Where should I put my validation code for user input

if (!strcmp(yn, "y")) {
  isCooked[x] = 1;
  printf("How much %s (in grams) will you cook? ", food[choice - 1]);
} else {
  isCooked[x] = 0;
  printf("How much %s (in grams) will you eat raw? ", food[choice - 1]);
}

scanf("%f", &grams);
userGrams[x] = grams;

Where should I put my if-else statement if I want to control the user input of grams? I want to put restrictions on my program, like the user should only input minmimum of 20g and the maximum of 200g.