Is there a function to generate a random int number in C? Or will I have to use a third party library?
Note: Don't use rand() for security. If you need a cryptographically secure number, see this answer instead.
#include <time.h>
#include <stdlib.h>
srand(time(NULL)); // Initialization, should only be called once.
int r = rand(); // Returns a pseudo-random integer between 0 and RAND_MAX.
On Linux, you might prefer to use random and srandom.
The rand() function in <stdlib.h> returns a pseudo-random integer between 0 and RAND_MAX. You can use srand(unsigned int seed) to set a seed.
It's common practice to use the % operator in conjunction with rand() to get a different range (though bear in mind that this throws off the uniformity somewhat). For example:
/* random int between 0 and 19 */
int r = rand() % 20;
If you really care about uniformity you can do something like this:
/* Returns an integer in the range [0, n).
*
* Uses rand(), and so is affected-by/affects the same seed.
*/
int randint(int n) {
if ((n - 1) == RAND_MAX) {
return rand();
} else {
// Supporting larger values for n would requires an even more
// elaborate implementation that combines multiple calls to rand()
assert (n <= RAND_MAX)
// Chop off all of the values that would cause skew...
int end = RAND_MAX / n; // truncate skew
assert (end > 0);
end *= n;
// ... and ignore results from rand() that fall above that limit.
// (Worst case the loop condition should succeed 50% of the time,
// so we can expect to bail out of this loop pretty quickly.)
int r;
while ((r = rand()) >= end);
return r % n;
}
}
If you need secure random characters or integers:
As addressed in how to safely generate random numbers in various programming languages, you'll want to do one of the following:
Use libsodium's randombytes API
Re-implement what you need from libsodium's sysrandom implementation yourself, very carefully
More broadly, use /dev/urandom, not /dev/random. Not OpenSSL (or other userspace PRNGs).
For example:
#include "sodium.h"
int foo()
{
char myString[32];
uint32_t myInt;
if (sodium_init() < 0) {
/* panic! the library couldn't be initialized, it is not safe to use */
return 1;
}
/* myString will be an array of 32 random bytes, not null-terminated */
randombytes_buf(myString, 32);
/* myInt will be a random number between 0 and 9 */
myInt = randombytes_uniform(10);
}
randombytes_uniform() is cryptographically secure and unbiased.
Lets go through this. First we use the srand() function to seed the randomizer. Basically, the computer can generate random numbers based on the number that is fed to srand(). If you gave the same seed value, then the same random numbers would be generated every time.
Therefore, we have to seed the randomizer with a value that is always changing. We do this by feeding it the value of the current time with the time() function.
Now, when we call rand(), a new random number will be produced every time.
#include <stdio.h>
int random_number(int min_num, int max_num);
int main(void)
{
printf("Min : 1 Max : 40 %d\n", random_number(1,40));
printf("Min : 100 Max : 1000 %d\n",random_number(100,1000));
return 0;
}
int random_number(int min_num, int max_num)
{
int result = 0, low_num = 0, hi_num = 0;
if (min_num < max_num)
{
low_num = min_num;
hi_num = max_num + 1; // include max_num in output
} else {
low_num = max_num + 1; // include max_num in output
hi_num = min_num;
}
srand(time(NULL));
result = (rand() % (hi_num - low_num)) + low_num;
return result;
}
If you need better quality pseudo random numbers than what stdlib provides, check out Mersenne Twister. It's faster, too. Sample implementations are plentiful, for example here.
The standard C function is rand(). It's good enough to deal cards for solitaire, but it's awful. Many implementations of rand() cycle through a short list of numbers, and the low bits have shorter cycles. The way that some programs call rand() is awful, and calculating a good seed to pass to srand() is hard.
