如何在 Objective-C 中创建随机浮点数？

Question

I'm trying to create a random float between 0.15 and 0.3 in Objective-C. The following code always returns 1:

int randn = (random() % 15)+15;
float pscale = (float)randn / 100;

What am I doing wrong?

Answer 1

Here is a function

- (float)randomFloatBetween:(float)smallNumber and:(float)bigNumber {
    float diff = bigNumber - smallNumber;
    return (((float) (arc4random() % ((unsigned)RAND_MAX + 1)) / RAND_MAX) * diff) + smallNumber;
}

Answer 2

Try this:

 (float)rand() / RAND_MAX

Or to get one between 0 and 5:

 float randomNum = ((float)rand() / RAND_MAX) * 5;

Several ways to do the same thing.

Answer 3

1. use arc4random() or seed your random values

2. try

   float pscale = ((float)randn) / 100.0f;

Answer 4

Your code works for me, it produces a random number between 0.15 and 0.3 (provided I seed with srandom() ). Have you called srandom() before the first call to random() ? You will need to provide srandom() with some entropic value (a lot of people just use srandom(time(NULL)) ).

For more serious random number generation, have a look into arc4random , which is used for cryptographic purposes. This random number function also returns an integer type, so you will still need to cast the result to a floating point type.

Answer 5

Easiest.

+ (float)randomNumberBetween:(float)min maxNumber:(float)max
{
    return min + arc4random_uniform(max - min + 1);
}

Answer 6

Using srandom() and rand() is unsafe when you need true randomizing with some float salt.

On MAC_10_7, IPHONE_4_3 and higher you can use arc4random_uniform(upper_bound)*. It allows to generate true random integer from zero to *upper_bound*.

So you can try the following

u_int32_t upper_bound = <some big enough integer>;

float r = 0.3 * (0.5 + arc4random_uniform(upper_bound)*1.0/upper_bound/2);

Answer 7

To add to @Caladain's answer, if you want the solution to be as easy to use as rand() , you can define these:

#define randf() ((CGFloat)rand() / RAND_MAX)
#define randf_scaled(scale) (((CGFloat)rand() / RAND_MAX) * scale)

Feel free to replace CGFloat with double if you don't have access to CoreGraphics.

Answer 8

I ended up generating to integers one for the actual integer and then an integer for the decimal. Then I join them in a string then I parse it to a floatvalue with the "floatValue" function... I couldn't find a better way and this works for my intentions, hope it helps :)

int integervalue = arc4random() % 2;
int decimalvalue = arc4random() % 9;   
NSString *floatString = [NSString stringWithFormat:@"%d.%d",integervalue,decimalvalue];
float randomFloat = [floatString floatValue];