I want to build a logical circit that performs addition and subtraction on two single digit numbers given as input. I shall soon try to send a block diagram of it.
Does your project require reinventing the wheel, or can you use logic ic's?. A 74LS83, for instance, is a four bit full adder with carry. To do a subtraction, compliment the number to subtract, and add it to the other number.
If you get the manual for LS TTL logic, you also get the internal logic of the chip, which describes how to build the function using AND/NAND & OR/NOR gates. If they're still using MIL-STD 806C logic symbology, that is.