I am working with combinatorics and I would like to know if there is an algorithm that prints all arrangments of subsequences of a given array. That is, if I give to this algorithm the sequence "ABCDEF" it will print :
A,
B,
C,
D,
E,
F,
AB,
AC,
AD,
AE,
AF,
BC,
BD,
BE,
BF,
CD,
CE,
CF,
DE,
DF,
EF,
ABC,
ABD,
ABE,
ABF,
ACD,
ACE,
ACF,
ADE,
ADF,
AEF,
BCD,
BCE,
BCF,
BDE,
BDF,
BEF,
CDE,
CDF,
CEF,
DEF,
ABCD,
ABCE,
ABCF,
ABDE,
ABDF,
ABEF,
ACDE,
ACDF,
ACEF,
ADEF,
BCDE,
BCDF,
BCEF,
BDEF,
CDEF,
ABCDE,
ABCDF,
ABCEF,
ABDEF,
ACDEF,
BCDEF,
ABCDEF,
or for a more simple case, if i give it 1234, it will print:
1,2,3,4,12,13,14,23,24,34,123,124,134,234,1234.
As you can see it is not an arbitrary permutation it is only the permutation of the last members of a subsequence in a way it still reains a subsequence.
I have tried to make a function in c that does this but i got really confused, my idea would be to make a int L that keeps the size of the subsequence,and another tree integers one that keeps the head of the subsequence, one that marks the separation from the head and one that slides trought the given number of characters, but it gets too confused too quickly.
Can anyone help me with this ?
my code is:
int Stringsize( char m[] ){
int k;
for(k=0;;k++){
if( m[k] == '\0') break;
}
return (k-1);
}
void StringOrdM(char m[]){
int q,r,s,k;
for(k=0;k<=Stringsize(m);k++)
for(q=0;q<=Stringsize(m);q++)
for(s=q;s<=Stringsize(m);s++ )
printf("%c",m[q]);
for(r=q+1; r<=Stringsize(m) && (r-q+1)<= k ;r++ )
printf("%c", m[r] );
}
And for ABCD it prints A,A,A,A,B,B,B,C,C,D,AA,AB,AC,AD,BC,BD,CC,CD,DD,... so it is not right because it keeps repeating the A 4 times the B three times and so on, when it should have been A,B,C,D,AB,AC,AD,BC,BD,CD,...
As I said in my comment above, one solution is simple: count in binary up to (1<<n)-1.
So if you have four items, count up to 15, with each bit pattern being a selection of the elements. You'll get 0001, 0010, 0011, 0100, 0101, 0110, 0111, 1000, 1001, 1010, 1011, 1100, 1101, 1110, 1111. Each bit is a true/false value as to whether to include that element of the array.
#include <stdio.h>
int main(void) {
////////////////////////////////////////////////////////////////////////
int A[] = { 1, 2, 3, 4, 5 };
////////////////////////////////////////////////////////////////////////
size_t len = sizeof A / sizeof A[0]; // Array length (in elements)
size_t elbp = (1<<len) - 1; // Element selection bit pattern
size_t i, j; // Iterators
// Cycle through all the bit patterns
for (i = 1; i<=elbp; i++) {
// For each bit pattern, print out the 'checked' elements
for (j = 0; j < len; j++) {
if (i & (1<<j)) printf("%d ", A[j]);
}
printf("\n");
}
return 0;
}
If you want the elements sorted shortest to longest, you could always store these results in a string array (using sprintf()) and then sort (using a stable sorting algorithm!) by string length.
I mentioned in a comment above that if you didn't want to use a bit pattern to find all permutations, and sort the results according to whatever criteria you'd like, you could also use a recursive algorithm.
I suspect this is a homework assignment, and you only asked for an algorithm, so I left some of the key code as an exercise for you to finish. However, the algorithm itself is complete (the key parts are just described in comments, rather than functional code being inserted).
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void printpermutations(const int *A, const size_t n, const char *pfix, const size_t rd);
int main(void) {
/////////////////////////////////////////////////////////////////////
int A[] = { 1, 2, 3, 4, 5 };
/////////////////////////////////////////////////////////////////////
size_t n = sizeof A / sizeof A[0]; // Array length (in elements)
size_t i; // Iterator
for (i = 1; i <= n; i++) {
printpermutations(A, n, "", i);
}
return 0;
}
// Recursive function to print permutations of a given length rd,
// using a prefix set in pfix.