The best way to generate random numbers in C is to use a third-party library like OpenSSL. For example,
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <openssl/rand.h>
/* Random integer in [0, limit) */
unsigned int random_uint(unsigned int limit) {
union {
unsigned int i;
unsigned char c[sizeof(unsigned int)];
} u;
do {
if (!RAND_bytes(u.c, sizeof(u.c))) {
fprintf(stderr, "Can't get random bytes!\n");
exit(1);
}
} while (u.i < (-limit % limit)); /* u.i < (2**size % limit) */
return u.i % limit;
}
/* Random double in [0.0, 1.0) */
double random_double() {
union {
uint64_t i;
unsigned char c[sizeof(uint64_t)];
} u;
if (!RAND_bytes(u.c, sizeof(u.c))) {
fprintf(stderr, "Can't get random bytes!\n");
exit(1);
}
/* 53 bits / 2**53 */
return (u.i >> 11) * (1.0/9007199254740992.0);
}
int main() {
printf("Dice: %d\n", (int)(random_uint(6) + 1));
printf("Double: %f\n", random_double());
return 0;
}
Why so much code? Other languages like Java and Ruby have functions for random integers or floats. OpenSSL only gives random bytes, so I try to mimic how Java or Ruby would transform them into integers or floats.
For integers, we want to avoid modulo bias. Suppose that we got some random 4 digit integers from rand() % 10000, but rand() can only return 0 to 32767 (as it does in Microsoft Windows). Each number from 0 to 2767 would appear more often than each number from 2768 to 9999. To remove the bias, we can retry rand() while the value is below 2768, because the 30000 values from 2768 to 32767 map uniformly onto the 10000 values from 0 to 9999.
For floats, we want 53 random bits, because a double holds 53 bits of precision (assuming it's an IEEE double). If we use more than 53 bits, we get rounding bias. Some programmers write code like rand() / (double)RAND_MAX, but rand() might return only 31 bits, or only 15 bits in Windows.
OpenSSL's RAND_bytes() seeds itself, perhaps by reading /dev/urandom in Linux. If we need many random numbers, it would be too slow to read them all from /dev/urandom, because they must be copied from the kernel. It is faster to allow OpenSSL to generate more random numbers from a seed.
More about random numbers:
Perl's Perl_seed() is an example of how to calculate a seed in C for srand(). It mixes bits from the current time, the process ID, and some pointers, if it can't read /dev/urandom.
OpenBSD's arc4random_uniform() explains modulo bias.
Java API for java.util.Random describes algorithms for removing bias from random integers, and packing 53 bits into random floats.
If your system supports the arc4random family of functions I would recommend using those instead the standard rand function.
The arc4random family includes:
uint32_t arc4random(void)
void arc4random_buf(void *buf, size_t bytes)
uint32_t arc4random_uniform(uint32_t limit)
void arc4random_stir(void)
void arc4random_addrandom(unsigned char *dat, int datlen)
arc4random returns a random 32-bit unsigned integer.
arc4random_buf puts random content in it's parameter buf : void *. The amount of content is determined by the bytes : size_t parameter.
arc4random_uniform returns a random 32-bit unsigned integer which follows the rule: 0 <= arc4random_uniform(limit) < limit, where limit is also an unsigned 32-bit integer.
arc4random_stir reads data from /dev/urandom and passes the data to arc4random_addrandom to additionally randomize it's internal random number pool.
arc4random_addrandom is used by arc4random_stir to populate it's internal random number pool according to the data passed to it.
If you do not have these functions, but you are on Unix, then you can use this code:
/* This is C, not C++ */
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h> /* exit */
#include <stdio.h> /* printf */
int urandom_fd = -2;
void urandom_init() {
urandom_fd = open("/dev/urandom", O_RDONLY);
if (urandom_fd == -1) {
int errsv = urandom_fd;
printf("Error opening [/dev/urandom]: %i\n", errsv);
exit(1);
}
}
unsigned long urandom() {
unsigned long buf_impl;
unsigned long *buf = &buf_impl;
if (urandom_fd == -2) {
urandom_init();
}
/* Read sizeof(long) bytes (usually 8) into *buf, which points to buf_impl */
read(urandom_fd, buf, sizeof(long));
return buf_impl;
}
The urandom_init function opens the /dev/urandom device, and puts the file descriptor in urandom_fd.
The urandom function is basically the same as a call to rand, except more secure, and it returns a long (easily changeable).
