笔记：随机生成数、字符串

//1.整型随机生成
 int x = arc4random() % (to - from + 1) + from;//[from，to]范围
 int number = arc4random() % (3 - 2 + 1) + 2;//[2,3]

//2.浮点数随机生成
#define ARC4RANDOM_MAX      0x100000000 
//然后就可以使用arc4random() 来获取0到100之间浮点数了（精度是rand（）的两倍），代码如下：

double val = floorf(((double)arc4random() / ARC4RANDOM_MAX) * 100.0f);

-(NSString *)randomStringWithLength:(NSInteger)len {
     NSString *letters = @"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    NSMutableString *randomString = [NSMutableString stringWithCapacity: len];
    
    for (NSInteger i = 0; i < len; i++) {
        [randomString appendFormat: @"%C", [letters characterAtIndex: arc4random_uniform([letters length])]];
    }
    return randomString;
}