Program to evaluate simple expressions - GeeksQuiz

Program to evaluate simple expressions - GeeksQuiz
You are given a string that represent an expression of digits and operands. E.g. 1+2*3, 1-2+4. You need to evaluate the string or the expression. NO BODMAS is followed. If the expression is of incorrect syntax return -1.
Test cases:
a) 1+2*3 will be evaluated to 9.
b) 4-2+6*3 will be evaluated to 24.
c) 1++2 will be evaluated to -1(INVALID).
Also, in the string spaces can occur. For that case we need to ignore the spaces. Like :- 1*2 -1 is equals to 1

int evaluate(char *exp)

{

    // Base Case: Given expression is empty

    if (*exp == '\0')  return -1;

 

    // The first character must be an operand, find its value

    int res = value(exp[0]);

 

    // Traverse the remaining characters in pairs

    for (int i = 1; exp[i]; i += 2)

    {

        // The next character must be an operator, and

        // next to next an operand

        char opr = exp[i], opd = exp[i+1];

 

        // If next to next character is not an operand

        if (!isOperand(opd))  return -1;

 

        // Update result according to the operator

        if (opr == '+')       res += value(opd);

        else if (opr == '-')  res -= value(opd);

        else if (opr == '*')  res *= value(opd);

        else if (opr == '/')  res /= value(opd);

 

        // If not a valid operator

        else                  return -1;

    }

    return res;

}

The above code doesn’t handle spaces. We can handle spaces by first removing all spaces from the given string. A better solution is to handle spaces in single traversa
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