A20 Answer

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

/* Puzzle A20 - print a triangle of stars, n rows tall */
int main(int argc, char *argv[])
{
  int row, j, spaces, stars;
  int n=12;
  
  for ( row=0; row < n; row++ )
  {
    stars = 2*row+1;
    spaces = (2*n-1-stars)/2;
    
    for ( j=0; j < spaces; j++ )
      printf(".");
    for ( j=0; j < stars; j++ )
      printf("*");
    for ( j=0; j < spaces; j++ )
      printf(".");
    printf("\n");
  }
    
  printf("\n");
  system("PAUSE");	
  return 0;
}

Comments: Each row of the picture has three parts: the first line of dots, the line of stars, the last line of dots. The three inside for-loops deal with each of these parts. Before they start, the number of characters each inside loop must output is calculated.