This question already has answers here:
srand() — why call it only once?
(7 answers)
Closed 7 years ago.
This is my code:
#include<stdio.h>
#include<stdlib.h>
#include <time.h>
int main(){
float m, n;
printf("Enter n, m:");
scanf("%f %f", &n, &m);
int l;
l=m-n;
int i;
for(i=0; i<4; i++){
srand(time(NULL));
double r=rand();
r/=RAND_MAX;
r*=l;
r+=n;
printf("%f ", r);
}
return 0;
}
Why it generates same numbers? and when I write srand(time(NULL));before the loop it generates different numbers! why it is like this? how does this program work?
srand() seeds the random number sequence.
The srand function uses the argument as a seed for a new sequence of pseudo-random numbers to be returned by subsequent calls to rand. If srand is then called with the same seed value, the sequence of pseudo-random numbers shall be repeated. ... C11dr §7.22.2.2 2
And time() is typically the same value - for a second #kaylum
[Edit]
Better to call srand() only once early in the code
int main(void) {
srand((unsigned) time(NULL));
...
or, if you want the same sequence every time, do not call srand() at all - useful for debugging.
int main(void) {
// If code is not debugging, then seed the random number generator.
#ifdef NDEBUG
srand((unsigned) time(NULL));
#endif
...
The call to time(NULL) returns the current calendar time (seconds since Jan 1, 1970).
So its the same seed you are giving. So, rand gives the same value.
Simply you may use:
srand (time(NULL)+i);
Related
This question already has answers here:
srand() — why call it only once?
(7 answers)
Closed 5 years ago.
I have made a function that creates a random number between 0 and 9 but when i call the function in main in a for loop with index i = 0 ; i < n ; i++ it prints 1 random number n types. but i want different values to be printed n times.
here is the code which i made :-
#include<stdio.h>
#include<stdlib.h>
#include<time.h>
void random_variable() {
int i;
srand(time(NULL));
i = rand()%10;
printf("%d ",i);
}
main() {
for(int j=0;j<10;j++) {
random_variable();
}
}
output is this :-
please click on the link to see the output :-
here instead of 8 ten times i needed 10 different values
srand() initializes the PRNG (pseudo random number generator) of your standard C library. A PRNG works by applying more-or-less complicated arithmetic operations to a value stored internally and returning part of the result as the next "random" number. srand() sets this internal state.
If you call srand() every time in your function, you won't get (sensible) random numbers because the internal state is reset every time. Call it once your program starts.
srand() seeds your random numbers. You want to do this once, not every time.
#include<stdio.h>
#include<stdlib.h>
#include<time.h>
void random_variable() {
int i;
i = rand()%10;
printf("%d ",i);
}
main() {
srand(time(NULL));
for(int j=0;j<10;j++) {
random_variable();
}
}
This question already has answers here:
srand() — why call it only once?
(7 answers)
Closed 5 years ago.
I tried to put different random number element in each array.
However that 2 arrays got exactly same 10 random number.
What's wrong with me and how can I solve this?
#define MAX 100
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
void number(int* array)
{
srand((long)time(NULL));
for (int i = 0; i < 20; i++)
array[i] = rand() % MAX + 1;
}
void printing(int* p_array)
{
for (int i = 0; i < 20; i++)
printf(" %d", p_array[i]);
puts("");
}
int main(void)
{
int score1[20];
int score2[20];
number(score1);
printing(score1);
number(score2);
printing(score2);
return 0;
}
srand((long)time(NULL));
When you are doing this, you are initializing srand with a seed based on the time the line has been called. The main point of a pseudo-number generator such as srand is that, whenever the seed is the same, the generator will always return the exact same serie of values, so a program using the generator with a specific seed can reproduce the exact same results several times.
The problem is that you initialize srand two times, and that the line is run two times at the exact same time (at least at the same second on such a short program), so the seed is the same. Your two arrays are, thus, strictly identical.
Call this line only once at the beginning of your main function, and you will use only one serie of numbers instead of two identical ones, leading to your two arrays having different values.
int main(void)
{
srand(time(NULL));
int score1[20];
int score2[20];
number(score1);
printing(score1);
number(score2);
printing(score2);
return 0;
}
Remove this statement from the number function
srand( (unsigned int)time(NULL));
and place it in main before the calls to number.
Otherwise the expression time(NULL) could yield the same value.
As 4386427 says, the problem is your call to srand(null). It resets the random number generator. If it didn't generate the same number series, then your number generator is broken. Remove it or call it with say the current time will fix the problem.
This is my first C program and I wanted to make a random password, but every time I run the program, it generates the same string. (always generates "pkDHTxmMR1...") This is not going to actually be used so the security of rand() doesn't really matter to me. Why would it output the same string every time that I run it?
