Simple Objective-C calculator program printing continuously - objective-c

Question

I'm writing my first real objective C program and it's to make a very simple calculator like in the book Programming in Objective-C 2.0 by Stephen Kochan.
Anyway, whenever I run the program it just continually prints the same thing over and over again, not giving me the option to type anything else. The code is below, and if anyone could help I think the problem is somewhere between the while loop and the switch function. Thank you in advance!
#import <Foundation/Foundation.h>
#interface Calculator : NSObject {
double number, accumulator;
char operator;
}
-(void) add: (double) n;
-(void) subtract: (double) n;
-(void) multiply: (double) n;
-(void) divide: (double) n;
#end
#implementation Calculator
-(void) add: (double) n {
accumulator += n;
NSLog(#"%fl", accumulator);
}
-(void) subtract: (double) n {
accumulator -= n;
NSLog(#"%fl", accumulator);
}
-(void) multiply: (double) n {
accumulator *= n;
NSLog(#"%fl", accumulator);
}
-(void) divide: (double) n {
if (n == 0)
NSLog(#"Error! You can't divide by 0!");
else
accumulator /= n;
NSLog(#"%fl", accumulator);
}
#end
int main(int argc, const char * argv[])
{
#autoreleasepool {
double number, accumulator;
char operator;
Calculator *myCalc = [[Calculator alloc] init];
NSLog(#"Begin calculations by typing a number then S");
scanf("%lf, %c", &accumulator, &operator);
while (operator != 'E') {
NSLog(#"%lf", accumulator);
NSLog(#"What would you like to do next?");
scanf("%lf, %c", &number, &operator);
switch (operator) {
case '+':
[myCalc add: number];
break;
case '-':
[myCalc subtract: number];
break;
case '*':
[myCalc multiply: number];
break;
case '/':
[myCalc divide: number];
break;
default:
break;
}
}
}
return 0;
}

Answer 1

scanf is generally a bad function to use. It's generally better to read an input line into a string and then use sscanf (or some other parser) on the string.
However, the fix in this case is simple. scanf returns the number of input items successfully assigned. You expect two. If there's an error, or end-of-file is reached, it will return less than two. Thus:
int rc = scanf("%lf, %c", &number, &operator);
if (rc < 2) {
break;
}

Answer 2

In short: don't use scanf(). It doesn't work how you think it does.
I'm already tried explaining what's wrong, but basically it doesn't like newlines and stuff and it's pedantic. Search SO for similar questions. The easy solution is to replace scanf() with something actually useful, such as
char buf[0x100];
char *end;
fgets(buf, sizeof buf, stdin);
accumulator = strtod(buf, &end);
while (isspace(*end))
end++;
operator = *end;
Also, your calculator logic is flawed. The myCalc object doesn't share the identically named accumulator variable with the main() function. Your program basically doesn't take into account the first number entered. Also, I don't see what purpose the "type 'S'" part serves, there's absolutely no check for entering "S" in the code, only "E" for end.
On a side note: we're in C (basically), but it's still not a good idea to use C++ keywords as identifiers. Let new and operator be reserved. Call that variable op.
Also, as a design improvement, you can abstract away the big switch statement into the calculator class, and that way you could write something like [myCalc performOp:'+' withNumber:number]; etc.