-D Compiler Flag

Dannys-MacBook-Pro-2:hw2 (master) dhermes$ gcc problem2_explicit.c -o problem2_explicit -Wall -std=gnu99 -lm
problem2_explicit.c:24:23: error: use of undeclared identifier 'HEY'
  printf("HEY: %d\n", HEY);
                      ^
1 error generated.
Dannys-MacBook-Pro-2:hw2 (master) dhermes$ gcc problem2_explicit.c -o problem2_explicit -Wall -std=gnu99 -lm -DHEY=10
Dannys-MacBook-Pro-2:hw2 (master) dhermes$ ./problem2_explicit 3 3
HEY: 10
Created expl_weighted_norms_3_3.txt
Created expl_solution_3_3.txt
Dannys-MacBook-Pro-2:hw2 (master) dhermes$ gcc problem2_explicit.c -o problem2_explicit -Wall -std=gnu99 -lm -DHEY=88
Dannys-MacBook-Pro-2:hw2 (master) dhermes$ ./problem2_explicit 3 3
HEY: 88
Created expl_weighted_norms_3_3.txt
Created expl_solution_3_3.txt