Question

(Recursive Function) Display the triangle pattern without using loop. Write a recursive function named Triangle to solve the problem.

void Triangle(int);

Write a main function to test it. For example, a function call Triangle(6) will displays the following 6 rows of a triangle pattern.

1

1 2 1

1 2 3 2 1

1 2 3 4 3 2 1

1 2 3 4 5 4 3 2 1

1 2 3 4 5 6 5 4 3 2 1

And a function call Triangle(4) will displays the 4 rows of the pattern as below.

1

1 2 1

1 2 3 2 1

1 2 3 4 3 2 1

A function call Triangle(9) will displays the 9 rows of the pattern as below.

1

1 2 1

1 2 3 2 1

1 2 3 4 3 2 1

1 2 3 4 5 4 3 2 1

1 2 3 4 5 6 5 4 3 2 1

1 2 3 4 5 6 7 6 5 4 3 2 1

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

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

(Hint: You may need to write another recursive functions to display one single row in the triangle pattern. And let your Triangle function invoke the second recursive function if necessary. E.g.,

void printRow(int m, int n)

When called with two actual parameters, the function call printRow(2,5) will display a sequence 2 3 4 5 4 3 2; The function call printRow(1,5)will display 1 2 3 4 5 4 3 2 1. )

Answer 1

// C implementation to print the given number sequences recursively

#include <stdio.h>
#include <stdlib.h>

void printRowExtend(int n)
{
    if (n < 1)
return;
  
   // print the remaining numbers of the nth row recursively in increasing order
   printf("%d ",n);
   printRowExtend(n-1);
}

void printRow(int n,int k)
{
   if (n < 1)
{
printRowExtend(k-1);
return;
}  
   // print the remaining numbers of the nth row recursively in decreasing order
printf("%d ",k-n+1);
   printRow(n-1,k);
}

// to recursively print the numbers in row number wise
void printRecur(int n,int k)
{
   if(n>k)
return;
  
   printRow(n,n); // pass n & n because we have to go first from 1 to n & then from n to 1
printf("\n"); // after each row, there should be new line
   printRecur(n+1,k);
}

// Triangle function as described in the question
void Triangle(int n)
{
printRecur(1,n);
}

// Driver program for the question given in description
int main()
{
   int n = 9;
   Triangle(n);
   return 0;
}