However, /dev/urandom can be a little slow, so it is recommended that you use it as a seed for a different random number generator.
If your system does not have a /dev/urandom, but does have a /dev/random or similar file, then you can simply change the path passed to open in urandom_init. The calls and APIs used in urandom_init and urandom are (I believe) POSIX-compliant, and as such, should work on most, if not all POSIX compliant systems.
Notes: A read from /dev/urandom will NOT block if there is insufficient entropy available, so values generated under such circumstances may be cryptographically insecure. If you are worried about that, then use /dev/random, which will always block if there is insufficient entropy.
If you are on another system(i.e. Windows), then use rand or some internal Windows specific platform-dependent non-portable API.
Wrapper function for urandom, rand, or arc4random calls:
#define RAND_IMPL /* urandom(see large code block) | rand | arc4random */
int myRandom(int bottom, int top){
return (RAND_IMPL() % (top - bottom)) + bottom;
}
STL doesn't exist for C. You have to call rand, or better yet, random. These are declared in the standard library header stdlib.h. rand is POSIX, random is a BSD spec function.
The difference between rand and random is that random returns a much more usable 32-bit random number, and rand typically returns a 16-bit number. The BSD manpages show that the lower bits of rand are cyclic and predictable, so rand is potentially useless for small numbers.
Have a look at ISAAC (Indirection, Shift, Accumulate, Add, and Count). Its uniformly distributed and has an average cycle length of 2^8295.
This is a good way to get a random number between two numbers of your choice.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define randnum(min, max) \
((rand() % (int)(((max) + 1) - (min))) + (min))
int main()
{
srand(time(NULL));
printf("%d\n", randnum(1, 70));
}
Output the first time: 39
Output the second time: 61
Output the third time: 65
You can change the values after randnum to whatever numbers you choose, and it will generate a random number for you between those two numbers.
I had a serious issue with pseudo random number generator in my recent application: I repeatedly called my C program via a Python script and I was using as seed the following code:
srand(time(NULL))
However, since:
rand will generate the same pseudo random sequence give the same seed in srand (see man srand);
As already stated, time function changes only second from second: if your application is run multiple times within the same second, time will return the same value each time.
My program generated the same sequence of numbers.
You can do 3 things to solve this problem:
mix time output with some other information changing on runs (in my application, the output name):
srand(time(NULL) | getHashOfString(outputName))
I used djb2 as my hash function.
Increase time resolution. On my platform, clock_gettime was available, so I use it:
#include<time.h>
struct timespec nanos;
clock_gettime(CLOCK_MONOTONIC, &nanos)
srand(nanos.tv_nsec);
Use both methods together:
#include<time.h>
struct timespec nanos;
clock_gettime(CLOCK_MONOTONIC, &nanos)
srand(nanos.tv_nsec | getHashOfString(outputName));
Option 3 ensures you (as far as I know) the best seed randomness, but it may create a difference only on very fast application.
In my opinion option 2 is a safe bet.
Well, STL is C++, not C, so I don't know what you want. If you want C, however, there is the rand() and srand() functions:
int rand(void);
void srand(unsigned seed);
These are both part of ANSI C. There is also the random() function:
long random(void);
But as far as I can tell, random() is not standard ANSI C. A third-party library may not be a bad idea, but it all depends on how random of a number you really need to generate.
You want to use rand(). Note (VERY IMPORTANT): make sure to set the seed for the rand function. If you do not, your random numbers are not truly random. This is very, very, very important. Thankfully, you can usually use some combination of the system ticks timer and the date to get a good seed.
FWIW, the answer is that yes, there is a stdlib.h function called rand; this function is tuned primarily for speed and distribution, not for unpredictability. Almost all built-in random functions for various languages and frameworks use this function by default. There are also "cryptographic" random number generators that are much less predictable, but run much slower. These should be used in any sort of security-related application.
This is hopefully a bit more random than just using srand(time(NULL)).
