Calculator (just entering numbers) class for Wells CS132

Calculator.h

#import <Foundation/Foundation.h>

FOUNDATION_EXPORT const char CLEAR_SCREEN_KEY; //The FOUNDATION_EXPORT part makes this truly global - available to all other files.

@interface Calculator : NSObject

@property (assign) int numberOnScreen;

-(void) clearScreen;

-(void) pressKey:(char) input;

@end

Calculator.m

#import "Calculator.h"

const char CLEAR_SCREEN_KEY = 'x';

@implementation Calculator

@synthesize numberOnScreen;

- (id)init
{
    self = [super init];
    if (self) {
        [self clearScreen];
    }
    return self;
}

-(NSString*) description
{
    return [NSString stringWithFormat:@"Calculator with %d on screen.", [self numberOnScreen]];
}

-(void) clearScreen
{
    NSLog(@"Clearing screen of <%@> at memory location [%p] upon receiving message {%@}",
          self, self, NSStringFromSelector(_cmd));
    
    [self setNumberOnScreen:0];
}

-(void) pressKey:(char)input
{
    int oldNumberOnScreen;
    int newNumberOnScreen;
    int numberEntered;
    
    if(input == CLEAR_SCREEN_KEY)
    {   //Clear the screen
        [self clearScreen];
    }
    else
    {   //Must be a digit
        
        //First, convert user input character into a number
        
        if(input < '0' || input > '9')
            numberEntered=NAN; //Not a digit!
        else
            numberEntered = (input - '0'); //Tricksy conversion
        
        //Second, update number on screen
        
        oldNumberOnScreen = [self numberOnScreen];
        newNumberOnScreen = 10*oldNumberOnScreen + numberEntered;
        [self setNumberOnScreen:newNumberOnScreen];
    }
}
@end

main.m

#import <Foundation/Foundation.h>
#import "Calculator.h"

int main(int argc, const char * argv[])
{

    @autoreleasepool {
        char userInput;
        Calculator* casioCFX9850g = nil;
        casioCFX9850g = [[Calculator alloc] init];
        

        while(YES)
        {
            printf("Please press a key:");
            scanf(" %c", &userInput);

            [casioCFX9850g pressKey:userInput];
            
            printf("You pressed %c. The number on screen is now %d\n",
                   userInput, [casioCFX9850g numberOnScreen]);
        }
    
        
    }
    return 0;
}