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And pretty good pseudo random source is the arc4random function that is available on many systems. If your system supports the arc4random family of functions I would recommend using those instead the standard rand function. The urandom function is basically the same as a call to rand , except more secure, and it returns a long easily changeable. If you are on another system i. Windows , then use rand or some internal Windows specific platform-dependent non-portable API.

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. 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. For example,. Why so much code? Other languages like Java and Ruby have functions for random integers or floats.

For integers, we want to avoid modulo bias. Each number from 0 to would appear more often than each number from to To remove the bias, we can retry rand while the value is below , because the values from to map uniformly onto the values from 0 to 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. It is faster to allow OpenSSL to generate more random numbers from a seed.

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. STL doesn't exist for C. You have to call rand , or better yet, random. These are declared in the standard library header stdlib. The difference between rand and random is that random returns a much more usable bit random number, and rand typically returns a bit number.

The BSD manpages show that the lower bits of rand are cyclic and predictable, so rand is potentially useless for small numbers. You want to use rand. 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. My minimalistic solution should work for random numbers in range [min, max.

Use srand time NULL before invoking the function. A pointer pointing to a memory location that has been deleted or freed is called dangling pointer. FWIW, the answer is that yes, there is a stdlib. 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.

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:. I don't know how uniform you need your random numbers to be, but the above appears uniform enough for most needs. 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.

If you need, say, secure random bits, the RFC 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 secure random bits and stretch it to a desired length, map it to human readable text, etc.

Stack Overflow for Teams — Start collaborating and sharing organizational knowledge. Create a free Team Why Teams? Collectives on Stack Overflow. Learn more. How to generate a random int in C? Ask Question.

Asked 12 years, 11 months ago. Modified 28 days ago. Viewed 2. Improve this question. Kredns Kredns See also srand : why call it only once. Add a comment. Sorted by: Reset to default. Highest score default Trending recent votes count more Date modified newest first Date created oldest first.

Help us improve our answers. Are the answers below sorted in a way that puts the best answer at or near the top? Improve this answer. Neuron - Freedom for Ukraine 4, 4 4 gold badges 28 28 silver badges 50 50 bronze badges. Also, in a threaded application, you might want to make sure that the generator's state is stored per thread, and seed the generator once for each thread.

Here's a reason: time only changes once per second. If you seed from time , for each call to rand , then you will get the same value for every call during a single second. But the bigger reason is that the properties of rand and functions like it are known best for the use case where they are seeded exactly once per run, and not on every single call.

Depending on "randomness" with untested or unproven properties leads to trouble. This is a deep subject. Start with reading Knuth Vol 2 Chapter 3 on random numbers as the best introduction to the mathematics and pitfalls. Keep in mind that this is still a weak way of seeing the PRNG. Just last year, a cryptolocker-type virus on Linux made the mistake of seeding with the time, and this dramatically reduced the search space.

All you had to do was get a decent idea of when the infection occurred and then try seeds from around that time. If all you really want, however, is for your program to act differently on each run, the above solution is fine. Show 15 more comments. Laurence Gonsalves Laurence Gonsalves k 32 32 gold badges silver badges bronze badges. It is a common practice alright, but not the correct one.

See this and this. Lazer: That's why I said "though bear in mind that this throws off the uniformity somewhat". Lazer the second link you posted is actually still not perfectly uniform. Casting to a double and back doesn't help. The first link you posted has a perfectly uniform solution, though it will loop a lot for small upper bounds. I've added a perfectly uniform solution to this answer that shouldn't loop as much even for small upper bounds.

Show 10 more comments. For example: include "sodium. Andrew 1, 14 14 silver badges 25 25 bronze badges. Scott Arciszewski Scott Arciszewski Don't worry about the RNG, it uses the kernel's. 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.

Chris Lutz Chris Lutz 70k 16 16 gold badges silver badges bronze badges. MH MH 1 1 gold badge 9 9 silver badges 13 13 bronze badges. If I were going to use a random number generator in a business application then I would definitely use this.

Relevant link. 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. Yun 2, 6 6 gold badges 7 7 silver badges 26 26 bronze badges. Koldar Koldar 1, 10 10 silver badges 32 32 bronze badges. Even with these heuristics, don't rely on rand for cryptographic data. At least for me, my application didn't involve cryptographic data, so for me it was ok the given method. Returns a non-negative Python integer with k random bits.

This method is supplied with the MersenneTwister generator and some other generators may also provide it as an optional part of the API. When available, getrandbits enables randrange to handle arbitrarily large ranges.

Return a random element from the non-empty sequence seq. If seq is empty, raises IndexError. Return a k sized list of elements chosen from the population with replacement. If the population is empty, raises IndexError. If a weights sequence is specified, selections are made according to the relative weights.

For example, the relative weights [10, 5, 30, 5] are equivalent to the cumulative weights [10, 15, 45, 50]. Internally, the relative weights are converted to cumulative weights before making selections, so supplying the cumulative weights saves work. If a weights sequence is supplied, it must be the same length as the population sequence.