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
srand((unsigned int)**main + (unsigned int)&argc + (unsigned int)time(NULL));
srand(rand());
for (int i = 0; i < 10; i++)
printf("%d\n", rand());
}
C Program to generate random number between 9 and 50
#include <time.h>
#include <stdlib.h>
int main()
{
srand(time(NULL));
int lowerLimit = 10, upperLimit = 50;
int r = lowerLimit + rand() % (upperLimit - lowerLimit);
printf("%d", r);
}
In general we can generate a random number between lowerLimit and upperLimit-1
i.e lowerLimit is inclusive or say r ∈ [ lowerLimit, upperLimit )
On modern x86_64 CPUs you can use the hardware random number generator via _rdrand64_step()
Example code:
#include <immintrin.h>
uint64_t randVal;
if(!_rdrand64_step(&randVal)) {
// Report an error here: random number generation has failed!
}
// If no error occured, randVal contains a random 64-bit number
rand() is the most convenient way to generate random numbers.
You may also catch random number from any online service like random.org.
#include <stdio.h>
#include <stdlib.h>
void main()
{
int visited[100];
int randValue, a, b, vindex = 0;
randValue = (rand() % 100) + 1;
while (vindex < 100) {
for (b = 0; b < vindex; b++) {
if (visited[b] == randValue) {
randValue = (rand() % 100) + 1;
b = 0;
}
}
visited[vindex++] = randValue;
}
for (a = 0; a < 100; a++)
printf("%d ", visited[a]);
}
Despite all the people suggestion rand() here, you don't want to use rand() unless you have to! The random numbers that rand() produces are often very bad. To quote from the Linux man page:
The versions of rand() and srand() in the Linux C Library use the same random number generator as random(3) and srandom(3), so the lower-order bits should be as random as the higher-order bits. However, on older rand() implementations, and on current implementations on different systems, the lower-order bits are much less random than the higher-order bits. Do not use this function in applications intended to be portable when good randomness is needed. (Use random(3) instead.)
Regarding portability, random() is also defined by the POSIX standard for quite some time now. rand() is older, it appeared already in the first POSIX.1 spec (IEEE Std 1003.1-1988), whereas random() first appeared in POSIX.1-2001 (IEEE Std 1003.1-2001), yet the current POSIX standard is already POSIX.1-2008 (IEEE Std 1003.1-2008), which received an update just a year ago (IEEE Std 1003.1-2008, 2016 Edition). So I would consider random() to be very portable.
POSIX.1-2001 also introduced the lrand48() and mrand48() functions, see here:
This family of functions shall generate pseudo-random numbers using a linear congruential algorithm and 48-bit integer arithmetic.
And a pretty good pseudo random source is the arc4random() function that is available on many systems. Not part of any official standard, appeared in BSD around 1997 but you can find it on systems like Linux and macOS/iOS.
#include <stdio.h>
#include <dos.h>
int random(int range);
int main(void)
{
printf("%d", random(10));
return 0;
}
int random(int range)
{
struct time t;
int r;
gettime(&t);
r = t.ti_sec % range;
return r;
}
#include<stdio.h>
#include<stdlib.h>
#include<time.h>
//generate number in range [min,max)
int random(int min, int max){
int number = min + rand() % (max - min);
return number;
}
//Driver code
int main(){
srand(time(NULL));
for(int i = 1; i <= 10; i++){
printf("%d\t", random(10, 100));
}
return 0;
}
For Linux C applications:
This is my reworked code from an answer above that follows my C code practices and returns a random buffer of any size (with proper return codes, etc.). Make sure to call urandom_open() once at the beginning of your program.
