stars.c

#include <stdio.h>

// we're going for something like this
//
//         *                     // row #0 //  1 star
//       * * *                   // row #1 //  3 stars
//     * * * * *                 // row #2 //  5 stars
//   * * * * * * *               // row #3 //  7 stars
// * * * * * * * * *             // row #4 //  9 stars

int num_stars(int row_index) {
  return 2*row_index+1;
}

void triangle(int num_rows) {
  int last_row = num_rows - 1;
  int num_stars_in_last_row = num_stars(last_row);
  int maximum_line_length = num_stars_in_last_row * 2;
  for (int this_row_index = 0; this_row_index < num_rows; this_row_index++) {
    int num_stars_in_this_row = num_stars(this_row_index);
    int line_length = num_stars_in_this_row * 2;
    int shift_distance = (maximum_line_length - line_length) / 2;
    for (int i = 0; i < shift_distance; i++) printf(" ");
    for (int i = 0; i < num_stars_in_this_row; i++) printf("* ");
    printf("\n");
  }
}

int main() {
  triangle(5);
}