You are looking for the disjunction between the set of all lockers and the set of rented lockers. So, in pseudo code is one way:

• I have an array of lockers, 1 .. 24, initialized to zeroes
• I have a database containing information
• Loop: Read a Record from the DB
• If the locker-id is present, set the value in my locker-array to 1.
• Go to Loop: until you exhaust the DB.
• scan the locker-array, if the value of the i-th element is 0, print the index (i).
• Turn the result into your Teacher, you have completed the assignment in a FORTRANly fashion.