This is a calculator I wrote for a student's project.

It still has a problem (one that I know of), where it can't handle inputs of the form 2*2*2... (more than one * or / sign). The case 2*2 was solved in hackish ways.

I'd appreciate any remarks or advice on the code and possibly how to solve that problem.

Relevant inputs are using +, -, *, /, (, ), etc.

EDIT: I fixed the problems listed above.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void getInput(char * in) {
    printf("> ");
    fgets(in, 256, stdin);
}

int isLeftParantheses(char p) {
    if (p == '(') return 1;
    else return 0;
}

int isRightParantheses(char p) {
    if (p == ')') return 1;
    else return 0;
}

int isOperator(char p) {
    if (p == '+' || p == '-' || p == '*' || p == '/') return p;
    else return 0;
}

int performOperator(int a, int b, char p) {
    switch(p) {
        case '+': return a+b;
        case '-': return a-b;
        case '*': return a*b;
        case '/':
            if (b == 0) { printf("Can't divide by 0, aborting...\n"); exit(1); } 
            return a/b;
    }
    return 0;
}


char isDigit(char p) {
    if (p >= '0' && p <= '9') return 1;
    else return 0;
}

int charToDigit(char p) {
    if (p >= '0' && p <= '9') return p - '0';
    else return 0;
}

int isNumber(char * p) {
    while(*p) {
        if (!isDigit(*p)) return 0;
        p++;
    }
    return 1;
}

int len(char * p) {
    return int(strlen(p));
}

int numOfOperands(char * p) {
    int total = 0;
    while(*p) {
        if (isOperator(*p)) total++;
        p++;
    }
    return total+1;
}

int isMDGRoup(char * p) {

    while(*p) {
        if (!isDigit(*p) && *p != '/' && *p != '*')
            return 0;
        p++;
    }
    return 1;
}

int getLeftOperand(char * p, char * l) {
    // Grab the left operand in p, put it in l,
    //and return the index where it ends.
    int i = 0;

    // Operand is part of multi-*/ group
    if (isMDGRoup(p)) {
        while(1) {
            if (*p == '*' || *p == '/') break;
            l[i++] = *p++;
        }
        return i;
    }

    // Operand is in parantheses
    if(isLeftParantheses(*p)) {
        int LeftParantheses = 1;
        int RightParantheses= 0;
        p++;
        while(1) {
            if (isLeftParantheses(*p))  LeftParantheses++;
            if (isRightParantheses(*p)) RightParantheses++;

            if (isRightParantheses(*p) && LeftParantheses == RightParantheses)
                break;
            l[i++] = *p++;
        }
        // while (!isRightParantheses(*p)) {
        //  l[i++] = *p++;
        // }
        l[i] = '\0';
        return i+2;
    }

    // Operand is a number
    while (1) {
        if (!isDigit(*p)) break;
        l[i++] = *p++;
    }
    l[i] = '\0';
    return i;
}

int getOperator(char * p, int index, char * op) {
    *op = p[index];
    return index + 1;
}

int getRightOperand(char * p, char * l) {
    // Grab the left operand in p, put it in l,
    //and return the index where it ends.
    while(*p && (isDigit(*p) || isOperator(*p) ||
                 isLeftParantheses(*p) || isRightParantheses(*p))) {
        *l++ = *p++;
    }
    *l = '\0';

    return 0;
}

int isEmpty(char * p) {
    // Check if string/char is empty
    if (len(p) == 0) return 1;
    else return 0;
}

int calcExpression(char * p) {
    // if p = #: return atoi(p)
    //
    // else:
    //  L = P.LeftSide
    //  O = P.Op
    //  R = P.RightSide
    //  return PerformOp(calcExpression(L), calcExpression(R), O)

    // ACTUAL FUNCTION

    // if p is a number, return it
    if (isNumber(p)) return atoi(p);

    // Get Left, Right and Op from p.
    char leftOperand[256] = ""; char rightOperand[256]= "";
    char op;

    int leftOpIndex   = getLeftOperand(p, leftOperand);
    int operatorIndex = getOperator(p, leftOpIndex, &op);
    int rightOpIndex  = getRightOperand(p+operatorIndex, rightOperand);

    printf("%s, %c, %s", leftOperand, op, rightOperand);
    getchar();

    if (isEmpty(rightOperand)) return calcExpression(leftOperand);

    return performOperator(
        calcExpression(leftOperand),
        calcExpression(rightOperand),
        op
    );
}

int main()
{
    char in[256];
    while(1) {
        // Read input from user
        getInput(in);
        if (strncmp(in, "quit", 4) == 0) break;

        // Perform calculations
        int result = calcExpression(in);
        printf("%d\n", result);
    }
}