Additional characters in NSString for network transfer

Question

Due to some protocol specifications for Quartz Composer, the string "\0\0\0" has to precede every character sent via UDP. The current value has this format: "1.23456". For the transfer the last three decimal places are not required, but the addition before every number, so it should look like this: "\0\0\01\0\0\0.\0\0\02\0\0\03". What's "the Objective-C way" to solve this?

Answer 1

If I understood you correctly, you want to send a sequence of characters (type char). In that case,

NSString *originalString = @"1.23456";

// It's not clear if you must remove the last three digits
// so I'm assuming that the string always has the last three
// characters removed

NSUInteger stringLength = [originalString length];
if (stringLength > 3)
    originalString = [originalString substringToIndex:stringLength - 3];

// I'm assuming ASCII strings so that one character maps to only one char
const char *originalCString = [originalString cStringUsingEncoding:NSASCIIStringEncoding];

if (! originalCString) {
    NSLog(@"Not an ASCII string");
    exit(1);
}

NSMutableData *dataToSend = [NSMutableData data];
char zeroPadding[] = { 0, 0, 0 };
NSUInteger i = 0;
char character;

while ((character = originalCString[i++])) {
    [dataToSend appendBytes:zeroPadding length:sizeof zeroPadding];
    [dataToSend appendBytes:&character length:1];
}

If you run

NSLog(@"%@", dataToSend);

the output should be

<00000031 0000002e 00000032 00000033>

where

00000031

means

00 00 00 31

and 31 is the ASCII code of ‘1’ (2e = ‘.’, 32 = ‘2’, 33 = ‘3’).

If you need to know the size (in bytes) of dataToSend, use [dataToSend length]. If you want to access the bytes themselves, use [dataToSend bytes].

Answer 2

One variation

NSMutableData* dataToSend;
char nullLeader[] = { 0, 0, 0 };

dataToSend = [NSMutableData dataWithBytes: nullLeader length: sizeof nullLeader];
[dataToSend appendData: [myStringIWantToSend dataUsingEncoding: NSUTF8StringEncoding]];

// send the data