Objective-c – Converting floats to NSData and back in Objective-C

iphoneobjective c

In an iphone application, I'm looking to convert a float to NSData for it to be sent over bluetooth and then converted back again when it's received. I have the bluetooth part working fine, but when I use this to convert to NSData:

NSData *data = [[NSData alloc]init];

float z = 9.8574; // Get the float value, 9.8574 is just an example

[data getBytes:&z length:sizeof(float)];

I can not convert it back to a float. I've tried a couple of methods but I'm wondering if this is the correct way to encode the float to NSData??

Thanks

Best Answer

Here is how to encode and decode a float with NSData:

encoding:

NSMutableData * data = [NSMutableData dataWithCapacity:0];
float z = ...;
[data appendBytes:&z length:sizeof(float)];

decoding:

NSData * data = ...; // loaded from bluetooth
float z;
[data getBytes:&z length:sizeof(float)];

A couple of things to note here:

1. You have to use NSMutableData if you are going to add things to the data object after creating it. The other option is to simply load the data all in one shot:

NSData * data = [NSData dataWithBytes:&z length:sizeof(float)];

2. the getBytes:length: method is for retrieving bytes from an NSData object, not for copying bytes into it.

Related Solutions

Objective-c – Generating random numbers in Objective-C

You should use the arc4random_uniform() function. It uses a superior algorithm to rand. You don't even need to set a seed.

#include <stdlib.h>
// ...
// ...
int r = arc4random_uniform(74);

The arc4random man page:

NAME
     arc4random, arc4random_stir, arc4random_addrandom -- arc4 random number generator

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdlib.h>

     u_int32_t
     arc4random(void);

     void
     arc4random_stir(void);

     void
     arc4random_addrandom(unsigned char *dat, int datlen);

DESCRIPTION
     The arc4random() function uses the key stream generator employed by the arc4 cipher, which uses 8*8 8
     bit S-Boxes.  The S-Boxes can be in about (2**1700) states.  The arc4random() function returns pseudo-
     random numbers in the range of 0 to (2**32)-1, and therefore has twice the range of rand(3) and
     random(3).

     The arc4random_stir() function reads data from /dev/urandom and uses it to permute the S-Boxes via
     arc4random_addrandom().

     There is no need to call arc4random_stir() before using arc4random(), since arc4random() automatically
     initializes itself.

EXAMPLES
     The following produces a drop-in replacement for the traditional rand() and random() functions using
     arc4random():

           #define foo4random() (arc4random() % ((unsigned)RAND_MAX + 1))

Objective-c – Shortcuts in Objective-C to concatenate NSStrings

An option:

[NSString stringWithFormat:@"%@/%@/%@", one, two, three];

Another option:

I'm guessing you're not happy with multiple appends (a+b+c+d), in which case you could do:

NSLog(@"%@", [Util append:one, @" ", two, nil]); // "one two"
NSLog(@"%@", [Util append:three, @"/", two, @"/", one, nil]); // three/two/one

using something like

+ (NSString *) append:(id) first, ...
{
    NSString * result = @"";
    id eachArg;
    va_list alist;
    if(first)
    {
        result = [result stringByAppendingString:first];
        va_start(alist, first);
        while (eachArg = va_arg(alist, id)) 
        result = [result stringByAppendingString:eachArg];
        va_end(alist);
    }
    return result;
}