Trouble with NSString: using a controller to set an NSTextField? - objective-c

Question

I am using Xcode 4 writing a simple program to generate text based on user input. Im reading from two text fields successfully, but I am unable to send the result to a separate NSTextField. I'm confident I've made all the right connections in IB, and I'm sending the NSTextField the setStringValue method. I have created a ScaleGenerator object that I believe may be improperly creating the string itself. My Controller header looks like this:
#import <Cocoa/Cocoa.h>
#interface Controller : NSObject
{
IBOutlet NSTextField * notesField;
IBOutlet NSTextField * midiField;
IBOutlet NSTextField * resultField;
IBOutlet id scalegenerator;
}
- (IBAction) generate: (id) sender;
#end // Controller
The Controller implementation looks like this:
#import "Controller.h"
#import "ScaleGenerator.h"
#import <Foundation/foundation.h>
#implementation Controller
- (IBAction) generate: (id) sender
{
int midinote;
int notes;
NSString * scale;
midinote = [midiField intValue];
if(midinote < 0 || midinote > 127) {
NSRunAlertPanel(#"Bad MIDI", #"That MIDI note is out of range! (0 - 127)", #"OK", nil, nil);
return;
}
notes = [notesField intValue];
if(notes < 2 || notes > 24) {
NSRunAlertPanel(#"Bad Scale Size", #"You must have at least two notes in your scale, and no more than 25 notes!", #"OK", nil, nil);
return;
}
scale = [scalegenerator generateScale: midinote and: notes];
[resultField setStringValue:scale];
}
#end
My ScaleGenerator code looks like this:
#import "ScaleGenerator.h"
#import <math.h>
#implementation ScaleGenerator
- (NSMutableString *) generateScale: (int) midinote and: (int) numberOfNotes
{
double frequency, ratio;
double c0, c5;
double intervals[24];
int i;
NSMutableString * result;
/* calculate standard semitone ratio in order to map the midinotes */
ratio = pow(2.0, 1/12.0); // Frequency Mulitplier for one half-step
c5 = 220.0 * pow(ratio, 3); // Middle C is three semitones above A220
c0 = c5 * pow(0.5, 5); // Lowest MIDI note is 5 octaves below middle C
frequency = c0 * pow(ratio, midinote); // the first frequency is based off of my midinote
/* calculate ratio from notes and fill the frequency array */
ratio = pow(2.0, 1/numberOfNotes);
for(i = 0; i < numberOfNotes; i++) {
intervals[i] = frequency;
frequency *= ratio;
}
for(i = 0; i < n; i++){
[result appendFormat: #"#%d: %f", numberOfNotes + 1, intervals[i]];
}
return (result);
}
#end // ScaleGenerator
My ScaleGenerator object has one function, which is where I believe I may be messing things up. What kind of string can I iteratively append formatted text to?? And what method do I call??

Answer 1

You have not allocated/initialized your NSMutableString. Therefore the message appendFormat: goes to nil. Replace the line
NSMutableString * result;
with
NSMutableString * result = [[NSMutableString alloc] init];