Weights are assumed to be non-negative and finite. A ValueError is raised if all weights are zero. For a given seed, the choices function with equal weighting typically produces a different sequence than repeated calls to choice. The algorithm used by choices uses floating point arithmetic for internal consistency and speed. The algorithm used by choice defaults to integer arithmetic with repeated selections to avoid small biases from round-off error.

The optional argument random is a 0-argument function returning a random float in [0. Note that even for small len x , the total number of permutations of x can quickly grow larger than the period of most random number generators. This implies that most permutations of a long sequence can never be generated. For example, a sequence of length is the largest that can fit within the period of the Mersenne Twister random number generator. Return a k length list of unique elements chosen from the population sequence or set.

Used for random sampling without replacement. Returns a new list containing elements from the population while leaving the original population unchanged. The resulting list is in selection order so that all sub-slices will also be valid random samples. This allows raffle winners the sample to be partitioned into grand prize and second place winners the subslices. Members of the population need not be hashable or unique. If the population contains repeats, then each occurrence is a possible selection in the sample.

Repeated elements can be specified one at a time or with the optional keyword-only counts parameter. To choose a sample from a range of integers, use a range object as an argument. If the sample size is larger than the population size, a ValueError is raised. Instances of set are no longer supported. The set must first be converted to a list or tuple , preferably in a deterministic order so that the sample is reproducible.

The following functions generate specific real-valued distributions. The low and high bounds default to zero and one. The mode argument defaults to the midpoint between the bounds, giving a symmetric distribution. Beta distribution. Returned values range between 0 and 1. Exponential distribution. It should be nonzero. Returned values range from 0 to positive infinity if lambd is positive, and from negative infinity to 0 if lambd is negative.

Gamma distribution. Not the gamma function! Normal distribution, also called the Gaussian distribution. This is slightly faster than the normalvariate function defined below. Multithreading note: When two threads call this function simultaneously, it is possible that they will receive the same return value. This can be avoided in three ways. Log normal distribution. Weibull distribution. Class that implements the default pseudo-random number generator used by the random module.

Class that uses the os. Not available on all systems. Does not rely on software state, and sequences are not reproducible. Accordingly, the seed method has no effect and is ignored. The getstate and setstate methods raise NotImplementedError if called.

Sometimes it is useful to be able to reproduce the sequences given by a pseudo-random number generator. By re-using a seed value, the same sequence should be reproducible from run to run as long as multiple threads are not running.

If your system supports the arc4random family of functions I would recommend using those instead the standard rand function. The urandom function is basically the same as a call to rand , except more secure, and it returns a long easily changeable. If you are on another system i. Windows , then use rand or some internal Windows specific platform-dependent non-portable API.

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.

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. For example,. Why so much code?

Other languages like Java and Ruby have functions for random integers or floats. For integers, we want to avoid modulo bias. Each number from 0 to would appear more often than each number from to To remove the bias, we can retry rand while the value is below , because the values from to map uniformly onto the values from 0 to 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. It is faster to allow OpenSSL to generate more random numbers from a seed. 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.

STL doesn't exist for C. You have to call rand , or better yet, random. These are declared in the standard library header stdlib. The difference between rand and random is that random returns a much more usable bit random number, and rand typically returns a bit number. The BSD manpages show that the lower bits of rand are cyclic and predictable, so rand is potentially useless for small numbers. You want to use rand. 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. My minimalistic solution should work for random numbers in range [min, max. Use srand time NULL before invoking the function.

A pointer pointing to a memory location that has been deleted or freed is called dangling pointer. FWIW, the answer is that yes, there is a stdlib. 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. 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:. I don't know how uniform you need your random numbers to be, but the above appears uniform enough for most needs. 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. If you need, say, secure random bits, the RFC 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 secure random bits and stretch it to a desired length, map it to human readable text, etc. Stack Overflow for Teams — Start collaborating and sharing organizational knowledge.

Create a free Team Why Teams? Collectives on Stack Overflow. Learn more. How to generate a random int in C? Ask Question. Asked 12 years, 11 months ago. Modified 28 days ago. Viewed 2. Improve this question. Kredns Kredns See also srand : why call it only once. Add a comment. Sorted by: Reset to default. Highest score default Trending recent votes count more Date modified newest first Date created oldest first.

Help us improve our answers. Are the answers below sorted in a way that puts the best answer at or near the top? Improve this answer. Neuron - Freedom for Ukraine 4, 4 4 gold badges 28 28 silver badges 50 50 bronze badges. Also, in a threaded application, you might want to make sure that the generator's state is stored per thread, and seed the generator once for each thread.

Here's a reason: time only changes once per second. If you seed from time , for each call to rand , then you will get the same value for every call during a single second. But the bigger reason is that the properties of rand and functions like it are known best for the use case where they are seeded exactly once per run, and not on every single call.

Depending on "randomness" with untested or unproven properties leads to trouble. This is a deep subject. Start with reading Knuth Vol 2 Chapter 3 on random numbers as the best introduction to the mathematics and pitfalls. Keep in mind that this is still a weak way of seeing the PRNG.

