I think you will want to investigate Reverse Polish Notation as it is applied to calculators. google has lots of information about it.

Sorry about lack of coding in previous post.
here we go :
Comments are included in the code.
Main points are the calculations for squareroot and reciprocal.
also the error check for alphabetic characters input in the first 22 strings.


// Calculator

//main things that need to be checked/corrected
scan 1st two strings for anything other than '-' and numbers
if so then give ERROR1
//check calculations for squareroot and reciprocal
//otherwise i think most stuff seems to work ok.


#include <stdlib.h>
#include <stdio.h>
//#include <math.h>
#include <conio.h>
#include <string.h>
//#include <ctype.h>
//#define RECIPROCAL (1/Num1)
//#define SQUAREROOT Num1^0.5
#define ISDIGIT(x) ((x) >= '0' && (x) <='9')
#define ERROR5 "Input strings too long. Applies to numbers and operators."
#define ERROR4 "Operation not possible."
#define ERROR3 "Division by zero."
#define ERROR2 "Unknown operation."
#define ERROR1 "Number contains alphabetic characters."
#define MAX 11
#define RECIPROCAL 1/x
#define SQUAREROOT
void main()
{
int Add, Subtract, Multiply;
float Divide = 0.0;
double Squareroot;
double Reciprocal;
int Num1;
int Num2;
char Num2atoi;
char Str2[MAX];
char Str1[MAX];
char Operator;
printf("\nPlease enter the required calculation\n");

scanf("%s %s", &Str1, &Str2);
//checks on num1 for numbers or - at the start only - 10 digits max
// if ((Num1)
// error check #5 - test string length is max10
{
if (strlen(Str1) > MAX-1)
printf(ERROR5);
else if
(strlen(Str2) > MAX-1)
printf(ERROR5);
else if
((*Str2) == 'S')
//what is the equation for this??????
printf("this is Squareroot \n" );
//squareroot=Num1^0.5;
// printf("=%lf ", Num1^y);

else if
((*Str2) == 'R')
//n1=(float) Num1;
printf("=Recip\n" );
//error1 - test for alphabetic characters
//check cos this doesnt work.
else if
ISDIGIT(*Str1)
//(*Str1<'0' || *Str1>'0' || *Str1=='-')
// Num1= atoi(Str1);
// Num2= atoi(Str2);
printf("its a digit");
else
printf("its not a valid number");
{
scanf(" %s", &Operator);
Num1= atoi(Str1);
Num2= atoi(Str2);
switch(Operator)
{
case '+':
printf("= %d", Num1+Num2);
break;
case '-':
printf("= %d \n", Num1-Num2);
break;
case '*':
printf("=%d ", Num1*Num2);
break;
case '/':
if(*Str2 == '0')
{
printf(ERROR3);
break;
}
else
//Num1= atof(Str1);
{ //Num2= atof(Str1);
//does this need to change for decimal. Divide=Num1/Num2;
printf("=%f", Divide);
break;
}
default :
printf(ERROR2);
}

}
getch();
}}

Use code tags and read this