Hi
I need help to generate some random numbers between 2 and 8.
#include <cstdlib>
using std::rand;
the following was out of my range,
int main() {
for (int i = 0; i < 50; i++){
int x = (int(rand())/444489786)*8;
cout << x << '\t' << endl;
}
}
it can be any quality random number.
thanks
Aug 1 '06
26  4113 
Gary Wessle wrote:
the thing is if you need to generate say 5 random numbers between 2
and 8 then this will not work, run this
for (int i= 0; i< 5; i++){
srand( time( 0));
int x= 2+ rand() %8;
int x = 2 + rand() % 6;
cout << x << '\n';
}
Best
Kai-Uwe Bux
Marco Wahl <mw@visenso.dew rites:
Gary Wessle <ph****@yahoo.c omwrites:
do you mean this, it still puts out all zeros, could you try it on
your machine and report back? thanks
#include <iostream>
#include <cstdlib>
using namespace std;
int main() {
srand( static_cast<uns igned(time( NULL )) );
int N = 8;
int x;
for (unsigned i=0; i<5; i++){
x = static_cast<int >(static_cast<d ouble>(N*rand() ) / (RAND_MAX+1));
cout << x << '\t';
}
}
I checked and also get all 0s. The output looks more interesting when using the line
x = static_cast<int >(N * static_cast<dou ble>(rand()) / (static_cast<do uble>(RAND_MAX) + 1));
There is an overflow on my machine at least when calculating RAND_MAX+1.
HTH
--
Marco Wahl
http://visenso.com
here is the final code for future readers.
*************** *************** *************** *************** ****
int randam(int from, int to) {
int f = from;
int t = to;
int a = to-from+1;
return static_cast<int (a * static_cast<dou ble>(rand())/
(static_cast<do uble>(RAND_MAX) + 1))+from;
}
int main() {
// print out 5 random numbers between 2 and 8
srand( static_cast<uns igned(time( NULL )) );
for(int i=0; i<5; i++) cout << randam(2,8) << " ";
}
"Gary Wessle" <ph****@yahoo.c omwrote in message
news:87******** ****@localhost. localdomain...
Marco Wahl <mw@visenso.dew rites:
>Gary Wessle <ph****@yahoo.c omwrites:
do you mean this, it still puts out all zeros, could you try it on
your machine and report back? thanks
#include <iostream>
#include <cstdlib>
using namespace std;
int main() {
srand( static_cast<uns igned(time( NULL )) );
int N = 8;
int x;
for (unsigned i=0; i<5; i++){
x = static_cast<int >(static_cast<d ouble>(N*rand() ) /
(RAND_MAX+1));
cout << x << '\t';
}
}
I checked and also get all 0s. The output looks more interesting when
using the line
x = static_cast<int >(N * static_cast<dou ble>(rand()) /
(static_cast<d ouble>(RAND_MAX ) + 1));
There is an overflow on my machine at least when calculating RAND_MAX+1.
HTH
--
Marco Wahl
http://visenso.com
here is the final code for future readers.
*************** *************** *************** *************** ****
int randam(int from, int to) {
int f = from;
int t = to;
int a = to-from+1;
return static_cast<int (a * static_cast<dou ble>(rand())/
(static_cast<do uble>(RAND_MAX) + 1))+from;
}
int main() {
// print out 5 random numbers between 2 and 8
srand( static_cast<uns igned(time( NULL )) );
for(int i=0; i<5; i++) cout << randam(2,8) << " ";
}
Hi Gary,
do you mean int x = from + rand() * (to-from) ?
Yes, that's what I mean. It a simple scaling and adding an offset to the
result of a random number in the [0,1).
The code above should give you what you are looking for. In a more compact
form you can write it like this:
return static_cast<int >( from + ( (to - from) * (rand() / (RAND_MAX +
1.0)) ));
But be careful - the statement above is different than writing
return static_cast<int >( from + ( (to - from) * (rand() / (RAND_MAX +
1)) ));
The "beast" that you were running into was the integer division of rand() by
another much larger integer. The result of this integer devision is always 0
and that's why you obtained zeros (or the offset from). However, by using
1.0 instead of 1 in the calculation you implicitly convert the divisor to a
float and consequently the division is performed and results in a float.
HTH
Chris
<ar************ ****@gmail.comw rote in message
news:11******** **************@ h48g2000cwc.goo glegroups.com.. .
Hello all,
why you are going far away?
first use srand() to randomize radom number, srand get a parameter as
seed, like:
srand(3); or you can use srand(time(0));
then use this method to have randome number in a certain range:
x=minum number +rand()% maximum number;
Yes, that surely works but brings back my original comment. Using the modulo
operation you interfere with the randomness and don't make use of the full
period of the random generator. Anyway, it depends on your application
whether this matters or not.
