{
  for (int a = 0; a < 5; a++)  
    cout << a << endl; // Body of the loop
  return 0;
}

The for loop causes lines 5 and 6 to be repeated five times. Therefore, we can say the loop has five iterations.

In the code above:

  • We use the int variable, a, as the counter and initialize it with 0.
  • We use a < 5 as a condition.
  • We update the value of a after each iteration.
  • The body of the loop starts after ).
  • The loop’s body contains only one statement, cout << a << endl;.

There are two repeatable lines in the program, but when we unfold the loop and write out each step as it happens, there are 1111 execution steps. The loop iterates all values based on the condition to execute each statement in the body of the loop.

Let’s take an example of generating the first five non-negative even numbers—0, 2, 4, 6, and 8.

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#include <iostream>
using namespace std;
int main()
{
  for (int a,e = 0; a < 5; a++)
  {
    e = a * 2; // Multiplying the value by 2
    cout << e << endl;
  }
  return 0;
}

In the for loop, we can have initialization of multiple variables separated by a comma, as shown in line 5 in the code above.

Lines 7–8 are the body of the loop.

Let’s take another example of generating the first 100 non-negative even numbers: 0,2,4,...1980, 2, 4, ... 198.

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#include <iostream>
using namespace std;
int main()
{
  for (int count = 0; count < 100; count++)
  {
    int evenNumber = count * 2;
    cout << evenNumber << endl;
  }
  return 0;
}

In the program above, the counter generates values from 00 to 9999, and we multiply each value by 22, displaying 0 * 2, 1 * 2, 2 * 2 … 99 * 2. The lines 6–9 constitute a block of two statements representing the body of the loop.

We may generate even numbers with the same sequence as above using an increment value of 2 in the counter. We may directly use the for (int c = 0; c < 200; c += 2), as shown in the following program:

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#include <iostream>
using namespace std;
int main()
{
  for (int c = 0; c < 200; c += 2)
    cout << c << endl;
  return 0;
}

If we want to generate the multiples of 7 between 1 and 700, we can use int i = 7; i < 700; i += 7. We use 7 as starting value so that every increment of 7 may take us to the next multiple of 7. Incorporating this in the for loop will allow us to show the generated values as output.

Programming practice for counter-controlled loops

The following are a few examples to help you practice writing the programs in C++. By clicking the “Show Solution” button (after you “Run” the code), you can see one of the possible solutions to the problem. There can be several ways to write correct solutions in programming.

Odd numbers

Write a program that prints the first 100 positive odd numbers 1,3,5...1991,3,5 ... 199.

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#include <iostream>
using namespace std;
int main()
{
  // Write your code here
  return 0;
}

Arithmetic sequence

An arithmetic sequence is an ordered set of numbers that have a common difference between each consecutive term.

Write a program that prints the terms of the sequence 1,6,11...<1001, 6, 11 ... < 100.

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#include <iostream>
using namespace std;
int main()
{
    // Write your code here
  return 0;
}

Table of nn

Write a program that takes an integer input by the user and prints a multiplication table for the first 20 multiples, as illustrated below.

Sample input

5

Sample output

5 x 1 = 5
5 x 2 = 10

.

.

.

5 x 20 = 100

#include <iostream>
using namespace std;

int main()
{
    // Write your code here
  return 0;
}
Display the table of n

Mean of nn inputs

Write a program that calculates the average of the numbers input by the user. The program first asks the user how many values they want to average. The number of values should be greater than 00.

Sample input

4
10
20
30
40

Sample output

The average is : 25

#include <iostream>
using namespace std;

int main()
{
    // Write your code here
  return 0;
}
Display the mean of n inputs

Factors of the user input

Write a program that shows all the factors of a number provided by the user.

Sample input

20

Sample output

1
2
4
5
10
20

#include <iostream>
using namespace std;

int main()
{
    // Write your code here
  return 0;
}
Display the factors of the user input

Prime or not

A prime number is divisible only by itself and 11.

Write a program that shows whether or not the natural number input by the user is a prime number.

Sample input 1

7

Sample output 1

7 is a prime number

Sample input 2

20

Sample output 2

20 is not a prime number

#include <iostream>
using namespace std;

int main()
{
    // Write your code here
  return 0;
}
Display whether or not the number is prime