// Arguments:
// int *A The integer array we're finding permutations in
// size_t n The size of the integer array
// char *pfix Computed output in higher levels of recursion,
// which will be prepended when we plunge to our
// intended recursive depth
// size_t rd Remaining depth to plunge in recursion
void printpermutations(const int *A, const size_t n, const char *pfix, const size_t rd) {
size_t i;
char newpfix[strlen(pfix)+22]; // 20 digits in 64-bit unsigned int
// plus a space, plus '\0'
if (n < rd) return; // Don't bother if we don't have enough
// elements to do a permutation
if (rd == 1) {
for (i = 0; i < n; i++) {
// YOUR CODE HERE
// Use printf() to print out:
// A string, consisting of the prefix we were passed
// Followed immediately by A[i] and a newline
}
}
else {
strcpy(newpfix, pfix);
for (i = 1; i <= n; i++) {
// YOUR CODE HERE
// Use sprintf() to put A[i-1] and a space into the new prefix string
// at an offset of strlen(pfix).
// Then, call printpermutations() starting with the ith offset into A[],
// with a size of n-i, using the new prefix, with a remaining
// recursion depth one less than the one we were called with
}
}
}
Depending on torstenvl's answer I did this code and It works perfectly.
If there is any problem let me know.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
char str[] = "1234";
size_t len = strlen(str); // Array length (in elements)
char *A = malloc(sizeof(char) * len);
strcpy(A,str);
size_t elbp = (1<<len) - 1; // Element selection bit pattern
size_t i, j; // Iterators
int a = 0, b = 0, n = 0;
char **arr = malloc(sizeof(char*) * (10000)); //allocating memory
if (A[0] >= 'A' && A[0] <= 'Z') //If the string given is "ABCD...." transfer 'A' to '1' ; 'C' to '3' ...etc
for(int i = 0; i < len; i++)
A[i] = A[i] - 'A' + '1';
// Cycle through all the bit patterns
for (i = 1; i<=elbp; i++)
{
arr[b] = malloc(sizeof(char) * len);
// For each bit pattern, store in arr[b] the 'checked' elements
for (j = 0, a = 0; j < len; j++)
if (i & (1<<j))
arr[b][a++] = A[j];
b++;
}
int *num = calloc(sizeof(int) ,10000);
for (i = 0; i < b; i++)
num[i] = strtol(arr[i], NULL, 10); //convert char to int
for (i = 0; i < b; i++) //sort array numbers from smallest to largest
for (a = 0; a < i; a++)
if (num[i] < num[a])
{
n = num[i];
num[i] = num[a];
num[a] = n;
}
char *result = calloc(sizeof(char),10000);
for (i = 0, a = 0; i<b; i++)
a += sprintf(&result[a], "%d,", num[i]); //convert int to char and store it in result[a]
result[a - 1] = '\0'; //remove the last ','
len = strlen(result);
if (str[0] >= 'A' && str[0] <= 'Z') //if the string given is "ABCD..." transfer '1' to 'A' ; '12' to 'AB' ; '13' to 'AC'.....etc
for (i = 0; i < len; i++)
if(result[i] != ',')
result[i] = 'A' + (result[i] - '1') ;
///test
printf("%s",result);
return 0;
}
the output for "1234":
1,2,3,4,12,13,14,23,24,34,123,124,134,234,1234
the output for "123456789":
1,2,3,4,5,6,7,8,9,12,13,14,15,16,17,18,19,23,24,25,26,27,28,29,34,35,36,37,38,39,45,46,47,48,49,56,57,58,59,67,68,69,78,79,89,123,124,125,126,127,128,129,134,135,136,137,138,139,145,146,147,148,149,156,157,158,159,167,168,169,178,179,189,234,235,236,237,238,239,245,246,247,248,249,256,257,258,259,267,268,269,278,279,289,345,346,347,348,349,356,357,358,359,367,368,369,378,379,389,456,457,458,459,467,468,469,478,479,489,567,568,569,578,579,589,678,679,689,789,1234,1235,1236,1237,1238,1239,1245,1246,1247,1248,1249,1256,1257,1258,1259,1267,1268,1269,1278,1279,1289,1345,1346,1347,1348,1349,1356,1357,1358,1359,1367,1368,1369,1378,1379,1389,1456,1457,1458,1459,1467,1468,1469,1478,1479,1489,1567,1568,1569,1578,1579,1589,1678,1679,1689,1789,2345,2346,2347,2348,2349,2356,2357,2358,2359,2367,2368,2369,