Just last year, a cryptolocker-type virus on Linux made the mistake of seeding with the time, and this dramatically reduced the search space. All you had to do was get a decent idea of when the infection occurred and then try seeds from around that time. If all you really want, however, is for your program to act differently on each run, the above solution is fine. Show 15 more comments. Laurence Gonsalves Laurence Gonsalves k 32 32 gold badges silver badges bronze badges. It is a common practice alright, but not the correct one.

See this and this. Lazer: That's why I said "though bear in mind that this throws off the uniformity somewhat". Lazer the second link you posted is actually still not perfectly uniform. Casting to a double and back doesn't help. The first link you posted has a perfectly uniform solution, though it will loop a lot for small upper bounds. I've added a perfectly uniform solution to this answer that shouldn't loop as much even for small upper bounds.

Show 10 more comments. For example: include "sodium. Andrew 1, 14 14 silver badges 25 25 bronze badges. Scott Arciszewski Scott Arciszewski Don't worry about the RNG, it uses the kernel's. 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. Chris Lutz Chris Lutz 70k 16 16 gold badges silver badges bronze badges.

MH MH 1 1 gold badge 9 9 silver badges 13 13 bronze badges. If I were going to use a random number generator in a business application then I would definitely use this. Relevant link. 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. Yun 2, 6 6 gold badges 7 7 silver badges 26 26 bronze badges. Koldar Koldar 1, 10 10 silver badges 32 32 bronze badges.

Even with these heuristics, don't rely on rand for cryptographic data. At least for me, my application didn't involve cryptographic data, so for me it was ok the given method. 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.

Class Random can also be subclassed if you want to use a different basic generator of your own devising: in that case, override the random , seed , getstate , and setstate methods. Optionally, a new generator can supply a getrandbits method — this allows randrange to produce selections over an arbitrarily large range. The random module also provides the SystemRandom class which uses the system function os. The pseudo-random generators of this module should not be used for security purposes.

For security or cryptographic uses, see the secrets module. Matsumoto and T. Complementary-Multiply-with-Carry recipe for a compatible alternative random number generator with a long period and comparatively simple update operations.

If a is omitted or None , the current system time is used. If randomness sources are provided by the operating system, they are used instead of the system time see the os. With version 2 the default , a str , bytes , or bytearray object gets converted to an int and all of its bits are used. With version 1 provided for reproducing random sequences from older versions of Python , the algorithm for str and bytes generates a narrower range of seeds.

Changed in version 3. Deprecated since version 3. Return an object capturing the current internal state of the generator. This object can be passed to setstate to restore the state. This method should not be used for generating security tokens. Use secrets. Return a randomly selected element from range start, stop, step. The positional argument pattern matches that of range. Keyword arguments should not be used because the function may use them in unexpected ways.

Currently randrange In the future, this will raise a TypeError. Returns a non-negative Python integer with k random bits. This method is supplied with the MersenneTwister generator and some other generators may also provide it as an optional part of the API.

When available, getrandbits enables randrange to handle arbitrarily large ranges. Return a random element from the non-empty sequence seq. If seq is empty, raises IndexError. Return a k sized list of elements chosen from the population with replacement. If the population is empty, raises IndexError. If a weights sequence is specified, selections are made according to the relative weights. For example, the relative weights [10, 5, 30, 5] are equivalent to the cumulative weights [10, 15, 45, 50].

Internally, the relative weights are converted to cumulative weights before making selections, so supplying the cumulative weights saves work. If a weights sequence is supplied, it must be the same length as the population sequence. Weights are assumed to be non-negative and finite. A ValueError is raised if all weights are zero. For a given seed, the choices function with equal weighting typically produces a different sequence than repeated calls to choice.

The algorithm used by choices uses floating point arithmetic for internal consistency and speed. The algorithm used by choice defaults to integer arithmetic with repeated selections to avoid small biases from round-off error. The optional argument random is a 0-argument function returning a random float in [0. Note that even for small len x , the total number of permutations of x can quickly grow larger than the period of most random number generators. This implies that most permutations of a long sequence can never be generated.

For example, a sequence of length is the largest that can fit within the period of the Mersenne Twister random number generator. Return a k length list of unique elements chosen from the population sequence or set. Used for random sampling without replacement. Returns a new list containing elements from the population while leaving the original population unchanged.

The resulting list is in selection order so that all sub-slices will also be valid random samples. This allows raffle winners the sample to be partitioned into grand prize and second place winners the subslices. Members of the population need not be hashable or unique. If the population contains repeats, then each occurrence is a possible selection in the sample. Repeated elements can be specified one at a time or with the optional keyword-only counts parameter. To choose a sample from a range of integers, use a range object as an argument.

If the sample size is larger than the population size, a ValueError is raised. Instances of set are no longer supported. The set must first be converted to a list or tuple , preferably in a deterministic order so that the sample is reproducible. The following functions generate specific real-valued distributions. The low and high bounds default to zero and one.

Changed in version randrange() is more sophisticated about producing equally distributed values. Formerly it used a style like int(random()*n) which could. The randint() method returns an integer number selected element from the specified range. Note: This method is an alias for randrange(start, stop+1). Return random integers from low (inclusive) to high (exclusive). Return random integers from the “discrete uniform” distribution of the specified dtype in the “.