macOS How can I analyze specific letters of a string entered in a NSTextField

[anonymous3025]

I wrote a non GUI simple calculator app in C. Here is the code.

#include <stdio.h>
main()
{
	float num1;
	float num2;
	char oper;
	float ans;
	printf("Enter an equation\n");
	scanf("%f%c%f\n" , &num1, oper, &num2);
	switch (oper)
	{ case ('+') :		//Case for addition
		ans=num1+num2;
		printf("The answer is %f\n" , ans);
	
	
	 case ('-') :		//Case for subtraction
		ans=num1-num2;
		printf("The answer is %f\n" , ans);
	
	
	 case ('*') :		//Case for multiplication
		ans=num1*num2;
		printf("The answer is %f\n" , ans);
	
	
	 case ('/') :		//Case for division
		ans=num1/num2;
		printf("The answer is %f\n" , ans);
		
		break;
		default :
		printf("\aThat is not an allowed operation\n");
	}
}

How would I be able to analyze certain characters in an NSTextField so I could make this a GUi in Cocoa with an NSTextField where the equation is entered and another NSTextField where the answer is displayed. Any help will be much appreciated.