2378,2379,2389,2456,2457,2458,2459,2467,2468,2469,2478,2479,2489,2567,2568,2569,2578,2579,2589,2678,2679,2689,2789,3456,3457,3458,3459,3467,3468,3469,3478,3479,3489,3567,3568,3569,3578,3579,3589,3678,3679,3689,3789,4567,4568,4569,4578,4579,4589,4678,4679,4689,4789,5678,5679,5689,5789,6789,12345,12346,12347,12348,12349,12356,12357,12358,12359,12367,12368,12369,12378,12379,12389,12456,12457,12458,12459,12467,12468,12469,12478,12479,12489,12567,12568,12569,12578,12579,12589,12678,12679,12689,12789,13456,13457,13458,13459,13467,13468,13469,13478,13479,13489,13567,13568,13569,13578,13579,13589,13678,13679,13689,13789,14567,14568,14569,14578,14579,14589,14678,14679,14689,14789,15678,15679,15689,15789,16789,23456,23457,23458,23459,23467,23468,23469,23478,23479,23489,23567,23568,23569,23578,23579,23589,23678,23679,23689,23789,24567,24568,24569,24578,24579,24589,24678,24679,24689,24789,25678,25679,25689,25789,26789,34567,34568,34569,34578,34579,34589,34678,34679,34689,34789,35678,35679,35689,35789,36789,45678,45679,45689,45789,46789,56789,123456,123457,123458,123459,123467,123468,123469,123478,123479,123489,123567,123568,123569,123578,123579,123589,123678,123679,123689,123789,124567,124568,124569,124578,124579,124589,124678,124679,124689,124789,125678,125679,125689,125789,126789,134567,134568,134569,134578,134579,134589,134678,134679,134689,134789,135678,135679,135689,135789,136789,145678,145679,145689,145789,146789,156789,234567,234568,234569,234578,234579,234589,234678,234679,234689,234789,235678,235679,235689,235789,236789,245678,245679,245689,245789,246789,256789,345678,345679,345689,345789,346789,356789,456789,1234567,1234568,1234569,1234578,1234579,1234589,1234678,1234679,1234689,1234789,1235678,1235679,1235689,1235789,1236789,1245678,1245679,1245689,1245789,1246789,1256789,1345678,1345679,1345689,1345789,1346789,1356789,1456789,2345678,2345679,2345689,2345789,2346789,2356789,2456789,3456789,12345678,12345679,12345689,12345789,12346789,12356789,12456789,13456789,23456789,123456789
the output for "ABCDEF":
A,B,C,D,E,F,AB,AC,AD,AE,AF,BC,BD,BE,BF,CD,CE,CF,DE,DF,EF,ABC,ABD,ABE,ABF,ACD,ACE,ACF,ADE,ADF,AEF,BCD,BCE,BCF,BDE,BDF,BEF,CDE,CDF,CEF,DEF,ABCD,ABCE,ABCF,ABDE,ABDF,ABEF,ACDE,ACDF,ACEF,ADEF,BCDE,BCDF,BCEF,BDEF,CDEF,ABCDE,ABCDF,ABCEF,ABDEF,ACDEF,BCDEF,ABCDEF
Combinations, or k-combinations, are the unordered sets of k elements chosen from a set of size n.
Source: http://www.martinbroadhurst.com/combinations.html
This is the code:
unsigned int next_combination(unsigned int *ar, size_t n, unsigned int k)
{
unsigned int finished = 0;
unsigned int changed = 0;
unsigned int i;
if (k > 0) {
for (i = k - 1; !finished && !changed; i--) {
if (ar[i] < (n - 1) - (k - 1) + i) {
/* Increment this element */
ar[i]++;
if (i < k - 1) {
/* Turn the elements after it into a linear sequence */
unsigned int j;
for (j = i + 1; j < k; j++) {
ar[j] = ar[j - 1] + 1;
}
}
changed = 1;
}
finished = i == 0;
}
if (!changed) {
/* Reset to first combination */
for (i = 0; i < k; i++) {
ar[i] = i;
}
}
}
return changed;
}
I'm looking for a function in ANSI C that would randomize an array just like PHP's shuffle() does. Is there such a function or do I have to write it on my own? And if I have to write it on my own, what's the best/most performant way to do it?
My ideas so far:
Iterate through the array for, say, 100 times and exchange a random index with another random index
Create a new array and fill it with random indices from the first one checking each time if the index is already taken (performance = 0 complexity = serious)
Pasted from Asmodiel's link to Ben Pfaff's Writings, for persistence:
#include <stdlib.h>
/* Arrange the N elements of ARRAY in random order.
