Main problem i am having is that with the program it can have a 2 string equation - eg 25 R or 25 S (Reciprocal and Squareroot)
or a 3 string equation where the second string will be a number and the third string will be the operator. eg 10 5 + or -10 20 -

I am not sure how to get this to work - i can get the coding to work if its just the 2 string equation but then if it is a 3 string equation i come unstuck.

Can anyone provide some assistance please.

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
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)
else if
(strlen(Str2) > MAX-1)
else if
((*Str2) == 'S')
//what is the equation for this??????
printf("this is Squareroot \n" );
// 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
//(*Str1<'0' || *Str1>'0' || *Str1=='-')
//              Num1= atoi(Str1);
//              Num2= atoi(Str2);
printf("its a digit");
printf("its not a valid number");
scanf(" %s", &Operator);
Num1= atoi(Str1);
Num2= atoi(Str2);
case '+':
printf("= %d", Num1+Num2);
case '-':
printf("= %d \n", Num1-Num2);
case '*':
printf("=%d ", Num1*Num2);
case '/':
if(*Str2 == '0')
//Num1= atof(Str1);
{        //Num2= atof(Str1);
//does this need to change for decimal.           Divide=Num1/Num2;
printf("=%f", Divide);
default :