You won't need an array. The simplest method to turn an integer into a grade from your table is to use a series of
jg instructions. For example:
cmp score, 90 jg Grade_A cmp score, 80 jg Grade_B cmp score, 70 jg Grade_C cmp score, 60 jg Grade_D - - and so on - -
I know you went to arrays for efficiency's sake, but try not to overthink these things. Keep it simple.
You'll need some way of converting the string you're getting from stdin into an integer (IIRC)