Only effective if N is much smaller than RAND_MAX;
if this may not be the case, use a better random
number generator. */
void shuffle(int *array, size_t n)
{
if (n > 1)
{
size_t i;
for (i = 0; i < n - 1; i++)
{
size_t j = i + rand() / (RAND_MAX / (n - i) + 1);
int t = array[j];
array[j] = array[i];
array[i] = t;
}
}
}
EDIT: And here's a generic version that works for any type (int, struct, ...) through memcpy. With an example program to run, it requires VLAs, not every compiler supports this so you might want to change that to malloc (which will perform badly) or a static buffer large enough to accommodate any type you throw at it:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
/* compile and run with
* cc shuffle.c -o shuffle && ./shuffle */
#define NELEMS(x) (sizeof(x) / sizeof(x[0]))
/* arrange the N elements of ARRAY in random order.
* Only effective if N is much smaller than RAND_MAX;
* if this may not be the case, use a better random
* number generator. */
static void shuffle(void *array, size_t n, size_t size) {
char tmp[size];
char *arr = array;
size_t stride = size * sizeof(char);
if (n > 1) {
size_t i;
for (i = 0; i < n - 1; ++i) {
size_t rnd = (size_t) rand();
size_t j = i + rnd / (RAND_MAX / (n - i) + 1);
memcpy(tmp, arr + j * stride, size);
memcpy(arr + j * stride, arr + i * stride, size);
memcpy(arr + i * stride, tmp, size);
}
}
}
#define print_type(count, stmt) \
do { \
printf("["); \
for (size_t i = 0; i < (count); ++i) { \
stmt; \
} \
printf("]\n"); \
} while (0)
struct cmplex {
int foo;
double bar;
};
int main() {
srand(time(NULL));
int intarr[] = { 1, -5, 7, 3, 20, 2 };
print_type(NELEMS(intarr), printf("%d,", intarr[i]));
shuffle(intarr, NELEMS(intarr), sizeof(intarr[0]));
print_type(NELEMS(intarr), printf("%d,", intarr[i]));
struct cmplex cmparr[] = {
{ 1, 3.14 },
{ 5, 7.12 },
{ 9, 8.94 },
{ 20, 1.84 }
};
print_type(NELEMS(intarr), printf("{%d %f},", cmparr[i].foo, cmparr[i].bar));
shuffle(cmparr, NELEMS(cmparr), sizeof(cmparr[0]));
print_type(NELEMS(intarr), printf("{%d %f},", cmparr[i].foo, cmparr[i].bar));
return 0;
}
The following code ensures that the array will be shuffled based on a random seed taken from the usec time. Also this implements the Fisher–Yates shuffle properly. I've tested the output of this function and it looks good (even expectation of any array element being the first element after shuffle. Also even expectation for being the last).
void shuffle(int *array, size_t n) {
struct timeval tv;
gettimeofday(&tv, NULL);
int usec = tv.tv_usec;
srand48(usec);
if (n > 1) {
size_t i;
for (i = n - 1; i > 0; i--) {
size_t j = (unsigned int) (drand48()*(i+1));
int t = array[j];
array[j] = array[i];
array[i] = t;
}
}
}
I’ll just echo Neil Butterworth’s answer, and point out some trouble with your first idea:
You suggested,
Iterate through the array for, say, 100 times and exchange a random index with another random index
Make this rigorous. I'll assume the existence of randn(int n), a wrapper around some RNG, producing numbers evenly distributed in [0, n-1], and swap(int a[], size_t i, size_t j),
void swap(int a[], size_t i, size_t j) {
int temp = a[i]; a[i] = a[j]; a[j] = temp;
}
which swaps a[i] and a[j].
Now let’s implement your suggestion:
void silly_shuffle(size_t n, int a[n]) {
for (size_t i = 0; i < n; i++)
swap(a, randn(n), randn(n)); // swap two random elements
}
Notice that this is not any better than this simpler (but still wrong) version:
void bad_shuffle(size_t n, int a[n]) {
for (size_t i = 0; i < n; i++)
swap(a, i, randn(n));
}
Well, what’s wrong? Consider how many permutations these functions give you: With n (or 2×_n_ for silly_shuffle) random selections in [0, n-1], the code will “fairly” select one of _n_² (or 2×_n_²) ways to shuffle the deck. The trouble is that there are n! = _n_×(n-1)×⋯×2×1 possible arrangements of the array, and neither _n_² nor 2×_n_² is a multiple of n!, proving that some permutations are more likely than others.
