在 Objective-C 中生成随机数

I'm a Java head mainly, and I want a way to generate a pseudo-random number between 0 and 74. In Java I would use the method:

Random.nextInt(74)

I'm not interested in a discussion about seeds or true randomness, just how you accomplish the same task in Objective-C. I've scoured Google, and there just seems to be lots of different and conflicting bits of information.

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))

Same as C, you would do

#include <time.h>
#include <stdlib.h>
...
srand(time(NULL));
int r = rand() % 74;

(assuming you meant including 0 but excluding 74, which is what your Java example does)

Edit: Feel free to substitute random() or arc4random() for rand() (which is, as others have pointed out, quite sucky).

According to the manual page for rand(3), the rand family of functions have been obsoleted by random(3). This is due to the fact that the lower 12 bits of rand() go through a cyclic pattern. To get a random number, just seed the generator by calling srandom() with an unsigned seed, and then call random(). So, the equivalent of the code above would be

#import <stdlib.h>
#import <time.h>

srandom(time(NULL));
random() % 74;

You'll only need to call srandom() once in your program unless you want to change your seed. Although you said you didn't want a discussion of truly random values, rand() is a pretty bad random number generator, and random() still suffers from modulo bias, as it will generate a number between 0 and RAND_MAX. So, e.g. if RAND_MAX is 3, and you want a random number between 0 and 2, you're twice as likely to get a 0 than a 1 or a 2.

I wrote my own random number utility class just so that I would have something that functioned a bit more like Math.random() in Java. It has just two functions, and it's all made in C.

Header file:

//Random.h
void initRandomSeed(long firstSeed);
float nextRandomFloat();

Implementation file:

//Random.m
static unsigned long seed;

void initRandomSeed(long firstSeed)
{ 
    seed = firstSeed;
}

float nextRandomFloat()
{
    return (((seed= 1664525*seed + 1013904223)>>16) / (float)0x10000);
}

It's a pretty classic way of generating pseudo-randoms. In my app delegate I call:

#import "Random.h"

- (void)applicationDidFinishLaunching:(UIApplication *)application
{
    initRandomSeed( (long) [[NSDate date] timeIntervalSince1970] );
    //Do other initialization junk.
}

Then later I just say:

float myRandomNumber = nextRandomFloat() * 74;

Note that this method returns a random number between 0.0f (inclusive) and 1.0f (exclusive).

Use the arc4random_uniform(upper_bound) function to generate a random number within a range. The following will generate a number between 0 and 73 inclusive.

arc4random_uniform(74)

arc4random_uniform(upper_bound) avoids modulo bias as described in the man page:

arc4random_uniform() will return a uniformly distributed random number less than upper_bound. arc4random_uniform() is recommended over constructions like ``arc4random() % upper_bound'' as it avoids "modulo bias" when the upper bound is not a power of two.

This will give you a floating point number between 0 and 47

float low_bound = 0;      
float high_bound = 47;
float rndValue = (((float)arc4random()/0x100000000)*(high_bound-low_bound)+low_bound);

Or just simply

float rndValue = (((float)arc4random()/0x100000000)*47);

Both lower and upper bound can be negative as well. The example code below gives you a random number between -35.76 and +12.09

float low_bound = -35.76;      
float high_bound = 12.09;
float rndValue = (((float)arc4random()/0x100000000)*(high_bound-low_bound)+low_bound);

Convert result to a rounder Integer value:

int intRndValue = (int)(rndValue + 0.5);

Better to use arc4random_uniform. However, this isn't available below iOS 4.3. Luckily iOS will bind this symbol at runtime, not at compile time (so don't use the #if preprocessor directive to check if it's available).

The best way to determine if arc4random_uniform is available is to do something like this:

#include <stdlib.h>

int r = 0;
if (arc4random_uniform != NULL)
    r = arc4random_uniform (74);
else
    r = (arc4random() % 74);

I thought I could add a method I use in many projects.

- (NSInteger)randomValueBetween:(NSInteger)min and:(NSInteger)max {
    return (NSInteger)(min + arc4random_uniform(max - min + 1));
}

If I end up using it in many files I usually declare a macro as

#define RAND_FROM_TO(min, max) (min + arc4random_uniform(max - min + 1))

E.g.

NSInteger myInteger = RAND_FROM_TO(0, 74) // 0, 1, 2,..., 73, 74

Note: Only for iOS 4.3/OS X v10.7 (Lion) and later

There are some great, articulate answers already, but the question asks for a random number between 0 and 74. Use:

arc4random_uniform(75)