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 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).
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);
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))
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.
//The following example is going to generate a number between 0 and 73.
int value;
value = (arc4random() % 74);
NSLog(@"random number: %i ", value);
//In order to generate 1 to 73, do the following:
int value1;
value1 = (arc4random() % 73) + 1;
NSLog(@"random number step 2: %i ", value1);
Output:
random number: 72
random number step 2: 52