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Once, my teacher taught me to use randomize() and random() function for generating pseudorandom numbers in C++ Builder. Now I prefer working in VS 2012, but when I tried to use these functions there it says that "identifier not found", even when I added #include <stdlib.h>. After some time of Googling I found that there are also rand() and srand() functions. What is the difference between them and which is it better to use?

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    you should use the random header especially since using rand() is considered harmful Commented Sep 10, 2013 at 18:15
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    "considered harmful" is a subjective measure. It depends on what you are doing. It is just s pseudo-random generator, after all. Commented Sep 10, 2013 at 18:24
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    It's a very crappy one, and it's not possible to prove any properties about the distribution (which is probably important if you're generating random numbers in the first place.) Commented Sep 10, 2013 at 20:36

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randomize() and random() are not part of the standard library. Perhaps your teacher wrote functions with these names for use in your class, or maybe you really mean random() and srandom() which are part of POSIX and not available on Windows. rand() and srand() are part of the standard library and will be provided by any standard conforming implementation of C++.

You should avoid rand() and srand() and use the new C++11 <random> library. <random> was added as part of the C++11 standard (and VS2012 does provide it).

Video explaining why: rand() Considered Harmful

  • rand() is sometimes a low quality pRNG and not suitable for applications that need a reasonable level of unpredictability. <random> provides a variety of engines with different characteristics suitable for many different use cases.

  • Converting the results of rand() into a number you can use directly usually relies on code that is difficult to read and easy to get wrong, whereas using <random> distributions is easy and produces readable code.

  • The common methods of generating values in a given distribution using rand() further decrease the quality of the generated data. % generally biases the data and floating point division still produces non-uniform distributions. <random> distributions are higher quality as well as more readable.

  • rand() relies on a hidden global resource. Among other issues this causes rand() to not be thread safe. Some implementations make thread safety guarantees, but this is not required by the standard. Engines provided by <random> encapsulate pRNG state as objects with value semantics, allowing flexible control over the state.

  • srand() only permits a limited range of seeds. Engines in <random> can be initialized using seed sequences which permit the maximum possible seed data. seed_seq also implements a common pRNG warm-up.

example of using <random>:

#include <iostream> #include <random> int main() { // create source of randomness, and initialize it with non-deterministic seed std::random_device r; std::seed_seq seed{r(), r(), r(), r(), r(), r(), r(), r()}; std::mt19937 eng{seed}; // a distribution that takes randomness and produces values in specified range std::uniform_int_distribution<> dist(1,6); for (int i=0; i<100; ++i) { std::cout << dist(eng) << '\n'; } }
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All things are relative, and depending on what you are doing, rand() may be largely sufficient. (Also, why all these presentations as videos? C++ is mainly used professionally, which means that we cannot activate the sound on our machines, and can't watch a video.)
@JamesKanze Headphones are handy in an office. There are probably also decent text resources but I don't know of any off hand. Even if quality and non-uniformity is not a concern one should still prefer <random> simply for readability.
@AndreyT There are no circumstances in which <random> is available but rand() should be prefered. And you're incorrect that no one considers rand() harmful. rand() is simply too conducive to incorrect usage (not just in terms of quality, but also creating data races) and does not provide any benefits over alternatives.
@JamesKanze "there was nothing in it I wasn't already thoroughly aware of" And there was nothing in it I wasn't aware of either. I guess once one person knows something we should burn all copies of that information and never mention it? "most people cannot use C++11 yet" It might be the case that most people don't use C++11, but often that's not because C++11 isn't available for their platform. Programmers, managers, etc. should be encouraged to move to C++11. Part of that is showing how it's better.
@JamesKanze "it's not available in production code, except in exceptional instances" Like I said, there are no circumstances in which <random> is available but rand() should be prefered. I also think 'exceptional' is an underestimate of its availability. "but it's the simplest solution" If by 'simpler' you mean the same sort of thing as people who say C should be prefered to C++ because it's 'simpler'. In terms of most directly expressing the meaning of a program I find <random> to be simpler. Getting the same level of abstraction out of rand() means implementing it oneself.
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Although there are (obviously, above) people who will assert with religious fervor that rand() is bad and random() is not, it turns out that your mileage may vary. Here's the gcc answer to the question of "What's the difference...", as supplied by the gcc version of stdlib.h (emphasis added):

/* These are the functions that actually do things. The `random', `srandom', `initstate' and `setstate' functions are those from BSD Unices. The `rand' and `srand' functions are required by the ANSI standard. **We provide both interfaces to the same random number generator.** */ /* Return a random long integer between 0 and RAND_MAX inclusive. */
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Functions rand() and random() are either defined by POSIX since at least POSIX.1-2001 (and randomize() is not standardized).

On older rand() implementations, and on current implementations on different systems, the lower-order bits are much less random than the higher-order bits.

When available, random() does not suffer of this issue.

In add, modern version of rand() use the same random number generator as random(). So rand() may be correct, but it is not garanteed.

So, always use random() instead of rand(). If random() is not available on your operating system, ask to operating system developers to provide newer standards API implementation (2001 standard is now old enough to expect any system to provide it).

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srand() is the C Standard library implementation for seeding the (pseudo) random number generator. rand() is the (pseudo) random number generator in the C Standard Library.