The Fisher-Yates shuffle is actually equivalent to your second suggestion, only with some optimizations that change (performance = 0, complexity = serious) to (performance = very good, complexity = pretty simple). (Actually, I’m not sure that a faster or simpler correct version exists.)
void fisher_yates_shuffle(size_t n, int a[n]) {
for (size_t i = 0; i < n; i++)
swap(a, i, i+randn(n-1-i)); // swap element with random later element
}
ETA: See also this post on Coding Horror.
There isn't a function in the C standard to randomize an array.
Look at Knuth - he has algorithms for the job.
Or look at Bentley - Programming Pearls or More Programming Pearls.
Or look in almost any algorithms book.
Ensuring a fair shuffle (where every permutation of the original order is equally likely) is simple, but not trivial.
Here a solution that uses memcpy instead of assignment, so you can use it for array over arbitrary data. You need twice the memory of original array and the cost is linear O(n):
void main ()
{
int elesize = sizeof (int);
int i;
int r;
int src [20];
int tgt [20];
for (i = 0; i < 20; src [i] = i++);
srand ( (unsigned int) time (0) );
for (i = 20; i > 0; i --)
{
r = rand () % i;
memcpy (&tgt [20 - i], &src [r], elesize);
memcpy (&src [r], &src [i - 1], elesize);
}
for (i = 0; i < 20; printf ("%d ", tgt [i++] ) );
}
The function you are looking for is already present in the standard C library. Its name is qsort. Random sorting can be implemented as:
int rand_comparison(const void *a, const void *b)
{
(void)a; (void)b;
return rand() % 2 ? +1 : -1;
}
void shuffle(void *base, size_t nmemb, size_t size)
{
qsort(base, nmemb, size, rand_comparison);
}
The example:
int arr[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
srand(0); /* each permutation has its number here */
shuffle(arr, 10, sizeof(int));
...and the output is:
3, 4, 1, 0, 2, 7, 6, 9, 8, 5
Assuming you may want to just access an array randomly instead of actually shuffling it, you can use the degenerative case of a linear congruential pseudo-random number generator
X_n+1 = (a Xn+c) mod N
where a is coprime to N
generates a random cycle over all values 0:N
Naturally you could store this sequence in an empty array.
uint32_t gcd ( uint32_t a, uint32_t b )
{
if ( a==0 ) return b;
return gcd ( b%a, a );
}
uint32_t get_coprime(uint32_t r){
uint32_t min_val = r>>1;
for(int i =0;i<r*40;i++){
uint64_t sel = min_val + ( rand()%(r-min_val ));
if(gcd(sel,r)==1)
return sel;
}
return 0;
}
uint32_t next_val(uint32_t coprime, uint32_t cur, uint32_t N)
{
return (cur+coprime)%N;
}
// Example output Array A in random order
void shuffle(float * A, uint32_t N){
uint32_t coprime = get_coprime(N);
cur = rand()%N;
for(uint32_t i = 0;i<N;i++){
printf("%f\n",A[cur]);
cur = next_val(coprime, cur, N);
}
Just run the following code first and modify it for your needs:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define arr_size 10
// shuffle array
void shuffle(int *array, size_t n) {
if (n > 1) {
for (size_t i = 0; i < n - 1; i++) {
size_t j = i + rand() / (RAND_MAX / (n - i) + 1);
int t = array[j];
array[j] = array[i];
array[i] = t;
}
}
}
// display array elements
void display_array(int *array, size_t n){
for (int i = 0; i < n; i++)
printf("%d ", array[i]);
}
int main() {
srand(time(NULL)); // this line is necessary
int numbers[arr_size] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
printf("Given array: ");
display_array(numbers, arr_size);
shuffle(numbers, arr_size);
printf("\nShuffled array: ");
display_array(numbers, arr_size);
return 0;
}
You would have something like:
You get different shuffled arrays every time you run the code:
The same answer like Nomadiq but the Random is kept simple.
The Random will be the same if you call the function one after another:
#include <stdlib.h>
#include <time.h>
void shuffle(int aArray[], int cnt){
int temp, randomNumber;
time_t t;
srand((unsigned)time(&t));
for (int i=cnt-1; i>0; i--) {
temp = aArray[i];
randomNumber = (rand() % (i+1));
aArray[i] = aArray[randomNumber];
aArray[randomNumber] = temp;
}
}
I saw the answers and I've discovered an easy way to do it
#include <stdio.h>
#include <conio.h>
#include <time.h>
int main(void){
int base[8] = {1,2,3,4,5,6,7,8}, shuffled[8] = {0,0,0,0,0,0,0,0};
int index, sorted, discart=0;
srand(time(NULL));
for(index = 0; index<8; index++){
discart = 0;
while(discart==0){
sorted = rand() % 8;
if (shuffled[sorted] == 0){
//This here is just for control of what is happening
printf("-------------\n");
printf("index: %i\n sorted: %i \n", index,sorted);
printf("-------------\n");
shuffled[sorted] = base[index];
discart= 1;
}
}
}
//This "for" is just to exibe the sequence of items inside your array
for(index=0;index<8; index++){
printf("\n----\n");
printf("%i", shuffled[index]);
}
return 0;
}
Notice that this method doesn't allow duplicated items.