Cheers
Chris
Chris Theis wrote:
>
<ar************ ****@gmail.comw rote in message
news:11******** **************@ h48g2000cwc.goo glegroups.com.. .
>Hello all,
why you are going far away?
first use srand() to randomize radom number, srand get a parameter as
seed, like:
srand(3); or you can use srand(time(0));
then use this method to have randome number in a certain range:
x=minum number +rand()% maximum number;
Yes, that surely works but brings back my original comment. Using the
modulo operation you interfere with the randomness
How, and why?
and don't make use of the full period of the random generator.
Huh? I see that that can happen. However, I do not see why the alternative
of chopping [0,RAND_MAX] into n intervals of equal length it a priory
better.
As far as I can see, any kind of mapping N values to n < N values can tamper
with the period or other measures of randomness. Whether a particular
mapping fares better or worse depends on the underlying random number
generator.
Anyway, it depends on your application whether this matters or not.
True, and I think up-thread it was stated that quality does not matter.
Best
Kai-Uwe Bux
Gary Wessle wrote:
Hi
I need help to generate some random numbers between 2 and 8.
#include <cstdlib>
using std::rand;
the following was out of my range,
int main() {
for (int i = 0; i < 50; i++){
int x = (int(rand())/444489786)*8;
cout << x << '\t' << endl;
}
}
it can be any quality random number.
thanks
This is one way of doing it:
#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;
template<typena me T>
class Random {
public:
Random(T min_, T max_);
T operator()(T min_, T max_); ///< Set new range for random numbers,
and return a random number.
T operator()(); ///< Return a random number.
private:
T _min, _max;
T _delta;
};
template<typena me T>
inline Random<T>::Rand om(T min_, T max_)
:_min(min_),
_max(max_){
srand(time(0));
srand(time(0));
_delta = RAND_MAX / (_max - _min);}
template<typena me T>
inline T Random<T>::oper ator()(T min_, T max_){
_min = min_;
_max = max_;
_delta = RAND_MAX / (_max - _min);
return(operator ()());}
template<typena me T>
inline T Random<T>::oper ator()(){
return((rand() / _delta) + _min);}
int main(){
Random<floatran dFloat(-10, -20);
cerr << "5 random floating point numbers between -10 and -20: ";
for(int i = 0; i < 5; i++)
cerr << fixed << randFloat() << " ";
cerr << endl;
randFloat(-100, 100);
cerr << "5 random floating point numbers between -100 and 100: ";
for(int i = 0; i < 5; i++)
cerr << fixed << randFloat() << " ";
cerr << endl;
Random<intrandI nt(20000, 10000);
cerr << "5 random integers between 20000 and 10000: ";
for(int i = 0; i < 5; i++)
cerr << fixed << randInt() << " ";
cerr << endl;
randInt(-10, 10);
cerr << "5 random integers between -10 and 10: ";
for(int i = 0; i < 5; i++)
cerr << fixed << randInt() << " ";
cerr << endl;
return 0;}
lars@ibmr52:~/programming/c$ g++ -g -Wall random.cpp -o random &&
../random
5 random floating point numbers between -10 and -20: -15.719680
-10.247149 -10.654663 -16.673820 -13.213199
5 random floating point numbers between -100 and 100: 63.869286
-77.031998 -25.940384 -4.096813 -49.285671
5 random integers between 20000 and 10000: 14281 19753 19346 13327
16787
5 random integers between -10 and 10: 6 -8 -3 -1 -5
--
mvh, Lars Rune Nøstdal http://lars.nostdal.org/
Gary Wessle <ph****@yahoo.c omwrote:
do you mean this, it still puts out all zeros, could you try it on
your machine and report back? thanks
*************** *
*************** *
#include <iostream>
#include <cstdlib>
using namespace std;
int main() {
srand( static_cast<uns igned(time( NULL )) );
int N = 8;
int x;
for (unsigned i=0; i<5; i++){
x = static_cast<int >(static_cast<d ouble>(N*rand() ) / (RAND_MAX+1));
cout << x << '\t';
}
}
*************** *
*************** *
Interesting, on my machine (Windows XP, VS .NET 2003), after I added
#include <ctime>, my output for repeated runs was (using spaces instead
of tabs):
2 5 1 1 1
2 6 6 7 2
2 6 2 2 7
etc.
--
Marcus Kwok
Replace 'invalid' with 'net' to reply
"Kai-Uwe Bux" <jk********@gmx .netwrote in message
news:ea******** **@murdoch.acc. Virginia.EDU...
Chris Theis wrote:
>>
<ar*********** *****@gmail.com wrote in message
news:11******* *************** @h48g2000cwc.go oglegroups.com. ..
>>Hello all,
why you are going far away?
first use srand() to randomize radom number, srand get a parameter as
seed, like:
srand(3); or you can use srand(time(0));
then use this method to have randome number in a certain range:
x=minum number +rand()% maximum number;
Yes, that surely works but brings back my original comment. Using the
modulo operation you interfere with the randomness
How, and why?
