Random Numbers - Revision history

Admin at 22:16, 21 June 2016

2016-06-21T22:16:15Z

← Older revision		Revision as of 22:16, 21 June 2016
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	1807295698		1807295698
	</pre>		</pre>


			[[Category:C++]]
			[[Category:Programming]]

Admin: /* More Curious Behavior */

2016-06-21T22:15:38Z

More Curious Behavior

← Older revision		Revision as of 22:15, 21 June 2016
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	<code>time(NULL)</code> only generates a unique integer once per second.		<code>time(NULL)</code> only generates a unique integer once per second.

	That means, if you run the program repeatedly with NO delays in-between, time(NULL) will always generate the same random number if it is still the same second, and so you will get the exact same numbers if you run the program over and over in a 1-second interval. For example:		That means, if you run the program repeatedly with NO delays in-between, <code>time(NULL)</code> will always generate the same random number if it is still the same second, and so you will get the exact same numbers if you run the program over and over in a 1-second interval. For example:

	<pre>		<pre>

Admin: /* Curious Behavior */

2016-06-21T22:11:22Z

Curious Behavior

← Older revision		Revision as of 22:11, 21 June 2016
Line 77:		Line 77:

	I hope the people who design cryptosystems know about this!		I hope the people who design cryptosystems know about this!

			=More Curious Behavior=

			<code>time(NULL)</code> only generates a unique integer once per second.

			That means, if you run the program repeatedly with NO delays in-between, time(NULL) will always generate the same random number if it is still the same second, and so you will get the exact same numbers if you run the program over and over in a 1-second interval. For example:

			<pre>
			charles @ cronus [ 2016-06-21 - 15:08:22 - 69 ] /temp $ ./a.out && ./a.out && ./a.out && ./a.out && ./a.out
			1584133365
			27210049
			2052760379
			1418900798
			1807295698

			1584133365
			27210049
			2052760379
			1418900798
			1807295698

			1584133365
			27210049
			2052760379
			1418900798
			1807295698

			1584133365
			27210049
			2052760379
			1418900798
			1807295698

			1584133365
			27210049
			2052760379
			1418900798
			1807295698
			</pre>

Admin: Created page with "Exploring some curious random number behavior in C++... =Simple Program= Put this code in a file called myrand.cpp

 #include      /* srand, rand..."

2016-06-21T22:01:46Z

Created page with "Exploring some curious random number behavior in C++... =Simple Program= Put this code in a file called <code>myrand.cpp</code> <pre> #include <stdlib.h> /* srand, rand..."

New page

Exploring some curious random number behavior in C++...

=Simple Program=

Put this code in a file called <code>myrand.cpp</code>

<pre>
#include <stdlib.h> /* srand, rand */
#include <time.h> /* time */
#include <iostream>

using std::cout;
using std::endl;

int main ()
{
int mynum;
srand (time(NULL)); /* initialize random seed */
for( int i = 0; i < 5; i++) {
cout << rand() << endl;
}
return 0;
}
</pre>

=Compilation=

Compile with g++:

<pre>
$ g++ myrand.cpp
</pre>

=Output=

Run it:

<pre>
$ ./a.out
</pre>

Here is the output from my terminal when I run this program:

<pre>
charles @ cronus [ 2016-06-21 - 15:00:12 - 41 ] /temp $ ./a.out
1575746672
805981500
1951708871
1705770619
2127589730

charles @ cronus [ 2016-06-21 - 15:00:13 - 42 ] /temp $ ./a.out
1575763479
1088456749
1426875297
543230630
1124215013

charles @ cronus [ 2016-06-21 - 15:00:14 - 43 ] /temp $ ./a.out
1575780286
1370931998
902041723
1528174288
120840296

charles @ cronus [ 2016-06-21 - 15:00:15 - 44 ] /temp $ ./a.out
1575797093
1653407247
377208149
365634299
1264949226
</pre>

=Curious Behavior=

Notice how the first random number that is generated is always starting at around 15757XXXXX. Seeding the random number generator with the current time seems to generate very similar (read: NOT random) numbers for the first random number.

I hope the people who design cryptosystems know about this!

Random Numbers - Revision history

Admin at 22:16, 21 June 2016

Admin: /* More Curious Behavior */

Admin: /* Curious Behavior */

Admin: Created page with "Exploring some curious random number behavior in C++... =Simple Program= Put this code in a file called myrand.cpp #include /* srand, rand..."

Admin: Created page with "Exploring some curious random number behavior in C++... =Simple Program= Put this code in a file called `myrand.cpp`
#include /* srand, rand..."