Hello friends I need your help.
My program is such an array size 1000 where the numbers should be between 0-999. These numbers should be determined randomly (rand loop) and the number must not be repeated. Would be considered the main part, I have to count how many times I used rand().
My idea is that: one loop where it initializes all the 1000 numbers, and if in this loop they check whether the number appears twice, if the number appears twice is set it again until that not appear twice (maybe this is not the best way but ...)
It is my exercise (Here I need your help)-
#include <stdio.h>
#include <stdlib.h>
int main()
{
int const arr_size = 1000;
int i, j, c;
int arr[arr_size];
int loop = 0;
for(i = 0; i<arr_size; i++)
{
arr[i] = rand() % 1000;
loop++;
if (arr[i] == arr[i - 1])
{
arr[i] = rand() % 1000;
loop++;
}
}
printf("%d\n",loop);
}
So if anyone can give me advice on how I can make it work I appreciate your help.
Thanks.
As suggested, shuffling the set will work but other indirect statistical quantities might be of interest, such as the distribution of the loop variable as a function of the array index.
This seemed interesting so I went ahead and plotted the distribution of the loop as a function of the array index, which generally increases as i increases. Indeed, as we get near the end of the array, the chance of getting a new random number that is not already in the set decreases (and hence, the value of the loop variable increases; see the code below).
Specifically, for an array size = 1000, I recorded the non-zero values generated for loop (there were around 500 duplicates) and then made a plot vs the index.
The plot looks like this:
The code below will produce an array with the unique random values, and then calculate the value for loop. The loop values could be stored in another array and then saved for later analysis, but I didn't include that in the code below.
Again, I'm not exactly sure this fits the application, but it does return information that would not necessarily be available from an approach using a shuffle algorithm.
NOTE: some folks expressed concerns about how long this might take but it runs pretty quick, on my 2011 Macbook Pro it took a about a second for an array size of 1000. I didn't do a big-O analysis as a function of the array size, but that would be interesting too.
NOTE 2: its more elegant to use recursion for the numberInSet() function but it seemed best to keep simple.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <stdbool.h> /* If C99 */
const int ARR_SIZE = 1000;
/* Check if the number is in the set up to the given position: */
bool numberInSet(int number, int* theSet, int position);
int main()
{
int* arr = malloc(sizeof(int)*ARR_SIZE);
srand((unsigned int)time(NULL));
/* Intialize array with rand entries, possibly duplicates: */
for(int i = 0; i < ARR_SIZE; i++)
arr[i] = rand() % ARR_SIZE;
/* Scan the array, look for duplicate values, replace if needed: */
for(int i = 0; i < ARR_SIZE; i++) {
int loop = 0;
while ( numberInSet(arr[i], arr, i-1) ) {
arr[i] = rand() % ARR_SIZE;
loop++;
}
/* could save the loop values here, e.g., loopVals[i] = loop; */
}
for(int i = 0; i < ARR_SIZE; i++)
printf("i = %d, %d\n",i,arr[i]);
/* Free the heap memory */
free(arr);
}
bool numberInSet(int number, int* theSet, int position) {
if (position < 0)
return false;
for(int i = 0; i <= position; i++)
if (number == theSet[i])
return true;
return false;
}
To make sure all random number you get in the same program are different, you must seed once the random generator:
srand (time(NULL)); //seed the random generator
//in the loop, rand will use the seeded value
rand() % 1000
Related
// What I mean by this is shown by my example:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int i;
int a;
for (a = 0;a <10;a ++) {
i = (rand()%10)+1; // generates a number from 1-10
printf("%d\n", i);
}
// I would like for the loop to generate a number that gives a number that was not generated before. For example, an output such as:
1,3,6,2,8,9,4,10,5,7
instead of:
3,9,10,3,7,9,2,7,10,1
In other words, I would like no copies.
You obviously don't just want no copies, but you want every number in a given set exactly once. This is, as commented by Robert, similar to shuffling a deck of cards. You don't have "decks" in C, but you can model one as an array:
int deck[] = {1,1,1,1,1,1,1,1,1,1};
This should represent 10 different "cards" (identified by their index in the array), each available one time. Now, just write code that "draws" cards:
int i = 0; // starting point for searching for the next card to draw
for (int n = 10; n > 0; --n) // how many cards are left
{
int skip = rand() % n; // randomly skip 0 .. n cards
while (1)
{
if (deck[i]) // card still available?
{
if (!skip) break; // none more to skip -> done
--skip; // else one less to skip
}
if (++i > 9) i = 0; // advance index, wrapping around to 0
}
deck[i] = 0; // draw the card
printf("%d\n", i+1); // and print it out
}
of course, seed the PRNG (e.g. srand(time(0))) first, so you don't get the same sequence every time.
