How to choose a method based on an element of an NSArray (Objective-C) - objective-c

Question

I'm writing a sort of calculator app. I have a UIPickerView (1 column) loading data from an NSArray of strings. The user will select one of these (it's selecting which type of calculator to use -- each uses a different method to calculate). The user inputs some things into some UITextFields and then presses a UIButton to do the calculations.
My NSArray is this:
calcNames = [NSArray alloc] initWithObjects:#"first", #"second", #"third", nil];
And my methods are called firstCalc(input1, input2, input3), secondCalc(input1, input2, input3), and so on. (The inputs are coming from the UITextFields.)
When I press the button, I would like to tell it to look at what the selection in the UIPickerView is and run the corresponding method without just typing an if-then statement for each one (it's very inconvenient to do this for reasons specific to my app, which are beyond the scope of this discussion).
So I have already defined a way to determine what the selected calc is:
selectedCalc = [[NSString alloc] initWithString:[calcNames objectAtIndex:row]]
where 'row' is the current selection in the UIPickerView.
Now I have a doCalculations method for when someone presses the UIButton:
-(IBAction)doCalculations:(id)sender {
// save the data input
double input1 = [input1Field.text doubleValue];
double input2 = [input2Field.text doubleValue];
double input3 = [input3Field.text doubleValue];
// do the calculations
int i;
for (i = 0; i < [calcNames count]; i++) {
if (selectedCalc == [calcNames objectAtIndex:i]) {
// do calculations here
double numResult = ??????
// if selectedCalc is "first", I want it to do firstCalc(input 1, input 2, input 3)
// if selectedCalc is "second", I want it to do secondCalc(input 1, input 2, input 3), and so on
// the rest is just for displaying the result
NSString* result = [NSString stringWithFormat:#"The answer is %f", numResult];
[resultLabel setText:result];
}
}
}
So basically, it runs a for loop until it finds which calculator is selected from the UIPickerView and when it finds it, runs the calculations and displays them.
I've been trying to understand if maybe function pointers or selectors (NSSelectorFromString?) are the right things to use here and how to use them, but I'm really struggling to understand where to go after a couple days of reading Apple's documentation, Stack Overflow questions, playing with sample code, and tinkering with my own code.
Sorry if the question is too lengthy, I thought it may be more helpful to others looking for assistance in the future to see the full idea. (At least I know sometimes I'm lost with these question pages.)
I would be very grateful for any assistance,
Ryan

Answer 1

You can dynamically invoke a method using a selector. You could for example have a secondary array to calcNames with selector called calcSelectors:
SEL calcSelectors[] = (SEL[3]){
#selector(first:arg:),
#selector(second:arg:),
#selector(third:arg:)};
Calling the right method would then be as simple as:
[self performSelector:calcSelectors[calcIndex] withObject:arg1 withObject:arg2];
If you need more then 2 arguments, then you also need to mess a bit with a NSInvocation instance to setup the call.

Answer 2

Example 1:
NSString *method=[calcNames objectAtIndex:0];//here play with objectatindex
SEL s=NSSelectorFromString(method);
[self performSelector:s];
which will call this method
-(void)first{
NSLog(#"first");
}
-----------------------------------------
Example 2:
NSString *totalMethodName;
totalMethodName=#"vijay";
totalMethodName=[totalMethodName stringByAppendingString:#"With"];
totalMethodName=[totalMethodName stringByAppendingString:#"Apple"];
SEL s=NSSelectorFromString(totalMethodName);
[self performSelector:s];
will call
-(void)vijayWithApple{
NSLog(#"vijayWithApple called");
}

Answer 3

You can make use of NSInvocation to dynamically bind multiple arguments to a selector. Follow this post to learn it.
If you are going to use NSInvocation you have to define your methods in the objective-C way something like the following.
- (double)firstCalcWithInput1:(double)input1 input2:(double)input2 andInput3:(double)input3;
- (double)secondCalcWithInput1:(double)input1 input2:(double)input2 andInput3:(double)input3;