C++ has implemented a newer (pseudo) random number generator in the <random> header file, which has a variety of different engines to use: http://en.cppreference.com/w/cpp/numeric/random

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It looks like you were using C-style functions, even though your question is labeled C++. Also, stdlib.h is a header file from C standard library. There's no such functions are random() and randomize() in C standard library. The C standard library has rand() and srand().

If you were using random() or something like that through stdlib.h, it must have been some non-standard library extension in Borland compiler package.

So, if you want to stick to C-style standard functions, that would be, again, rand() and srand(). But if you are writing in C++, you might have better (and more appropriate) options in C++ standard library.

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Does it mean that random() and randomize() are only included in C++ Borland compiler?
@Sunrise: That is what non-standard means, yes.
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I am not familiar with randomize() and random() but they are not part of the standard library. You should avoid using rand() this video explains why using rand() is considered harmful.

You should be using the random header introduced in C++11, here is example using both std::uniform_real_distribution and std::uniform_int_distribution:

#include <iostream> #include <random> int main() { std::random_device rd; std::mt19937 e2(rd()); std::uniform_int_distribution<> dist(1, 6); std::uniform_real_distribution<> distReal(1, 6); for( int i = 0 ; i < 10; ++i ) { std::cout << dist(e2) << ","; } std::cout << std::endl ; for( int i = 0 ; i < 10; ++i ) { std::cout << distReal(e2) << ","; } std::cout << std::endl ; return 0 ; }
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This is a work-around using random numbers form outside of c++.

This is the original program in C, copied from "http://www.programmingsimplified.com/" This program does not run because the " temp1 = 1 + random ( 588 );" "temp2 = 1 + random ( 380 );" statements do not work. 'random' is not a function of graphics.h, conio.h, or stdlib.h Nor, does it work if you include random.h. Below this listing is a work-around for the random function.

#include<graphics.h> #include<conio.h> #include<stdlib.h> main() { int gd = DETECT, gm, area, temp1, temp2, left = 25, top = 75; void *p; initgraph(&gd,&gm,"C:\\TC\\BGI"); setcolor(YELLOW); circle(50,100,25); setfillstyle(SOLID_FILL,YELLOW); floodfill(50,100,YELLOW); setcolor(BLACK); setfillstyle(SOLID_FILL,BLACK); fillellipse(44,85,2,6); fillellipse(56,85,2,6); ellipse(50,100,205,335,20,9); ellipse(50,100,205,335,20,10); ellipse(50,100,205,335,20,11); area = imagesize(left, top, left + 50, top + 50); p = malloc(area); setcolor(WHITE); settextstyle(SANS_SERIF_FONT,HORIZ_DIR,2); outtextxy(155,451,"Smiling Face Animation"); setcolor(BLUE); rectangle(0,0,639,449); while(!kbhit()) { temp1 = 1 + random ( 588 ); temp2 = 1 + random ( 380 ); getimage(left, top, left + 50, top + 50, p); putimage(left, top, p, XOR_PUT); putimage(temp1 , temp2, p, XOR_PUT); delay(100); left = temp1; top = temp2; } getch(); closegraph(); return 0; }

The random numbers are generated with a simple MS Excel macro, listed here:

Sub Macro1() Dim i For i = 1 To 400 Randomize Range("a" & i) = Int(Rnd * 588) + 1 Range("b" & i) = Int(Rnd * 380) + 1 Next i End Sub

This generates 2 columns of random numbers. Each column is copied and pasted into its own *.txt file , i.e. rnd1.txt and rnd2.txt and placed in a directory where they can be accessed by the c++ program that follows. Substitute the "c:\PATH\rnd1.txt" and "C:\PATH\rnd2.txt" with the correct path.

#include<iostream> #include<fstream> #include<graphics.h> using namespace std; int i,j,k; int main(int argc, char** argv) { std::ifstream infile1; infile1.open("c:\\PATH\\rnd1.txt",ios::in); ifstream infile2; infile2.open("c:\\PATH\\rnd2.txt",ios::in); int gd = DETECT, gm, area, temp1, temp2, left = 25, top = 75; void *p; initgraph(&gd,&gm,"C:\\TC\\BGI"); setcolor(YELLOW); circle(50,100,25); setfillstyle(SOLID_FILL,YELLOW); floodfill(50,100,YELLOW); setcolor(BLACK); setfillstyle(SOLID_FILL,BLACK); fillellipse(44,85,2,6); fillellipse(56,85,2,6); ellipse(50,100,205,335,20,9); ellipse(50,100,205,335,20,10); ellipse(50,100,205,335,20,11); area = imagesize(left, top, left + 50, top + 50); p = malloc(area); setcolor(WHITE); settextstyle(SANS_SERIF_FONT,HORIZ_DIR,2); outtextxy(155,451,"Smiling Face Animation "); setcolor(BLUE); rectangle(0,0,639,449); while(!kbhit()) { infile1 >> j; temp1 = j; infile2 >> k; temp2 = k; if(infile2.eof()) { closegraph(); void close(); return 0; } getimage(left, top, left + 50, top + 50, p); putimage(left, top, p, XOR_PUT); putimage(temp1 , temp2, p, XOR_PUT); delay(100); left = temp1; top = temp2; } }

This program will run for about 40 seconds and then terminate.

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2 Comments

Using Excel to generate random numbers for a C++ application... bleh. Much better to find a decent library if you don't want to use the 'built-in' options. obligitory xkcd link
graphics.h, conio.h and... EXEL? Seriously?

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