Parsing a NSString to a byte array - ios

Question

I need to parse an NSString to a byte array and am having some trouble doing it. I have a padded byte array in a method and convert that into a mutablestring, then I have a method that needs to place those numbers back into a byte array.
In C# it would be as simple as:
do
{
val = byte.Parse(str.Substring(i, 3));
byteArr[j++] = val;
i += 3;
}
Here is the code snippit Note** Ive been trying a lot of different things in the do loop so its a mess in there right now:
-(NSData*) StrToByteArray: (NSString*)str
{
NSLog(#"StrToByteArray. String: %#", str);
if([str length]==0)
NSLog(#"Invailid String");
int val;
Byte byteArr[[str length]/3];
int i = 0;
int j = 0;
NSRange range;
do {
range = NSMakeRange(i, 3);
val = (int)[str substringFromIndex:i];
NSLog(#"StrToByteArray. VAR: %i", val);
byteArr[j++] = val;
NSLog(#"byteArr: %i",byteArr[i]);
i+=3;
}while(i<str.length);
NSData* wrappedByteArr = [NSData dataWithBytes:&byteArr length: sizeof(byteArr)];
return wrappedByteArr;
}
Here is the loop that makes the padded string:
for(int i = 0; i<=len;i++)
{
val = byteArr[i];
NSLog(#"byteArr to string original: %i", val);
if(val<(Byte)10)
{
[tempStr appendString:(#"00")];
[tempStr appendString:[NSString stringWithFormat:#"%d",val]];
}
else if(val<(Byte)100)
{
[tempStr appendString:(#"0")];
[tempStr appendString:[NSString stringWithFormat:#"%d",val]];
}
else {
[tempStr appendString:[NSString stringWithFormat:#"%d",val]];
}
}
NSLog(#"string: %#", tempStr);
return tempStr;

Answer 1

Take 2
Now that I know what the data looks like and how you want to parse it, I would approach it like this:
- (NSData *) parseStringToData:(NSString *) str
{
if ([str length] % 3 != 0)
{
// raise an exception, because the string's length should be a multiple of 3.
}
NSMutableData *result = [NSMutableData dataWithLength:[str length] / 3];
unsigned char *buffer = [result mutableBytes];
for (NSUInteger i = 0; i < [result length]; i++)
{
NSString *byteString = [str substringWithRange:NSMakeRange(i * 3, 3)];
buffer[i] = [byteString intValue];
}
return result;
}
Edit:
Your padding method could be simplified as well by providing the correct format specifier that automatically pads integers.
for(int i = 0; i<=len;i++)
{
val = byteArr[i];
NSLog(#"byteArr to string original: %i", val);
[tempStr appendFormat:#"%03d", val];
}