And at the end you can use either numbers and letters, just replacing them into the string.
This function will shuffle array based on random seed:
void shuffle(int *arr, int size)
{
srand(time(NULL));
for (int i = size - 1; i > 0; i--)
{
int j = rand() % (i + 1);
int tmp = arr[i];
arr[i] = arr[j];
arr[j] = tmp;
}
}
In the code example, I have a function that takes as parameters a pointer to an int ordered_array and a pointer to int shuffled_array and a number representing the length of both arrays. It picks in each loop a random number from the ordered_array and inserts it into the shuffled array.
void shuffle_array(int *ordered_array, int *shuffled_array, int len){
int index;
for(int i = 0; i < len; i++){
index = (rand() % (len - i));
shuffled_array[i] = ordered_array[index];
ordered_array[index] = ordered_array[len-i];
}
}
I didn't see it among answers so I propose this solution if it can help anybody:
static inline void shuffle(size_t n, int arr[])
{
size_t rng;
size_t i;
int tmp[n];
int tmp2[n];
memcpy(tmp, arr, sizeof(int) * n);
bzero(tmp2, sizeof(int) * n);
srand(time(NULL));
i = 0;
while (i < n)
{
rng = rand() % (n - i);
while (tmp2[rng] == 1)
++rng;
tmp2[rng] = 1;
arr[i] = tmp[rng];
++i;
}
}
I'm looking for a function in ANSI C that would randomize an array just like PHP's shuffle() does. Is there such a function or do I have to write it on my own? And if I have to write it on my own, what's the best/most performant way to do it?
My ideas so far:
Iterate through the array for, say, 100 times and exchange a random index with another random index
Create a new array and fill it with random indices from the first one checking each time if the index is already taken (performance = 0 complexity = serious)
Pasted from Asmodiel's link to Ben Pfaff's Writings, for persistence:
#include <stdlib.h>
/* Arrange the N elements of ARRAY in random order.
Only effective if N is much smaller than RAND_MAX;
if this may not be the case, use a better random
number generator. */
void shuffle(int *array, size_t n)
{
if (n > 1)
{
size_t i;
for (i = 0; i < n - 1; i++)
{
size_t j = i + rand() / (RAND_MAX / (n - i) + 1);
int t = array[j];
array[j] = array[i];
array[i] = t;
}
}
}
EDIT: And here's a generic version that works for any type (int, struct, ...) through memcpy. With an example program to run, it requires VLAs, not every compiler supports this so you might want to change that to malloc (which will perform badly) or a static buffer large enough to accommodate any type you throw at it:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
/* compile and run with
* cc shuffle.c -o shuffle && ./shuffle */
#define NELEMS(x) (sizeof(x) / sizeof(x[0]))
/* arrange the N elements of ARRAY in random order.
* Only effective if N is much smaller than RAND_MAX;
* if this may not be the case, use a better random
* number generator. */
static void shuffle(void *array, size_t n, size_t size) {
char tmp[size];
char *arr = array;
size_t stride = size * sizeof(char);
if (n > 1) {
size_t i;
for (i = 0; i < n - 1; ++i) {
size_t rnd = (size_t) rand();
size_t j = i + rnd / (RAND_MAX / (n - i) + 1);
memcpy(tmp, arr + j * stride, size);
memcpy(arr + j * stride, arr + i * stride, size);
memcpy(arr + i * stride, tmp, size);
}
}
}
#define print_type(count, stmt) \
do { \
printf("["); \
for (size_t i = 0; i < (count); ++i) { \
stmt; \
} \
printf("]\n"); \
} while (0)
struct cmplex {
int foo;
double bar;
};
int main() {
srand(time(NULL));
int intarr[] = { 1, -5, 7, 3, 20, 2 };
print_type(NELEMS(intarr), printf("%d,", intarr[i]));
shuffle(intarr, NELEMS(intarr), sizeof(intarr[0]));
print_type(NELEMS(intarr), printf("%d,", intarr[i]));
struct cmplex cmparr[] = {
{ 1, 3.14 },
{ 5, 7.12 },
{ 9, 8.94 },
{ 20, 1.84 }
};
print_type(NELEMS(intarr), printf("{%d %f},", cmparr[i].foo, cmparr[i].bar));
shuffle(cmparr, NELEMS(cmparr), sizeof(cmparr[0]));
print_type(NELEMS(intarr), printf("{%d %f},", cmparr[i].foo, cmparr[i].bar));
return 0;
}
The following code ensures that the array will be shuffled based on a random seed taken from the usec time. Also this implements the Fisher–Yates shuffle properly. I've tested the output of this function and it looks good (even expectation of any array element being the first element after shuffle. Also even expectation for being the last).