The idea shown in the question is to print numbers within a range, without repetition. Here is one way to do that, by putting each value into an array and swapping its elements around.
A variation could be that you don't want to use all the possible numbers, in that case just change PICKED.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define ARRLEN 10
#define PICKED 10
int main(void) {
int array[ARRLEN];
srand((unsigned)time(NULL)); // seed the PRNG
for(int i = 0; i < ARRLEN; i++) { // generate the numbers
array[i] = i + 1;
}
for(int i = 0; i < ARRLEN; i++) { // shuffle the array
int index = rand() % ARRLEN;
int temp = array[i];
array[i] = array[index]; // by randomly swapping
array[index] = temp;
}
for(int i = 0; i < PICKED; i++) { // output the numbers
printf("%d ", array[i]);
}
printf("\n");
}
Program output:
9 8 4 5 1 10 7 3 6 2
The library's PRNG is not very random, but for many cases that is not important. If it is, better algorithms are available.
So I'm just learning C and I would like to know how you could prevent a variable randomized with the rand() function from repeating the same number. I have a script which simply randomizes and prints a variable in a for loop 4 times. How could I make it so the variable never gets the same number after each time it uses the rand() function?
#include <stdio.h>
#include <stdlib.h>
int randomInt;
int main()
{
srand(time(0));
for (int i = 0; i < 4; ++i) {
randomInt = rand() % 4;
printf("%d\n", randomInt);
}
return 0;
}
On most machines, int is 32 bits. So after 232 iterations, you are sure that you'll get some repetition (and probably much before).
If you restrict yourself to much less loops, consider e.g. keeping an array of previously met random numbers (or some hash table, or some binary tree, or some other container).
For a loop repeated only 4 times, keeping an array of (at most 4-1) previously emitted numbers is quite simple, and efficient enough.
Read also about the pigeonhole principle.
A slightly different approach.
int set[] = {0, 1, 2, 3 } ;
srand(time(0));
shuffle(set,4);
using the shuffle algorithm given in this question
https://stackoverflow.com/a/6127606/9288531
I'm guessing that you are getting the same numbers because your are running your program multiple times within the same second. If time(0) hasn't changed, you will have the same seed and the same random numbers generated. Unless your program runs extremely quickly, I imagine using a seed based on microseconds instead of seconds would work:
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
int randomInt;
int main()
{
struct timeval my_microtimer;
gettimeofday(&t1, NULL);
srand(t1.tv_sec * my_microtimer.tv_usec);
for (int i = 0; i < 4; ++i) {
randomInt = rand() % 4;
printf("%d\n", randomInt);
}
return 0;
}
What you could do is keeping track of each number you already generated.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int hasMyNumberAlreadyBeenGenerated(int number, int generatedNumbers[], int size){
for(int i = 0; i < size + 1; i++){
//If you already generated the number, it should be present somewhere in your array
if(generatedNumbers[i] == number) return 1;
//If you did not, find the first available space in your array, and put the number you generated into that space
if(generatedNumbers[i] == 0){
generatedNumbers[i] = number;
break; //No need to continue to check the array
}
}
return 0;
}
int main()
{
int randomInt;
int generatedNumbers[4];
//We set "0" in all the array, to be sure that the array doesn't contain unknown datas when we create it
memset(generatedNumbers, 0x0, sizeof(generatedNumbers));
srand(time(0));
for (int i = 0; i < 4; ++i) {
randomInt = rand() % 4 + 1;
//As long as the number you generate has already been generated, generate a new one
while(hasMyNumberAlreadyBeenGenerated(randomInt, generatedNumbers, i) == 1){
randomInt = rand() % 4 + 1;
}
printf("generated : %d\n", randomInt);
}
return 0;
}
The problem with this method is that you can't generate a 0, because if you do you'll endlessly loop.
You can bypass this problem using a dynamic array using malloc() function.
If you want to write clean code you should define how many numbers you want to generate with a #define.
What you seem to be asking is a non-random set of numbers 0 to 3 in a random order. Given that;
int set[] = {0, 1, 2, 3 } ;
int remaining = sizeof(set) / sizeof(*set) ;
while( remaining != 0 )
{
int index = rand() % sizeof(set) / sizeof(*set) ;
if( set[index] > 0 )
{
printf( "%d\n", set[index] ) ;
set[index] = -1 ;
remaining-- ;
}
}
For very large sets, this approach may not be practical - the number of iterations necessary to exhaust the set is non-deterministic.
Hi I want to know is there a way to store randnumber without prompting user for input. In this example we want user to store 16 randnumbers in array storerand without prompting the user for randnums for example:
#include<stdio.h>
#include<stdlib.h>
void main() {
int randnum;
int i=0;
int storerand[16]; // storing randnumbers
randnum=rand();
while(i <16){
storerand[i]=randnum; // why is this not storing 16 rand number how can i store 16 randnumbers
i++;
}
return 0;
An easy way to generate random numbers in C is to use the rand() function which comes with <stdlib.h>. Note that if you do not seed with the time library, you will receive the same random numbers every time you run your program.
