I'm trying to write a calculator code to evaluate addition, subtraction, and squares. For example, the expression "1000 6^ - 5^ 1;" should evaluate to 1012. For some reason my code gives me the wrong answer :( It evaluates to 63. Where does the issue occur in my code?

#include <iostream>
using namespace std;

int main()
{
    int answer = 0, num;
    char sign;
    cin >> num; 

    while (cin >> sign)
    {
        if (sign == '^')
        {
            answer  = num * num;
            cin >> sign >> num;
            
            if (sign == ' ')
            {
                answer  = num;
            }
            else if (sign == '-')
            {
                answer -= num;
            }
        }
        else 
        {
            if (sign == ' ')
            {
                cin >> num;
                answer  = num;
            }
            else if (sign == '-')
            {
                cin >> num;
                answer -= num;
            }
        }
    }

    cout << answer << endl;
    
    return 0;
}

CodePudding user response：

Let's take a step-by-step approach. Taking the input as 1000 6^ - 5^ 1;, which is actually 1000<spacebar> <spacebar>6^<spacebar>-<spacebar>5^<spacebar> <spacebar>1;

1. Firstly, after you take 1000 as num, <spacebar> follow as a sign. As <spacebar> does not match any of the sign, this 1000 is discarded because it does not hit any of the if case.

2. Then, is took as a sign, making <spacebar>6 -> 6 as num, so num becomes the 6. Then, ^ is encountered. That adds 36 to the answer, making the answer to 42.

3. Then again <spacebar> is encountered, making the answer untouched. Then, - sign is encountered. reading <spacebar>5 -> 5 as num, it will subtract 5 from 42, making the answer 37.

4. Next, ^ will be encountered, adding 25 to the answer, the answer will be 62.

5. Then, <spacebar> is again encountered as a sign, having no effect to the answer.

6. Lastly, will be encountered, adding <spacebar>1 -> 1 to the answer, Therefore, the final answer will be 63 for this expression. Additional ; will be treated as a sign with no matches, therefore will not affect the final answer.

As you see, because of the <spacebar> and the fact that cin >> <char> only takes exactly 1 character as the input, you get unexpected result.

Remember: Computer exactly does what you programmed, not what you actually intended.

If you want to parse the mathematical expression correctly, study about

Shunting-yard algorithm, https://en.wikipedia.org/wiki/Shunting-yard_algorithm

And Postfix notation. https://en.wikipedia.org/wiki/Reverse_Polish_notation

Those two algorithms are extensively used when the real-world calculator is implemented.

CodePudding user response：

I am not an expert but this type of problem (calculator) requires usage of stack, because there are many things to consider like operator precedence. For example, if there is a bracket then expression within brackets should be solved first. So, I advise to look in this further as there will be many good article explaining this.

CodePudding user response：

your current structure is wrong! and u need to add fist number to answer at beginning

try this struct:

|- while loop
|- take sign & num
         |
         check operator ^ is present after that
                     |
                      if present -> answer (sign)= num * num
                      |  
                      else       -> answer (sign)= num