int gUrandomFd = -1;
int urandom_open(void)
{
if (gUrandomFd == -1) {
gUrandomFd = open("/dev/urandom", O_RDONLY);
}
if (gUrandomFd == -1) {
fprintf(stderr, "Error opening /dev/urandom: errno [%d], strerrer [%s]\n",
errno, strerror(errno));
return -1;
} else {
return 0;
}
}
void urandom_close(void)
{
close(gUrandomFd);
gUrandomFd = -1;
}
//
// This link essentially validates the merits of /dev/urandom:
// http://sockpuppet.org/blog/2014/02/25/safely-generate-random-numbers/
//
int getRandomBuffer(uint8_t *buf, int size)
{
int ret = 0; // Return value
if (gUrandomFd == -1) {
fprintf(stderr, "Urandom (/dev/urandom) file not open\n");
return -1;
}
ret = read(gUrandomFd, buf, size);
if (ret != size) {
fprintf(stderr, "Only read [%d] bytes, expected [%d]\n",
ret, size);
return -1;
} else {
return 0;
}
}
Here is my approach (a wrapper around rand()):
I also scale to allow a case where min is INT_MIN and max is INT_MAX, which is normally not possible with rand() alone since it returns values from 0 to RAND_MAX, inclusive (1/2 that range).
Use it like this:
const int MIN = 1;
const int MAX = 1024;
// Get a pseudo-random number between MIN and MAX, **inclusive**.
// Seeding of the pseudo-random number generator automatically occurs
// the very first time you call it.
int random_num = utils_rand(MIN, MAX);
Definitions and doxygen descriptions:
#include <assert.h>
#include <stdbool.h>
#include <stdlib.h>
/// \brief Use linear interpolation to rescale, or "map" value `val` from range
/// `in_min` to `in_max`, inclusive, to range `out_min` to `out_max`, inclusive.
/// \details Similar to Arduino's ingenious `map()` function:
/// https://www.arduino.cc/reference/en/language/functions/math/map/
///
/// TODO(gabriel): turn this into a gcc statement expression instead to prevent the potential for
/// the "double evaluation" bug. See `MIN()` and `MAX()` above.
#define UTILS_MAP(val, in_min, in_max, out_min, out_max) \
(((val) - (in_min)) * ((out_max) - (out_min)) / ((in_max) - (in_min)) + (out_min))
/// \brief Obtain a pseudo-random integer value between `min` and `max`, **inclusive**.
/// \details 1. If `(max - min + 1) > RAND_MAX`, then the range of values returned will be
/// **scaled** to the range `max - min + 1`, and centered over the center of the
/// range at `(min + max)/2`. Scaling the numbers means that in the case of scaling,
/// not all numbers can even be reached. However, you will still be assured to have
/// a random distribution of numbers across the full range.
/// 2. Also, the first time per program run that you call this function, it will
/// automatically seed the pseudo-random number generator with your system's
/// current time in seconds.
/// \param[in] min The minimum pseudo-random number you'd like, inclusive. Can be positive
/// OR negative.
/// \param[in] max The maximum pseudo-random number you'd like, inclusive. Can be positive
/// OR negative.
/// \return A pseudo-random integer value between `min` and `max`, **inclusive**.
int utils_rand(int min, int max)
{
static bool first_run = true;
if (first_run)
{
// seed the pseudo-random number generator with the seconds time the very first run
time_t time_now_sec = time(NULL);
srand(time_now_sec);
first_run = false;
}
int range = max - min + 1;
int random_num = rand(); // random num from 0 to RAND_MAX, inclusive
if (range > RAND_MAX)
{
static_assert(
sizeof(long int) > sizeof(int),
"This must be true or else the below mapping/scaling may have undefined overflow "
"and not work properly. In such a case, try casting to `long long int` instead of "
"just `long int`, and update this static_assert accordingly.");
random_num = UTILS_MAP((long int)random_num, (long int)0, (long int)RAND_MAX, (long int)min,
(long int)max);
return random_num;
}
// This is presumably a faster approach than the map/scaling function above, so do this faster
// approach below whenever you don't **have** to do the more-complicated approach above.
random_num %= range;
random_num += min;
return random_num;
}
See also:
[I discovered this Q&A after writing my answer above, but it is obviously very relevant, and they do the same thing I do for the non-scaling range case] How do I get a specific range of numbers from rand()?
[I NEED TO STUDY AND READ THIS ANSWER MORE STILL--seems to have some good points about retaining good randomness by not using modulus alone] How do I get a specific range of numbers from rand()?
http://c-faq.com/lib/randrange.html
If you need, say, 128 secure random bits, the RFC 1750 compliant solution is to read hardware source that is known to generate useable bits of entropy (such as a spinning disk). Better yet, good implementations should combine multiple sources using a mixing function, and finally de-skew the distribution of their output, by re-mapping or deleting outputs.
