C语言 rand() 与 time() 问题一起播种
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rand() seeding with time() problem
提问by Ock
I'm having difficulty figuring out how to use rand() and seeding it with time() using Xcode. I want to generate random decimal numbers between 0 and 1.
我很难弄清楚如何使用 rand() 并使用 Xcode 将其与 time() 一起播种。我想生成 0 到 1 之间的随机十进制数。
The code gives me seemingly random numbers for elements 1 and 2, but element 0 is always somewhere around 0.077. Any ideas why this would be?
该代码为我提供了元素 1 和 2 的看似随机数，但元素 0 始终在 0.077 附近。任何想法为什么会这样？
My code is:
我的代码是：
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(void)
{
double array[3];
double rand_max = RAND_MAX;
srand((int)time(NULL));
for (int iii = 0; iii < 3; iii++)
array[iii] = rand() / rand_max;
for (int iii = 0; iii < 3; iii++)
printf("Element %d is: %lf.\n", iii, array[iii]);
return(0);
}
The output for several runs was as follows:
几次运行的输出如下：
[Session started at 2010-09-11 21:19:07 -0500.]
Element 0 is: 0.076918.
Element 1 is: 0.753664.
Element 2 is: 0.824467.
The Debugger has exited with status 0.
[Session started at 2010-09-11 21:19:12 -0500.]
Element 0 is: 0.076957.
Element 1 is: 0.411353.
Element 2 is: 0.602494.
The Debugger has exited with status 0.
[Session started at 2010-09-11 21:19:16 -0500.]
Element 0 is: 0.076988.
Element 1 is: 0.937504.
Element 2 is: 0.624915.
The Debugger has exited with status 0.
[Session started at 2010-09-11 21:19:24 -0500.]
Element 0 is: 0.077051.
Element 1 is: 0.989806.
Element 2 is: 0.669757.
回答by Alex
try randomize random generator without casting it to in
尝试随机化随机生成器而不将其强制转换为
/* initialize random seed: */
srand ( time(NULL) );
Holly molly... this one is not easy. I remember long time ago in my stochastic models I had this problem. The solution was to use some pragma derective, as far as I remember.
Holly molly……这个不容易。我记得很久以前在我的随机模型中我遇到过这个问题。据我所知，解决方案是使用一些 pragma 指令。
anyways below is solution and it works. Random numbers every time... kind of cheezy though.
反正下面是解决方案，它的工作原理。每次都是随机数……虽然有点俗气。
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(void)
{
double array[10];
double rand_max = RAND_MAX;
srand(time(NULL));
for (int iii = 0; iii < 9; iii++){
rand();
array[iii] = rand() / rand_max;
}
for (int iii = 0; iii < 9; iii++){
printf("Element %d is: %lf.\n", iii, array[iii]);
}
return(0);
}
回答by jamesdlin
Typical randimplementations simply aren't very good. You could throw away the first few results, or you could switch to a better pseudo-random number generator. Also see: I need a random number generator.from the comp.lang.c FAQ.
典型的rand实现并不是很好。您可以丢弃前几个结果，或者您可以切换到更好的伪随机数生成器。另请参阅：我需要一个随机数生成器。来自 comp.lang.c 常见问题解答。
回答by tc.
rand() is usually a linear congruential generator. I suspect it's rand-fbsd.c. time() is not a very good seed either, since only the bottom few bits change significantly over a short time.
rand() 通常是一个线性同余生成器。我怀疑它是rand-fbsd.c。time() 也不是一个很好的种子，因为只有底部的几位在短时间内发生了显着变化。
random() is a bit better. Also consider seeding using sranddev() or srandomdev().
random() 好一点。还可以考虑使用 sranddev() 或 srandomdev() 进行播种。
