C++ Program to Print Number Triangles

The program to print number triangles uses two loops (usually while or for loop) and prints a triangle of numbers. In this example, we will see four such patterns of number triangle.

This program is written using Dev C++ version 4.9.9.2 installed on a Windows 7 64-bit PC. You may use any standard C++ compiler and the code in this program will still work.

The intent of this program is to develop an understanding of C++ loops for beginner C++ programmers.

How a loop starts and terminates?

How does a nested loop works?

These are some of the questions, we tried to answer using these examples. However, you as a learner must try to write the program on your own and then compare your results given here.

Problem Definition

We want to following 4 number triangles using nested for loops. You may try a different combination of loops for better learning. The number patterns are given below.

Each of this program take two “for” loop. The outer loop is for rows and inner loop is for columns.

Program Code – Pattern 1

The program code for pattern 1 is given below.

/* C++ Program to print the following number triangle 
1 
2 1 
3 2 1 
4 3 2 1 
5 4 3 2 1 
6 5 4 3 2 1 
7 6 5 4 3 2 1 
8 7 6 5 4 3 2 1 
9 8 7 6 5 4 3 2 1 */

#include <conio.h> 
#include <iostream.h>
#include <cstdlib>

int main()
{       
   int i,j,n;    
   n = 10;       
   for( i = 1; i < n; i++) 
   { 
      for( j=i;j >= 1; j--)         
      {             
         cout << j;         
      }             
   cout << "\n";     
   } 
getch(); 
return 0; 
}

Output – Pattern 1

1 
2 1 
3 2 1 
4 3 2 1 
5 4 3 2 1 
6 5 4 3 2 1 
7 6 5 4 3 2 1 
8 7 6 5 4 3 2 1 
9 8 7 6 5 4 3 2 1

Program Code – Pattern 2

The program code for pattern 2 is given below.

/* C++ program to print following number triangle
1 
12 
123 
1234 
12345 
123456 
1234567 
12345678 
123456789 */ 

#include <conio.h> 
#include <iostream.h>
#include <cstdlib>

using namespace std; 
int main() 
{  
   int i,j,n;      
   n = 10;       
   for(i = 1;i < n;i++)     
   {         
      for(j = 1;j<=i;j++)         
      {             
         cout << j;         
      }             
   cout << "\n";     
   } 
getch(); 
return EXIT_SUCCESS; 
}

Output – Pattern 2

1 
12 
123 
1234 
12345 
123456 
1234567 
12345678 
123456789

Program Code – Pattern 3

The program code for pattern 1 is given below.

/* C++ Program to Print 
987654321 
87654321 
7654321 
654321 
54321 
4321 
321 
21 
1 */ 

#include <conio.h> 
#include <iostream.h>
#include <cstdlib>

using namespace std; 

int main() 
{   

    int i,j,n; 

    n = 10;   

    for(i = 1; i <=10;i++) { for( j= n-i;j>= 1;j--) 
        { 

            cout << j; 

        } 

            cout << "\n"; 
    }   

getch(); 
return EXIT_SUCCESS; 

}

Output – Pattern 3

Program Code – Pattern 4

The program code for pattern 4 is given below.

/* C++ Program to print 
123456789 
12345678 
1234567 
123456 
12345 
1234 
123 
12 
1 */ 

#include <conio.h> 
#include <iostream.h>
#include <cstdlib>

using namespace std; 

int main() 
{   

    int i,j,n; 

    n = 10; 
  
    for(i = 1; i <= n;i++) 
    { 

        for(j = 1; j <= n - i; j++) 
        { 

            cout << j; 

        } 

            cout << "\n"; 
    } 

getch(); 

return EXIT_SUCCESS; 

}

Output – Pattern 4