void shuffle(int *array, size_t n) {
struct timeval tv;
gettimeofday(&tv, NULL);
int usec = tv.tv_usec;
srand48(usec);
if (n > 1) {
size_t i;
for (i = n - 1; i > 0; i--) {
size_t j = (unsigned int) (drand48()*(i+1));
int t = array[j];
array[j] = array[i];
array[i] = t;
}
}
}
I’ll just echo Neil Butterworth’s answer, and point out some trouble with your first idea:
You suggested,
Iterate through the array for, say, 100 times and exchange a random index with another random index
Make this rigorous. I'll assume the existence of randn(int n), a wrapper around some RNG, producing numbers evenly distributed in [0, n-1], and swap(int a[], size_t i, size_t j),
void swap(int a[], size_t i, size_t j) {
int temp = a[i]; a[i] = a[j]; a[j] = temp;
}
which swaps a[i] and a[j].
Now let’s implement your suggestion:
void silly_shuffle(size_t n, int a[n]) {
for (size_t i = 0; i < n; i++)
swap(a, randn(n), randn(n)); // swap two random elements
}
Notice that this is not any better than this simpler (but still wrong) version:
void bad_shuffle(size_t n, int a[n]) {
for (size_t i = 0; i < n; i++)
swap(a, i, randn(n));
}
Well, what’s wrong? Consider how many permutations these functions give you: With n (or 2×_n_ for silly_shuffle) random selections in [0, n-1], the code will “fairly” select one of _n_² (or 2×_n_²) ways to shuffle the deck. The trouble is that there are n! = _n_×(n-1)×⋯×2×1 possible arrangements of the array, and neither _n_² nor 2×_n_² is a multiple of n!, proving that some permutations are more likely than others.
The Fisher-Yates shuffle is actually equivalent to your second suggestion, only with some optimizations that change (performance = 0, complexity = serious) to (performance = very good, complexity = pretty simple). (Actually, I’m not sure that a faster or simpler correct version exists.)
void fisher_yates_shuffle(size_t n, int a[n]) {
for (size_t i = 0; i < n; i++)
swap(a, i, i+randn(n-1-i)); // swap element with random later element
}
ETA: See also this post on Coding Horror.
There isn't a function in the C standard to randomize an array.
Look at Knuth - he has algorithms for the job.
Or look at Bentley - Programming Pearls or More Programming Pearls.
Or look in almost any algorithms book.
Ensuring a fair shuffle (where every permutation of the original order is equally likely) is simple, but not trivial.
Here a solution that uses memcpy instead of assignment, so you can use it for array over arbitrary data. You need twice the memory of original array and the cost is linear O(n):
void main ()
{
int elesize = sizeof (int);
int i;
int r;
int src [20];
int tgt [20];
for (i = 0; i < 20; src [i] = i++);
srand ( (unsigned int) time (0) );
for (i = 20; i > 0; i --)
{
r = rand () % i;
memcpy (&tgt [20 - i], &src [r], elesize);
memcpy (&src [r], &src [i - 1], elesize);
}
for (i = 0; i < 20; printf ("%d ", tgt [i++] ) );
}
The function you are looking for is already present in the standard C library. Its name is qsort. Random sorting can be implemented as:
int rand_comparison(const void *a, const void *b)
{
(void)a; (void)b;
return rand() % 2 ? +1 : -1;
}
void shuffle(void *base, size_t nmemb, size_t size)
{
qsort(base, nmemb, size, rand_comparison);
}
The example:
int arr[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
srand(0); /* each permutation has its number here */
shuffle(arr, 10, sizeof(int));
...and the output is:
3, 4, 1, 0, 2, 7, 6, 9, 8, 5
Assuming you may want to just access an array randomly instead of actually shuffling it, you can use the degenerative case of a linear congruential pseudo-random number generator
X_n+1 = (a Xn+c) mod N
where a is coprime to N
generates a random cycle over all values 0:N
Naturally you could store this sequence in an empty array.