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
//this is a program to generate a random password
int main()
{
int counter = 0;
srand(time(NULL));
char randChar;
int passwordLength;
printf("Type in a password Length \n");
scanf("%d", &passwordLength);
while(counter < passwordLength)
{
//seed random based on time
srand(time(NULL));
randChar = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"[random () % 62];
printf("%c", randChar);
counter++;
}
return 0;
}
Oh dear. Everybody has got the answer wrong, including me before I tried the questioner's code for myself.
In fact, yes there should be no call to srand() in the loop because it will reseed the random number generator on each iteration. However, there should also be no call to srand() outside the loop either because the function used to generate actual random numbers is random() not rand(). The correct code is
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main()
{
int counter = 0;
srandom(time(NULL)); // Correct seeding function for random()
char randChar;
int passwordLength;
printf("Type in a password Length \n");
scanf("%d", &passwordLength);
while(counter < passwordLength)
{
randChar = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"[random () % 62];
printf("%c", randChar);
counter++;
}
printf("\n"); // Stops the output from being on the same line as the prompt
return 0;
}
Your loop takes less than a second to run.
Therefore, time(NULL) always returns the same value, so your random numbers all have the same seed.
Don't do that.
The standard:
The srand function uses the argument as a seed for a new sequence of
pseudo-random numbers to be returned by subsequent calls to rand. If
srand is then called with the same seed value, the sequence of
pseudo-random numbers shall be repeated.
It is very likely that the time_t on your system is based on seconds or something like that. But the execution time between srand() calls is far far less than one second, so you keep feeding it the same seed value.
Always just call srand() once in your whole program.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
calling rand() returning non-random results
In my workshop I need to takes 2 different random numbers but I get 2 same random number.
#include<stdio.h>
#include<stdlib.h>
#include<time.h>
int random1_6(){
int k;
srand(time(0));
k=((rand()%6)+1);
return k;
}
int main(void){
int a,b;
a=random1_6();
printf("%d.\n",a);
b=random1_6();
printf("%d.\n",b);
return 0;
}
How to get 2 different random number?
A non-cryptographic random number generator (RNG) is not truely random but it generates random-like numbers based on a seed.
What you do is initializing the RNG with the same seed two times, so you get the same results. Seed the RNG just once, e.g. at program start, and you will get random-like different results.
Edit: a code like follows should work:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int random1_6(){
return ((rand() % 6) + 1);
}
int main(void){
int a,b;
srand(time(NULL));
a = random1_6();
printf("%d.\n",a);
b=random1_6();
printf("%d.\n",b);
return 0;
}
Don't do srand(time(0)); on every call. Only call it once.
You must initialize the random number generator seed with srand() only once : upon each call you are re-initializing the RNG with the same seed since is it most likely that the two subsequent calls to time(0) will return the same timestamp (seconds-level precision), hence rand() will return the same number twice.
If you call srand() only once at the beginning of your program (in the main() entry-point), then every call to rand() will return a different number.
You always initialize the random number generator with the same seed, so you'll get the same random sequence, it is pseudo random anyway. Typically you will only want to call srand once in the beginning to initialize the generator.
Also, you only have 6 different possible outcomes, so it is perfectly legitimate to get same number twice, there is 1/6 chance for that.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Why do I always get the same sequence of random numbers with rand()?
So yeah, this might seem slightly noobish, but since I'm teaching myself C after becoming reasonable at Java, I've already run into some trouble. I'm trying to use the rand() function in C, but I can only call it once, and when it do, it ALWAYS generates the same random number, which is 41. I'm using Microsoft Visual C++ 2010 Express, and I already set it up so it would compile C code, but the only thing not working is this rand() function. I've tried including some generally used libraries, but nothing works. Here's the code:
#include "stdafx.h"
#include "stdio.h"
#include "conio.h"
#include "stdlib.h"
int main(void)
{
printf("%d", rand()); //Always prints 41
return 0;
}
This because the rand() initializes its pseudo-random sequence always from the same point when the execution begins.
You have to input a really random seed before using rand() to obtain different values. This can be done through function srand that will shift the sequence according to the seed passed .
Try with:
srand(clock()); /* seconds since program start */
srand(time(NULL)); /* seconds since 1 Jan 1970 */
You have to seed rand().
srand ( time(NULL) ); is usually used to initialise random seed. Otherwise,
You need a seed before you call rand(). Try calling "srand (time (NULL))"
You must first initialise the random seed using srand().
#include "stdafx.h"
#include "stdio.h"
#include "conio.h"
#include "stdlib.h"
int main(void)
{
srand(time(NULL)); // Initialise the random seed.
printf("%d", rand());
return 0;
}
rand() gives a random value but you need to seed it first. The problem is that if you execute your code more than once (probably), with the same seed srand(time(NULL)), then rand(); will give always the same value.
Then, the second option is to execute, as Thiruvalluvar says, srand(time(NULL)) and then rand().
The one above will give you 1000 random numbers. Try to execute this code and see what happens:
srand (time(NULL));
for (int i=0; i<1000; i++)
{
printf ("Random number: %d\n", rand());
}
Hope it helps!