If you need more bits than that, the compliant thing to do is start with sequence of 128 secure random bits and stretch it to a desired length, map it to human readable text, etc.
If you want to generate a secure random number in C I would follow the source code here:
https://wiki.sei.cmu.edu/confluence/display/c/MSC30-C.+Do+not+use+the+rand%28%29+function+for+generating+pseudorandom+numbers
Note that for Windows BCryptGenRandom is used, not CryptGenRandom which has become unsecure within the past two decades. You can confirm for yourself that BCryptGenRandom is compliant with RFC 1750.
For POSIX-compliant operating systems, e.g. Ubuntu (a flavor of Linux), you can simply read from /dev/urandom or /dev/random, which is a file-like interface to a device that generates bits of entropy by combining multiple sources in an RFC 1750 compliant fashion. You can read a desired number of bytes from these "files" with read or fread just like you would any other file, but note that reads from /dev/random will block until a enough new bits of entropy are available, whereas /dev/urandom will not, which can be a security issue. You can get around that by checking the size of the available entropy pool, either my reading from entropy_avail, or by using ioctl.
The glibc-specific function (that should be found in most of Linux environments) related to this is random(), or you may be interested with its thread-safe version random_r(). You have to initialize the struct random_data with initstate_r() prior to passing it to random_r().
Here is quick code sample :
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
void xxx (void) {
unsigned int seed = (unsigned int) time(NULL);
char rnd_state[17] = {0};
struct random_data rnd_st_buf = {0};
initstate_r(seed, &rnd_state[0], 17, &rnd_st_buf);
for(size_t idx = 0; idx < 8; idx++) {
int32_t rnd_int = 0;
char rnd_seq_str[6] = {0};
random_r(&rnd_st_buf, &rnd_int);
memcpy((char *)&rnd_seq_str[0], (char *)&rnd_int, 4);
printf("random number : 0x%08x, \n", rnd_int);
}
}
You can generate random chars, then view them as int :
#include <stdlib.h>
#include <stdio.h>
typedef double rand_type; // change double to int
rand_type my_rand() {
char buff[sizeof(rand_type)];
for (size_t i = 0 ; i < sizeof(rand_type) ; ++i)
buff[i] = (char) rand();
return *(rand_type *) buff;
}
int main() {
int i ; // srand as you want
for (i = 0 ; i < 10 ; ++i)
printf("%g\n", my_rand()); // change %g to %d
return 0 ;
}
You can also use mathgl library #include <mgl2/mgl_cf.h> (though first you need to install it, I own installed through MSYS2) with function mgl_rnd(). It also have kinds of distribution like uniform, guassian and more. It's ez to use. But I dont know about it's characteristic.
Hearing a good explanation of why using rand() to produce uniformly distributed random numbers in a given range is a bad idea, I decided to take a look at how skewed the output actually is. My test case was fair dice throwing. Here's the C code:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(int argc, char *argv[])
{
int i;
int dice[6];
for (i = 0; i < 6; i++)
dice[i] = 0;
srand(time(NULL));
const int TOTAL = 10000000;
for (i = 0; i < TOTAL; i++)
dice[(rand() % 6)] += 1;
double pers = 0.0, tpers = 0.0;
for (i = 0; i < 6; i++) {
pers = (dice[i] * 100.0) / TOTAL;
printf("\t%1d %5.2f%%\n", dice[i], pers);
tpers += pers;
}
printf("\ttotal: %6.2f%%\n", tpers);
}
and here's its output:
$ gcc -o t3 t3.c
$ ./t3
1666598 16.67%
1668630 16.69%
1667682 16.68%
1666049 16.66%
1665948 16.66%
1665093 16.65%
total: 100.00%
$ ./t3
1667634 16.68%
1665914 16.66%
1665542 16.66%
1667828 16.68%
1663649 16.64%
1669433 16.69%
total: 100.00%
I don't know how uniform you need your random numbers to be, but the above appears uniform enough for most needs.
