conversion from string to variables

3 days trying to find a solution for this problem but i've failed :(, its the first problem that i wasnt enable to do.so i thought about finding some help here.

first, its all about a program which has an input from the user we can say its an equation like:

x = a+b-c*d/e

at first, we want to cut each variable alone then we had to scan them from the user after this we replace the values with the variables in the equation so we get the total in x.

the problem here comes when i've tried to convert from a normal string or text into a variables which the compiler can understand.

i've made alot of searches about this and i found 2 functions which are: atoi() or strtol()

but lets try to do it without the help of those functions

waiting for your help guyz..