>and don't make use of the full period of the random generator.
Huh? I see that that can happen. However, I do not see why the alternative
of chopping [0,RAND_MAX] into n intervals of equal length it a priory
better.
Well we're getting into a hot topic here which has been discussed
extensively. On one hand using modulo you might skew the results towards
lower values and on the other hand you can have an effect on the randomness.
The reason is that the modulo operation uses low order bits and depending on
the type of random generator the high-order bits might show better
randomness. However, this is something which depends on the implementation
of rand and is the subject of endless discussions! For a thorough discussion
on this including the mathematics you might refer to Knuth's book The Art of
computer programming Vol.2 and/or Numerical Recipes.
As far as I can see, any kind of mapping N values to n < N values can
tamper
with the period or other measures of randomness. Whether a particular
mapping fares better or worse depends on the underlying random number
generator.
Yes and no. Of course you will never get good results if your random
generator is crap. However, the simple scaling and offsetting does not
interfere with any property of the RNG and therefore does not affect the
"randomness ". In opposition to this the modulo mapping can affect the
randomness because you're introducing a limitation.
>
>Anyway, it depends on your application whether this matters or not.
True, and I think up-thread it was stated that quality does not matter.
Best
Kai-Uwe Bux
Cheers
Chris
Chris Theis wrote:
"Kai-Uwe Bux" <jk********@gmx .netwrote in message
news:ea******** **@murdoch.acc. Virginia.EDU...
>>
Huh? I see that that can happen. However, I do not see why the alternative
of chopping [0,RAND_MAX] into n intervals of equal length it a priory
better.
Well we're getting into a hot topic here which has been discussed
extensively. On one hand using modulo you might skew the results towards
lower values and on the other hand you can have an effect on the randomness.
The reason is that the modulo operation uses low order bits
That's not the reason. Consider a drastically simplified case: a rand()
function that produces values between 0 and 5 inclusive, and an attempt
to generate values from 0 to 4 inclusive. Using the remainder operator
(C and C++ do not have a modulus operator, but for positive arguments
remainder and modulus do the same thing), the generated value 0, 1, 2,
3, and 4 each map to themselves, and the generated value 5 maps to 0. So
you get twice as many 0's in the output.
When you use floating point, you're still mapping six values into five
slots, and one of the slots will come up twice as often as the others.
Try it.
In general, this problem occurs whenever the number of values in the
range returned by the generator is not an exact multiple of the number
of values in the target range. The solution, regardless of how you
implement the range conversion, is to discard some values from the
generator, so that you end up with a range whose size is a multiple of
the size of the target range. In the case of the simple example above,
throw away every 5 that you get from the generator.
and depending on
the type of random generator the high-order bits might show better
randomness. However, this is something which depends on the implementation
of rand and is the subject of endless discussions! For a thorough discussion
on this including the mathematics you might refer to Knuth's book The Art of
computer programming Vol.2 and/or Numerical Recipes.
That is, indeed, one of the assertions made about random number
generators. The mismatch in range sizes is far more significant.
>
>>As far as I can see, any kind of mapping N values to n < N values can
tamper
with the period or other measures of randomness. Whether a particular
mapping fares better or worse depends on the underlying random number
generator.
Yes and no. Of course you will never get good results if your random
generator is crap. However, the simple scaling and offsetting does not
interfere with any property of the RNG and therefore does not affect the
"randomness ".
It does affect the randomness of the resulting sequence, as shown above.
Chris Theis wrote:
"Gary Wessle" <ph****@yahoo.c omwrote in message
news:87******** ****@localhost. localdomain...
>>
here is the final code for future readers.
************* *************** *************** *************** ******
int randam(int from, int to) {
int f = from;
int t = to;
int a = to-from+1;
return static_cast<int (a * static_cast<dou ble>(rand())/
(static_cast<do uble>(RAND_MAX) + 1))+from;
}
int main() {
// print out 5 random numbers between 2 and 8
srand( static_cast<uns igned(time( NULL )) );
for(int i=0; i<5; i++) cout << randam(2,8) << " ";
}
The code above should give you what you are looking for. In a more compact
form you can write it like this:
As I pointed out in another message, this doesn't compensate for
mismatched range sizes. The bias is less obvious than when using
remainainder because remainder puts the excess values down at the low
end of the target range, while the floating-point approach distributes
them more evenly. But try that code with a custom rand function that
generates values between 0 and 8, inclusive. One of the values will turn
up twice as often as the others. (I haven't done the analysis to figure
out which one.)
To avoid this bias, compute the largest multiple of the number of values
in the target range (m * (to - from + 1)) that is less than or equal to
the number of values in the generator's range (loosely, RAND_MAX+1, but
beware of overlows). Each time you call rand, check whether the value is
greater than or equal to this largest multiple; if it is, call rand again. This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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