Edit: it would be a good idea to initialize the PRNG with something better than time(NULL).
My minimalistic solution should work for random numbers in range [min, max). Use srand(time(NULL)) before invoking the function.
int range_rand(int min_num, int max_num) {
if (min_num >= max_num) {
fprintf(stderr, "min_num is greater or equal than max_num!\n");
}
return min_num + (rand() % (max_num - min_num));
}
I'm working on Project Euler #14 in C and have figured out the basic algorithm; however, it runs insufferably slow for large numbers, e.g. 2,000,000 as wanted; I presume because it has to generate the sequence over and over again, even though there should be a way to store known sequences (e.g., once we get to a 16, we know from previous experience that the next numbers are 8, 4, 2, then 1).
I'm not exactly sure how to do this with C's fixed-length array, but there must be a good way (that's amazingly efficient, I'm sure). Thanks in advance.
Here's what I currently have, if it helps.
#include <stdio.h>
#define UPTO 2000000
int collatzlen(int n);
int main(){
int i, l=-1, li=-1, c=0;
for(i=1; i<=UPTO; i++){
if( (c=collatzlen(i)) > l) l=c, li=i;
}
printf("Greatest length:\t\t%7d\nGreatest starting point:\t%7d\n", l, li);
return 1;
}
/* n != 0 */
int collatzlen(int n){
int len = 0;
while(n>1) n = (n%2==0 ? n/2 : 3*n+1), len+=1;
return len;
}
Your original program needs 3.5 seconds on my machine. Is it insufferably slow for you?
My dirty and ugly version needs 0.3 seconds. It uses a global array to store the values already calculated. And use them in future calculations.
int collatzlen2(unsigned long n);
static unsigned long array[2000000 + 1];//to store those already calculated
int main()
{
int i, l=-1, li=-1, c=0;
int x;
for(x = 0; x < 2000000 + 1; x++) {
array[x] = -1;//use -1 to denote not-calculated yet
}
for(i=1; i<=UPTO; i++){
if( (c=collatzlen2(i)) > l) l=c, li=i;
}
printf("Greatest length:\t\t%7d\nGreatest starting point:\t%7d\n", l, li);
return 1;
}
int collatzlen2(unsigned long n){
unsigned long len = 0;
unsigned long m = n;
while(n > 1){
if(n > 2000000 || array[n] == -1){ // outside range or not-calculated yet
n = (n%2 == 0 ? n/2 : 3*n+1);
len+=1;
}
else{ // if already calculated, use the value
len += array[n];
n = 1; // to get out of the while-loop
}
}
array[m] = len;
return len;
}
Given that this is essentially a throw-away program (i.e. once you've run it and got the answer, you're not going to be supporting it for years :), I would suggest having a global variable to hold the lengths of sequences already calculated:
int lengthfrom[UPTO] = {};
If your maximum size is a few million, then we're talking megabytes of memory, which should easily fit in RAM at once.
The above will initialise the array to zeros at startup. In your program - for each iteration, check whether the array contains zero. If it does - you'll have to keep going with the computation. If not - then you know that carrying on would go on for that many more iterations, so just add that to the number you've done so far and you're done. And then store the new result in the array, of course.
Don't be tempted to use a local variable for an array of this size: that will try to allocate it on the stack, which won't be big enough and will likely crash.
Also - remember that with this sequence the values go up as well as down, so you'll need to cope with that in your program (probably by having the array longer than UPTO values, and using an assert() to guard against indices greater than the size of the array).
If I recall correctly, your problem isn't a slow algorithm: the algorithm you have now is fast enough for what PE asks you to do. The problem is overflow: you sometimes end up multiplying your number by 3 so many times that it will eventually exceed the maximum value that can be stored in a signed int. Use unsigned ints, and if that still doesn't work (but I'm pretty sure it does), use 64 bit ints (long long).
This should run very fast, but if you want to do it even faster, the other answers already addressed that.