NSString problems - string

Question

I have problems with a simple NSString on Mac OS X:
NSString *letters = #"abcdefghijklmnopqrstuvwxyz0123456789";
myString = [myString stringByAppendingFormat:#"%c", [letters characterAtIndex:3]];
When I try to access this string again, Xcode is returning EXC_BAD_ACCESS
This error just occures, when I'm using the format #"%c"
When I'm using #"%#", sometimes the same error, sometimes this string: control 0x10040a480 and sometimes this:
{(
<CFRunLoopObserver 0x10015ac60 [0x7fff70731ee0]>{locked = No, valid = Yes, activities = 0x21, repeats = Yes, order = 0, callout = _ZL15FlushAllBuffersP19__CFRunLoopObservermPv (0x7fff88a147d4), context = <CFRunLoopObserver context 0x0>}
)}
The errors occur randomly even, if I don't change anything in the code and re-run it.
I try to get a random String by doing:
randomString = #"";
NSString *letters = #"abcdefghijklmnopqrstuvwxyz0123456789";
srand(time(NULL));
for (int i=0; i<5; i++)
{
randomString = [randomString stringByAppendingFormat:#"%c", [letters characterAtIndex:(rand()%[letters length])]];
}
randomString is declared in header.h
I also tried using a NSMutableString but that didn't work too.
Every time I try to access the string (or mutable string) via #"%#" I'm getting EXC_BAD_ACCESS
Any idea?
Hope somebody can help me!
Greets,
Julian

Answer 1

Your problem is that myString gets autoreleased before you access it.
You need to change:
myString = [myString stringByAppendingFormat:#"%c", [letters characterAtIndex:3]];
to:
myString = [[myString stringByAppendingFormat:#"%c", [letters characterAtIndex:3]] retain];
Remember to run [myString release]; when you are done with it.

Answer 2

this works for me:
define letters at top of file:
#define letters #"abcdefghijklmnopqrstuvwxyz0123456789"
then append a random character like this:
NSString *randomString = #"";
for (int i = 0; i < 5; i++)
{
UInt64 index = rand() % [letters length];
NSString *randomCharacter = [letters substringWithRange: NSMakeRange(index, 1)];
randomString = [randomString stringByAppendingString:randomCharacter];
}
NSLog(#"%#", randomString);