uint32_t gcd ( uint32_t a, uint32_t b )
{
if ( a==0 ) return b;
return gcd ( b%a, a );
}
uint32_t get_coprime(uint32_t r){
uint32_t min_val = r>>1;
for(int i =0;i<r*40;i++){
uint64_t sel = min_val + ( rand()%(r-min_val ));
if(gcd(sel,r)==1)
return sel;
}
return 0;
}
uint32_t next_val(uint32_t coprime, uint32_t cur, uint32_t N)
{
return (cur+coprime)%N;
}
// Example output Array A in random order
void shuffle(float * A, uint32_t N){
uint32_t coprime = get_coprime(N);
cur = rand()%N;
for(uint32_t i = 0;i<N;i++){
printf("%f\n",A[cur]);
cur = next_val(coprime, cur, N);
}
Just run the following code first and modify it for your needs:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define arr_size 10
// shuffle array
void shuffle(int *array, size_t n) {
if (n > 1) {
for (size_t i = 0; i < n - 1; i++) {
size_t j = i + rand() / (RAND_MAX / (n - i) + 1);
int t = array[j];
array[j] = array[i];
array[i] = t;
}
}
}
// display array elements
void display_array(int *array, size_t n){
for (int i = 0; i < n; i++)
printf("%d ", array[i]);
}
int main() {
srand(time(NULL)); // this line is necessary
int numbers[arr_size] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
printf("Given array: ");
display_array(numbers, arr_size);
shuffle(numbers, arr_size);
printf("\nShuffled array: ");
display_array(numbers, arr_size);
return 0;
}
You would have something like:
You get different shuffled arrays every time you run the code:
The same answer like Nomadiq but the Random is kept simple.
The Random will be the same if you call the function one after another:
#include <stdlib.h>
#include <time.h>
void shuffle(int aArray[], int cnt){
int temp, randomNumber;
time_t t;
srand((unsigned)time(&t));
for (int i=cnt-1; i>0; i--) {
temp = aArray[i];
randomNumber = (rand() % (i+1));
aArray[i] = aArray[randomNumber];
aArray[randomNumber] = temp;
}
}
I saw the answers and I've discovered an easy way to do it
#include <stdio.h>
#include <conio.h>
#include <time.h>
int main(void){
int base[8] = {1,2,3,4,5,6,7,8}, shuffled[8] = {0,0,0,0,0,0,0,0};
int index, sorted, discart=0;
srand(time(NULL));
for(index = 0; index<8; index++){
discart = 0;
while(discart==0){
sorted = rand() % 8;
if (shuffled[sorted] == 0){
//This here is just for control of what is happening
printf("-------------\n");
printf("index: %i\n sorted: %i \n", index,sorted);
printf("-------------\n");
shuffled[sorted] = base[index];
discart= 1;
}
}
}
//This "for" is just to exibe the sequence of items inside your array
for(index=0;index<8; index++){
printf("\n----\n");
printf("%i", shuffled[index]);
}
return 0;
}
Notice that this method doesn't allow duplicated items.
And at the end you can use either numbers and letters, just replacing them into the string.
This function will shuffle array based on random seed:
void shuffle(int *arr, int size)
{
srand(time(NULL));
for (int i = size - 1; i > 0; i--)
{
int j = rand() % (i + 1);
int tmp = arr[i];
arr[i] = arr[j];
arr[j] = tmp;
}
}
In the code example, I have a function that takes as parameters a pointer to an int ordered_array and a pointer to int shuffled_array and a number representing the length of both arrays. It picks in each loop a random number from the ordered_array and inserts it into the shuffled array.
void shuffle_array(int *ordered_array, int *shuffled_array, int len){
int index;
for(int i = 0; i < len; i++){
index = (rand() % (len - i));
shuffled_array[i] = ordered_array[index];
ordered_array[index] = ordered_array[len-i];
}
}
I didn't see it among answers so I propose this solution if it can help anybody:
static inline void shuffle(size_t n, int arr[])
{
size_t rng;
size_t i;
int tmp[n];
int tmp2[n];
memcpy(tmp, arr, sizeof(int) * n);
bzero(tmp2, sizeof(int) * n);
srand(time(NULL));
i = 0;
while (i < n)
{
rng = rand() % (n - i);
while (tmp2[rng] == 1)
++rng;
tmp2[rng] = 1;
arr[i] = tmp[rng];
++i;
}
}