C++ calculator with modification.

#include <ctype.h>
#include <string.h>
#include <conio.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

// Defining the constant values for error
#define ERROR "Error"

//Check the error if number contains alphabetic characters
#define ERROR1 "ERROR Number 1"

//Check the ERROR for unknown operation
#define EROR2 "ERROR Number 2"

//ERROR for division by Zero
#define EROR3 "ERROR Number 3"

//ERROR if operation not possible
#define EROR4 "ERROR Number 4"

//ERROR if input strings are longer than limit
#define EROR5 "ERROR Number 5"

//Defining prototypes for functions
int DATA_RECALL(int *num_1, int *num_2, char *operation);
void CALCULATE(int num1, int num2,  char operator_);
int VALID_CHECK_1 (char input[11], int *output);
int VALID_CHECK_2(char input[11], int *output, char*valid_operator);
int ERROR_CHECK (int retval);
int OPERATOR_VERIFICATION(char optor_in, char *optor_out);
 
void main ()
{
  printf("Please enter the values for required calculation :\n");
  char string[5];
  int num_1 = 0, num_2 = 0;
  while(strcmp(string,"QUIT"))
  {
        flushall();
        char operation = ' ';
        if (!ERROR_CHECK( DATA_RECALL(&num_1, &num_2, &operation)))
          CALCULATE (num_1, num_2, operation);
          printf("\n-----------------------------\n");
          printf("Type 'QUIT' to exit.\n");
          printf("Press any key to continue\n");
          printf("And press Enter key: ");
          scanf("%s",string);
          printf("-----------------------------\n");
          printf("\nPlease enter the values for required calculation :\n");
          puts("");

  }
}

int DATA_RECALL(int *num_1, int *num_2, char *operation)
{
    int value= 0;
    int counter = 0;
    char buffer[48], overflow[24]="", tmpNum2[12]="\0", tmp_operator = ' ', tmpNum1[12]= "\0";
    
    gets (buffer);
    sscanf(buffer, "%11s", tmpNum1);
    sscanf(buffer, "%*s %11s", tmpNum2);
    sscanf(buffer, "%*s %*s %c", &tmp_operator );
    sscanf(buffer, "%*s %*s %*c %20s", overflow);
    
    if ( tmpNum1!= "\0")
        {
              counter = 1;
              if ( tmpNum2!= "\0")
              {
                counter = 2;
                if ( tmp_operator!= ' ')
                  counter = 3;
              }
        }
    value =  VALID_CHECK_1(tmpNum1, num_1);
    
    if (value == 0)
    value = VALID_CHECK_2(tmpNum2, num_2, operation);
    
    if ((*operation == ' ') && (value == 0))
    {
        value = OPERATOR_VERIFICATION(tmp_operator, operation);
    }
    
    if ((value == 0) && (strlen(overflow) > 0))
        value = 5;
    return value;
}

void CALCULATE(int num1, int num2,  char optr)
{
    float a,b,c=0.00001,d,e;
    e=num1;
    float srt;
    
    switch (optr)
    {
        case '+':    /*step for the Addition */
         printf("%d\n", num1 + num2);
         break;
        case '-':    /*step for the Subtraction*/
         printf("%d\n", num1 - num2);
         break;
        case '*':  /*step for the Multiplication*/
         printf("%d\n", num1 * num2);
         break;
        case '/':  /*step for the Divison*/
         if (num2 == 0)
         {
            puts(EROR3);
            break;
         }
         else
           {
            printf("%f\n",(float)num1 / num2);
            break;
         }
        case 'S':  /* step for the Square Root*/
         if (num1 < 0)
         {
            puts(EROR4);
            break;
         }
         else
         {
            a=e;d=a*a;
            while(d-e>=c)
              {
                b=(a+(e/a))/2;
                a=b;
                  d=a*a;
                srt=a;
            }
                  printf("%f\n",srt);
           break;
                }
        case 'R':/*Calculatting the reciprocal*/
         if (num1 == 0)
         {
            puts(EROR3);
            break;
         }
         else
         {
            printf("%f\n",1/(float) num1);
            break;
         }
    }
}

int VALID_CHECK_1 (char input[20], int *output)
{
      int length = 0;
      int ret_value = 0;
      length = strlen(input);
      
      if (length == 0)
      {
        ret_value = 2; /*Nothing was Entered*/
      }
      else if (length <= 10)
      {
        for (int i = 0; i < length; i++)
        {
            if ((length ==1)&& (input[0]=='-'))
            {
              ret_value = 1;
              break;
            }
            if (!(((i == 0) && (input[i] == '-')) || (isdigit(input[i]))))
            {
              ret_value = 1;  /*detects -sign and check if there are no alphabetic
                              characters. */
              break;
            }
        }
        if (ret_value == 0)
            *output = atoi(input);/*All valid,convert to 'int'*/
      }
      else
        ret_value = 5;/*more then 10 digits*/
        return ret_value;
}

int VALID_CHECK_2(char input[20], int *output, char *valid_operator)
{
  char *str = "RS", tmp_operator = *valid_operator; /*modification: assign *valid operator to tmp_operator */
  int length = 0;
  int ret_value = 0;
  
  /* Following check if:the numbers consist of  a maximum of 10 characters*/
  length = strlen(input);
  if (length == 0)
  {
      ret_value = 4;
  }
  else if(length == 1)
  {
      if (strrchr(str, input[0]))
      {
        *valid_operator = input[0];/*one digit and an operator*/
      }
      else if (isdigit(input[0]))
      {
        *output = atoi(input);/* If the input value is a number*/
      }
      else if ((input[0]== '+' )||(input[0]== '-')||(input[0]=='/')||(input[0]=='*'))
        ret_value = 4;
      else if ((tmp_operator == ' ')&&((input[0]!='R')||(input[0]!= 'S')))
        ret_value = 2;
      else
        ret_value = 1;
  }
  else if (length <=10)
  {
      for (int i=0; i < length; i++)
      {
        if ((tmp_operator =='R')||(tmp_operator =='S'))
        {
            ret_value = 4;
            break;
        }
        if (!(((i == 0) && (input[i] == '-')) || (isdigit(input[i]))))
        {
          ret_value = 1;
          break;
        }
      }
  }
  else
  ret_value = 5;
  if (ret_value ==0)
  {
    *output = atoi(input); //Converting the value to int if valid
  }
  return ret_value;
}

int OPERATOR_VERIFICATION(char operIn, char *valid_operator)
{
    char *str = "/*-+";
    int ret_value = 0;
    if (operIn =='\0') /*considering that return value isnt true*/
      ret_value = 4;
    else if (strrchr(str, operIn))
      *valid_operator = operIn;
    else
      ret_value = 2;
    return ret_value;
}
 

int root(int num1)
{
    return(sqrt((double)num1));
}


int ERROR_CHECK (int ERROR_value)
{
    switch (ERROR_value)
    {
      case 0:
        ERROR_value = 0;
        break;
      case 1:
        puts(ERROR1);
        break;
      case 2:
        puts(EROR2);
        break;
      case 3:
        puts(EROR3);
        break;
      case 4:
        puts(EROR4);
        break;
      case 5:
        puts(EROR5);
        break;
      default:
        puts(ERROR);
        break;
    }
    return ERROR_value;
}