calculator in C - input a whole formula

Question

I'm trying to code a calculator in C, and want to make one that can calculate a multiple inputs, eg, (5*9 + 1 -2). These inputs can be completely random and I'm stuck on how to do this.

I know how to initialize a variable, and ask the user input a number and all that, but if the user wanted to add up 50 random numbers, that calculator wouldn't be able to do that.

Hope you can help, or share some tips

Thanks !

Answer 1

You will need to implement an expression parser that will take operator precedence into account. To me the two simplest ways to do this would be to either implement a recursive decent parser or to implement the Shunting yard algorithm.

See this for an example.

Answer 2

In order to do this, you need to read the entire line (it shouldn't be too hard), then you need to parse it and store it into some data structures.

Here are two ways I know to store and use it :

• The first one : it's easy to do, easy to use, but not beautiful nor fast : A double linked-list with each link containing an operator or a number and a priority if it's an operator (you can use an enum + union if you want something cleaner):

  struct list {
      struct list *prev;
      struct list *next;
      char operator;
      int number;
      unsigned int priority;
  }
  

  You loop trough your string and apply an easy algorithm for priority (pseudocode):

  var priority = 0
  var array = cut string // ["5", "+", "3", "*", "(", "6", "-", "2", ")"]
  
  check string // verify the string is correct
  
  for each element of array :
      if (element is number)
          store in list
  
      else if (element is operator)
          store in list
          if (element is '*' or '/' or '%')
              set his priority to priority + 1
          else
              set his priority to priority
  
      else if (element is open parenthesis)
          priority += 2
  
      else if (element is close parenthesis)
          priority -= 2
  

  For example :

  string:

  5 + 3 * (6 - 2) - 1
  

  priorities:

    0   1    2    0
  

  Then, to do your calculations :

  while list isn't empty:
      find operator with the highest priority // if there is more than one, take the first
      calculate this operator // with the element prev and next
      replace by result in list // operator, but also prev and next
  

  An example, again, with 5 + 3 * (6 - 2) - 1 :

  first iteration:

  5 + 3 * 4 - 1
  

  then

  5 + 12 - 1
  

  then

  17 - 1       
  

  then

  16
  

• The other (and better, even though it is a little bit harder if you're not familiar with recursion) one : A binary tree using the reverse polish notation (see here and here)

  This one is more common, so I won't explain it.

Answer 3

You can use a string when you read and divide that string when you find {+,-,*,/}. And what you find between them are your numbers.

Try to add your code!

Good luck!

Answer 4

It's not a complete solution but hey it's something... try this code:

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

#define MAX_SIZE (1000)

static char tempNumStr[50] = {0};
static double parsedDoubleArr[5] = {0};
static char parsedOperators[5] = {0};

void flushTempArr(char tmpArr[50]){
    for(int i=0; i< 50; i++)
        tmpArr[i]  = 0 ;
}

int tempNumStrIterator=0;

int parsedDoubleArrIterator=0;
int parsedOperatorsIterator=0;


int main(void)
{


    char operator;
    char sourceStr[] = "(17.5 + 8)";

    for(int i = 0; sourceStr[i] != '\0'; ++i) /*iterate over string till \0 */
    {
        if (IsDigit(sourceStr[i]))
        {
            while(sourceStr[i] != '\0' && IsDigit(sourceStr[i]))
            {
                tempNumStr[tempNumStrIterator++] = sourceStr[i];
                ++i;
            }

            sscanf(tempNumStr, "%lf", &parsedDoubleArr[parsedDoubleArrIterator++]);
            flushTempArr(tempNumStr);
            tempNumStrIterator = 0;
        }

        if (IsCalcOperator(sourceStr[i]))
        {
            parsedOperators[parsedOperatorsIterator++] = sourceStr[i];
        }

        else if (IsBracket(sourceStr[i]))
        {
            //do something
            continue;
        }
    }

    //do what you want with parsedDoubleArr and parsedOperators

    return EXIT_SUCCESS;
}