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I believe the struct member stuList somehow doesn’t exist. I can’t figure out why not, and it is likely a simple thing but I am completely frustrated tying to make it go.

typedef struct {                                             // Struct to hold a class
	int classID;
	NSString *classSec, *className;
	NSMutableArray *stuList, *stuData;       // The problem variable.  Supposed to grow using addObject to hold strings
} Classes;

int i, j, perNum;
NSString *line, *str;
NSArray *parts;

Classes classList[7];                                      // Array of 7 Class structures

    NSOpenPanel *openPanel = [NSOpenPanel openPanel];

for (i = 0; i < 7; i++) {
	classList[i].classID = 0;

    if ([openPanel runModalForTypes: [NSArray arrayWithObjects: @"grd",@"GRD",NULL]]) {
	NSString *theWholeEnchilada = [NSString stringWithContentsOfFile: [openPanel filename]]; 
	NSScanner *myScanner = [NSScanner scannerWithString: theWholeEnchilada], *myOtherScanner;
	while ([myScanner scanUpToString: @"\n" intoString: &line ]){ 
		parts = [line componentsSeparatedByString: @"\t"];
		if ( parts objectAtIndex: 0] isEqualToString: @"N300"] ) {
			// 3000 = first TA course number
			// 2000 = first Special Ed course number
			if ( [[parts objectAtIndex: 2] intValue] > 2000 || [[parts objectAtIndex: 2] intValue] < 3000 ) {
				classList[ [[parts objectAtIndex: 4] intValue] ].classID = [[parts objectAtIndex: 2] intValue];
				classList[ [[parts objectAtIndex: 4] intValue] ].classSec = [parts objectAtIndex: 3];
				myOtherScanner = [[[NSScanner scannerWithString: [parts objectAtIndex: 5]];
				[myOtherScanner scanUpToString: @" -" intoString: &str];
				str = [str stringByAppendingString: @")"];
				if ( [str characterAtIndex: 0] == '(' ) str = [str substringFromIndex: 3];
		if ( parts objectAtIndex: 0] isEqualToString: @"N320"] ) {
			if ( [ [parts objectAtIndex: 2] intValue ] > 2000 || [ [parts objectAtIndex: 2] intValue ] < 3000 ) {
				for ( i = 5 ; i < [parts count] ; i++ ) {
					perNum = [ [parts objectAtIndex: 4] intValue ];
					[ classList[perNum].stuList addObject: [ parts objectAtIndex: i] ];         // This should add an element, right?
					j = [ classList[perNum].stuList count ];                                                 // But j gets 0 here -- what gives?

*There’s no evidence anywhere in your code that you’ve created an [[NSMutableArray for stuList to point to. Do you have, elsewhere in your code, something resembling classList[perNum].stuList=[[NSMutableArray alloc] init]? If not, you may be finding that j==0 because you’re calling a method on a nil object (the result of a method call to a nil object is nil by definition in ObjectiveC). *I also have a…philosophico-stylistic question for you. Why are you using C structs to organise your data when you could be using Cocoa’s wonderful modelling classes everywhere? Why not replace your Class struct with an NSDictionary or a custom class?

Yes, you’re never alloc/initting your mutable array.

If you’re hellbent on using structs, instead of something NSObject-derived ( which I think is your best option ) you should write a constructor function, something like:

void Classes_init( Classses *instance ) { instance->classID = 0; instance->stuList = [[NSMutableArray alloc] init]; instance->stuData = [[NSMutableArray alloc] init]; }

void Classes_destroy( Classes *instance ) { [instance->stuList release]; [instance->stuData release]; }

//and then in your code // the const int might only work in C++…

const int nClasses = 7; Classes classList[ nClasses ];

NSOpenPanel *openPanel = [NSOpenPanel openPanel];

for (i = 0; i < nClasses; i++) { Classes_init( &classList[i]); }

This is basically the kind of thing C programmers do when they need to do something object-oriented. Of course, they’ve got APIs like GObject to streamline it, but why bother when you could just have an NSObject version?

@interface ClassInfo : NSObject { int classID; NSString *classSec, *className; NSMutableArray *stuList, *stuData;

+(ClassInfo*) classInfoWithClassID: (int) classID;

… and so on


@implementation ClassInfo

+(ClassInfo*) classInfoWithClassID: (int) classID { ClassInfo *ci = [[[ClassInfo alloc] init] autorelease]; ci->classID = classID; }

… and so on


Sure, it’s a little more code. But think of it this way: your program/data is obviously modeled around this Classes struct. It an be construed then that it is important. Therefore, you might as well spend a little time wrapping it in a class with a concise public interface, so that you can run test-units on the Classes object itself and be reasonably assured of its robustness. If you take the struct approach you’ll open yourself up to a lot of room for errors, since every time you assign a string or array you’ll have to explicitly release/retain – which means that if you assign in ten different places in your program, you’ve got ten opportunities to screw up.

Let me put it this way: there’s no reason not to use structs in Objective-C. But, my rule is this: any structure which requires some sort of memory management, be it a malloc-ed array or NSStrings or NSMutableDictionary or whatnot, should be refactored into a class, with memory management handled behind the public interface.


Having coded in C that was what I fell back on, but I can see that objC has some more nifty features that I need to learn about. Thank you very much!

– Charlie

Or, if you are comfortable with C++, you can fall back on that as well using Objective-C++. It is what I use for almost all of my internal code and it allows me to avoid “heap based” objects when I really want silly-fast stack bound objects.