Question

I would like to ask how could I convert a string which inserted by user to a Math equation so xCode would understand.

For example if user enter this to a text field. “2*sin(2*M_PI*t)” which the “t” inside the equation is a variable inside the code and Xcode would able understand the equation like

double answer = 2*sin(2*M_PI*t);

Many Thanks
Kin

Answer 1

You can do it with just the standard cocoa classes, but the syntax gets somewhat ridiculous, and the amount of set-up is staggering too.

@interface MyExprTest : NSObject
-(void)run;
@end

@implementation MyExprTest

-(void)run {
    NSExpression *e = [(NSComparisonPredicate *)[NSPredicate
        predicateWithFormat:@"2*FUNCTION(2*PI*$t,'sin')==0"]
        leftExpression];
    NSMutableDictionary *vars = [NSMutableDictionary
                             dictionaryWithObjectsAndKeys:
                             [NSNumber numberWithDouble:0.04], @"t",
                             nil];
    NSLog(@"%@", [e expressionValueWithObject:self context:vars]);
}

-(double)PI {
    return M_PI;
}

@end

@interface NSNumber (Trigonometry)
- (NSNumber*)sin;
@end

@implementation NSNumber (Trigonometry)

- (NSNumber*) sin {
    double result = sin([self doubleValue]);
    return [NSNumber numberWithDouble:result];
}

@end

int main (int argc, const char * argv[]) {
    MyExprTest *t = [[MyExprTest alloc] init];
    [t run];
    return 0;
}

A few pointers to note:

• The t in your expression became $t so that you could reference it from the vars dictionary
• The M_PI became simply PI; note the added method to support it
• There are no built-in trigonometric functions; see how FUNCTION and a category is used to get around it.