#include <stdlib.h>
#include <time.h>
int main (void) {
//Uses system time to seed a proper random number.
srand(time(NULL));
//rand() generates a random integer
int a = rand();
//Use mod to restrict range:
int b = rand()%5; //random integer from 0-4
return 0;
}
You also need to make sure to increment your index i within your while loop.
while(i < 16) {
storerand[i] = rand();
i++;
}
You initialise randnum with a random value here:
randnum=rand();
And then you run your loop to put its value into each slot of your array. That means you're putting the same value into each slot.
while(i <16){
storerand[i]=randnum;
i++;
}
The solution is to call rand() each time around the loop:
for( i = 0; i < 16; i++ ) {
storerand[i] = rand();
}
Edit:
Sorry for the inconvinience that made because of my undetailed question.
So, i have an array of numbers (int) with 1000 cells (index 0-999)
and I need to fill all the cells of the array with unique random numbers by calling the rand() function in C, but i have to do it with only 1000 calls to the fuction.. (Every number that generated is inserted to the array), there cant be duplicated numbers in the array.
Any ideas how I can do it?
Note:
Here is a sample code to fill the array without limiting the number of calls to rand.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define DO_RAND rand()%1000
#define ARR_SIZE 1000
int main()
{
int i,callToRand=0,j;
int array[ARR_SIZE];
srand(time(NULL));
for(i=0;i<ARR_SIZE;i++) //Runs on the whole array
{
array[i]=DO_RAND; //inserting random number to the array
callToRand++; //increasing the number of calls to rand() function
for(j=0;j<i;j++) //running on the array till the current cell
{
if(array[j]==array[i])
//checking if the number has already in the array.
{
array[i]=DO_RAND;
callToRand++;
j=-1; //staring the checker loop again
}
}
}
printf("The number of calls to the function rand() were: %d\n",callToRand);
system("PAUSE");
return (0);
}
Having finally stated that you want an array of unique numbers within the range of the array size, try this. Note that the numbers are not randomised - they are defined - the sequence is.
Your posted attempt was very inefficient, using two nested loops. This uses one loop to initialise the array and another to randomise the sequence. No need to count the calls to rand() which are obviously ARR_SIZE.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define ARR_SIZE 100 //1000
int main()
{
int i, j, temp;
int array[ARR_SIZE];
srand((unsigned)time(NULL));
for(i=0; i<ARR_SIZE; i++) // set up unique array
array[i] = i;
for(i=0; i<ARR_SIZE; i++) { // randomize the sequence
j = rand() % ARR_SIZE; // pick another (or same) index
temp = array[i]; // and swap the values
array[i] = array[j];
array[j] = temp;
}
for(i=0; i<ARR_SIZE; i++) // show results
printf ("%5d", array[i]);
printf ("\n");
return (0);
}
I have a simple test program in C to scramble an array of values on the heap. Sidenote: I know the random logic here has a flaw that will not allow the "displaced" value to exceed RAND_MAX, but that is not the point of this post.
The point is that when I run the code with N = 10000, every once in a while it will crash with very little information (screenshots posted below). I'm using MinGW compiler. I can't seem to reproduce the crash for lower or higher N values (1000 or 100000 for example).
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
const int N = 10000;
int main() {
int i, rand1, rand2, temp, *values;
/* allocate values on heap and initialize */
values = malloc(N * sizeof(int));
for (i = 0; i < N; i++) {
values[i] = i + 1;
}
/* scramble */
srand(time(NULL));
for (i = 0; i < N/10; i++) {
rand1 = (int)(N*((double)rand()/(double)RAND_MAX));
rand2 = (int)(N*((double)rand()/(double)RAND_MAX));
temp = values[rand1];
values[rand1] = values[rand2];
values[rand2] = temp;
}
int displaced = 0;
for (i = 0; i < N; i++) {
if (values[i] != (i+1)) {
displaced++;
}
}
printf("%d numbers out of order\n", displaced);
free(values);
return 0;
}
it may be because rand() generates a random number from 0 to RAND_MAX inclusive so (int)(N*((double)rand()/(double)RAND_MAX)) can be N, which exceeds the array boundary. however, i don't see why that would vary with array size (it does explain why it only crashes sometimes, though).
try /(1+(double)RAND_MAX) (note that addition is to the double, to avoid overflow, depending on the value of RAND_MAX) (although i'm not convinced that will always work, depending on the types involved. it would be safer to test for N and try again).
also, learn to use a tool from Is there a good Valgrind substitute for Windows? - they make this kind of thing easy to fix (they tell you exactly what went wrong when you run your program).