How to input a mathematical expression from keyboard? [closed]

Question

I have a simple C program

int main()
{

    int n, maxn = 21;
    float SN, x;

    printf("input x:");
    scanf("%f", &x);

    printf("input maxn:");
    scanf("%d", &maxn);

    for(n=0;n<=maxn;n++){
        SN = SN + pow(x,n);
        n = n + 1;
    }

    printf("%f", SN);
    getch();
    return 0;
}

I need to make it versatile so the user can enter any expression to replace pow(x, n) with whatever the user inputs from keyboard. How do I put an expression into the program?

Answer 1

C and C++ don't offer you this feature already implemented because there is a clear cut between the compile time (when expressions are analyzed syntactically) and run time (when they're evaluated).

Implementing a parser and compiler/evaluator for expression is a good exercise but it's not trivial and would probably require a lot more experience than you have at this moment (given the question you asked).

The easiest to understand approach to this problem is in my opinion a recursive descent parser but for just the four operations, numbers and variables I'd guess it would require a hundred or so lines of code.

yacc and bison are tools that were designed to generate automatically parsing/evaluation code from a definition of a grammar. I personally prefer to hand-write parsers because of the finer control you get (especially on error handling or when the syntactic and semantic level interact, but that's me and I'm also a bad case of NIH syndrome).

Higher level languages like Python or Javascript instead offer you this feature already implemented as eval.