Question

I have been stumped by this problem. I need to create a pattern such as:

1
21
221
2221
22221

Using nested for loops. I have something that does (A)

222221
222221
222221
222221
222221

and used to have something that did (B)

/* 1
* 21
* 221
* 2221
* 22221
* 222221
* 2222222
*/

#include <stdio.h>

main()
{
    int n, c, k;

    printf("Enter number of rows\n");
    scanf("%d",&n);

    for ( c = 1 ; c <= n ; c++ )
    {
        printf("1\n");

        for( k = 1 ; k <= c ; k++ )
        printf("2");


    }

    return 0;
}

Any hints would be helpful.

Solution – Thanks to the intelligent people that decided to help.
I appreciate your help!

#include <stdio.h>

main()
{
    int n, c, k;

    printf("Enter number of rows");
    scanf("%d",&n);

    for ( c = 1 ; c <= n ; c++ )
    {


        for( k = 1 ; k < c ; k++ )
        {
            printf("2");
        }

        printf("1\n");
    }

    return 0;
}

Answer 1

This can be done using nested for loops. Let’s examine the formulae for one line of the output:

line 1:

1

Which can be made using a simple for loop like this:

for (int i = 0; i < 1; i++)
{
    putc('1', stdout);
    putc('\n', stdout);
}

line 2:

21

Hmm, this requires change to our structure, as we can’t break the output of iteration #1, but we still need to be able to add the ‘2’ in there… Something like this should work:

for (int i = 0; i < 2; i++)
{
    if (i > 0)
       putc('2', stdout);

    putc('1', stdout);
    putc('\n', stdout);
}

line 3:

221

Wait, now we need two ‘2’s in there! How can we do this without breaking line’s 2 and three? Well something like this should do it:

 for (int i = 0; i < 3; i++)
 {
     int j = i;
     while (j--)
     {
         putc('2', stdout);
     }

     putc('1', stdout);
     putc('\n', stdout);
 } 

Notice that I used a while loop instead of a for loop. It is an exercise to the reader to figure out how to turn that while loop into a for loop.

Hopefully this helped you to understand the process behind solving similar problems like this in the future – as it is an important programming skill to have.