Using a String representing the name of a variable to set the variable

Question

This is a basic example that I know can be simplified, but for testing sake, I would like to do this in such a way. I want to set a variable based on an appending string (the variables "cam0" and "pos1" are already declared in the class). The appending string would essentially be an index, and i would iterate through a loop to assign cameras (cam0, cam1..) to different positions (pos0, pos1..).

cam0 is defined as an UIImageView
pos1 is defined as a CGRect

This works for a NSString Variable named coverIndex:

NSString* text = [[NSString alloc] initWithString:@""];
NSLog(@"%@",(NSString *)[self performSelector:NSSelectorFromString([text stringByAppendingString:@"coverIndex"])]);

The correct string that I set for coverIndex was logged to the Console.

Now back to my UIImageView and CGRect. I want this to work.

NSString* camText = [[NSString alloc] initWithString:@"cam"];
NSString* posText = [[NSString alloc] initWithString:@"pos"];

[(UIImageView *)[self performSelector:NSSelectorFromString([camText stringByAppendingString:@"0"])] setFrame:(CGRect)[self performSelector:NSSelectorFromString([posText stringByAppendingString:@"1"])]];

My error is "Conversion to non-scalar type requested"

This is the only way I found to do this sort of thing (and get the NSLog to work), but I still believe there is an easier way.

Thank you so much for any help :)

What is the @property line for the ivars cam & pos in the header file. — Black Frog
– Black Frog, Commented Apr 15, 2011 at 18:02
Did you know that [[NSString alloc] initWithString:@"cam"] is exactly the same as @"cam" ? — hooleyhoop
– hooleyhoop, Commented Apr 16, 2011 at 8:57

Bartosz Ciechanowski · Accepted Answer · 2013-06-01 10:52:11Z

4

Use KVC, it's an amazing piece of technology that will do just what you want:

for (int index = 0; index < LIMIT; index++) {

    NSString *posName = [NSString stringWithFormat:@"pos%d", index];
    CGRect pos = [[self valueForKey:posName] CGRectValue];
    NSString *camName = [NSString stringWithFormat:@"cam%d", index];

    UIImageView *cam = [self valueForKey:camName];
    cam.frame = pos;
}

edited Jun 1, 2013 at 10:52

answered Apr 15, 2011 at 21:16

Bartosz Ciechanowski

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Nektarios · Accepted Answer · 2011-04-15 17:57:33Z

2

One way you can do this would be to create your cameras in a dictionary and use those special NSStrings to key in to it. Like,

NSMutableDictionary *myCams;
myCams = [[myCams alloc] init];
[myCams addObject:YOUR_CAM0_OBJECT_HERE forKey:@"cam[0]"];
[myCams addObject:YOUR_CAM1_OBJECT_HERE forKey:@"cam[1]"];

NSString camString = @"cam[0]"; // you'd build your string here like you do now
id theCamYouWant = [myCams objectForKey:camString];

answered Apr 15, 2011 at 17:57

Nektarios

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2 Comments

johntraver Over a year ago

that would make sense! I am so used to using.. vars()[string] = someValue .. in python to set variables that need to be defined in iteration. Thanks Again!!

Nektarios Over a year ago

No problem, if it solves your need remember to accept as correct answer. Welcome to SO

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