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How write a recursive print program

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How write a recursive print program

Gurus,

I want to know how to write a recursive function that prints

1
12
123
1234
…
……

For eg: display(4) should print

1
12
123
1234

Code

#include <stdio.h>

void print(int n)
{
        if(n != 0)
        {
                print(n-1);
                printf("\n");
                print(n-1);
                printf("%d",n);
        }
}
int main()
{
        print(3);
}

Output

1

12

123

Issues

I wanted to write a pure recursive function but unable to filter unwanted prints.
Hope someone will help me out!!!

Answer 1

May this be what you want?

#include <stdio.h>

void print(int n)
{
    int i;
    if (n > 0)                  // [2]
    {
        //call print recursively
        print(n-1);             // [3]
        for (i=1; i<=n; i++)    // [4]
            printf("%d",i);
        printf("\n");
    }
}
int main(int argc, char *argv[])
{
    // If no argument, default to 8
    int value=8;
    if (argc > 1 )
        value = atoi(argv[1]);
    print(value);                // [1]
    return 0;
}

Compile it and execute like this:

display 3

or without any argument (which defaults to display 8):

display

The trick is to call print(n-1) before printing the output for the current n. This is the program flow when executed as display 3:

<main() function is running>
main() calls print(3) in [1]
   <print(3) is running, n is 3>
   since n>0 in [2] print(3) calls print(2) in [3]
      <print(2) is running, n is 2>
      since n>0 print(2) calls print(1)
         <print(1) is running, n is 1>
         since n>0 print(1) calls print(0)  
            <print(0) is running, n is 0>
            since n=0 print(0) doesn't print anything and returns
            now the function that called print(0) (that is, print(1)) takes over
         <print(1) is running, n is 1, continues executing in [4]>
         print (1) enters the for loop, prints "1\n" and returns
         now the function that called print(1) (that is, print(2)) takes over
      <print(2) is running, n is 2, continues executing in [4]>
      print(2) enters the for loop, prints "12\n" and returns
      now the function that called print(2) (that is, print(3)) takes over
   <print(3) is running, n is 3, continues executing in [4]>
   print (3) enters the for loop, prints "123\n" and returns
   now the function that called print(3) (that is, main()) takes over
<main() is running>
main executes return(0) and exits

This is what gets printed to the console:

1
12
123

If you move print(n-1) after printf("\n") you will have:

123
12
1