[X] My Assistant

[BrainStew]

I think I'm going crazy... I'm trying to initialize an array to a random size between 1 and 20. I must be too tired or something.

CODE

#include <iostream>
#include <ctime>

int main(void)
{
    srand((unsigned)time(NULL));

    int iRand_Num = rand()%20+1;
    int iArray[iRand_Num] = {0};

    system("pause");
}

Errors:

CODE

------ Build started: Project: Sort Array, Configuration: Debug Win32 ------
Compiling...
main.cpp
c:\documents and settings\james\desktop\sort array\sort array\main.cpp(9) : error C2057: expected constant expression
c:\documents and settings\james\desktop\sort array\sort array\main.cpp(9) : error C2466: cannot allocate an array of constant size 0
Build log was saved at "file://c:\Documents and Settings\James\Desktop\Sort Array\Sort Array\Debug\BuildLog.htm"
Sort Array - 2 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Thanks... I'm going to go jump off my roof now.

This post has been edited by skyhawk133: 21 Aug, 2007 - 04:48 PM

[Bench]

I think I'm going crazy... I'm trying to initialize an array to a random size between 1 and 20. I must be too tired or something.

CODE

#include <iostream>
#include <ctime>

int main(void)
{
    srand((unsigned)time(NULL));

    int iRand_Num = rand()%20+1;
    int iArray[iRand_Num] = {0};

    system("pause");
}

Thanks... I'm going to go jump off my roof now.

The size of a statically allocated array must be constant i'm afraid.

You have two options - the first (preferable) is to forget arrays, because 'C' style arrays are a bit of a nuisance, and go with a vector from the C++ STL (standard template library). A vector is basically a C++ array, although it automatically resizes itself to suit the number of elements you wish to add to it, and has accessor methods with bounds checking.

CODE

#include <iostream>
#include <vector>
#include <ctime>
#include <cmath>

int main()
{
    srand( time(0) );
    int size = rand() % 20 + 1;
    std::vector<int> my_vec(size);
    std::cout << my_vec.size();
}

The other way (less preferable, but if you insist on having a raw C-style array..) is to create it dynamically with new

CODE

int rand_num = rand() %20 + 1;
int* arr = new int[rand_num];

Doing this, you must remember to delete [] the array when you're done with it, else your program will have a memory leak. (The great thing about vectors is that they clean up after themselves and don't "leak").

[BrainStew]

No I need to use a raw c array. In a number of C++ equivilency tests I keep seeing it asks you to sort an array. I was just goofing around with sorting arrays, blah blah blah.

Yeah I know STLs are a lot better but can't use them for what I was doing.

I'll just go with the pointer, didn't want to but meh.

Thanks

This post has been edited by BrainStew: 21 Aug, 2007 - 12:30 PM

[Amadeus]

Although I'm sure you're aware, the use of the system function to call a pause.exe reduces the portability of your code. I'm assuming you've placed it there to hold an execution window open?

[Bench]

Although I'm sure you're aware, the use of the system function to call a pause.exe reduces the portability of your code. I'm assuming you've placed it there to hold an execution window open?

Indeed - and for anyone using Visual C++ (Which is what it looks like, based on the debugger output shown), its unnecessary, because [CTRL] + [F5] .. 'a.k.a.' Start without debugging automatically pauses the console output window from VC++

[BrainStew]

Although I'm sure you're aware, the use of the system function to call a pause.exe reduces the portability of your code. I'm assuming you've placed it there to hold an execution window open?

Indeed - and for anyone using Visual C++ (Which is what it looks like, based on the debugger output shown), its unnecessary, because [CTRL] + [F5] .. 'a.k.a.' Start without debugging automatically pauses the console output window from VC++

didn't know about the ctrl+f5, thanks. Learn new tricks every day.

[BrainStew]

ps: Whats best iso/ansi standard way to keep a console window open?

[MorphiusFaydal]

http://www.dreamincode.net/code/snippet1067.htm
http://www.dreamincode.net/code/snippet582.htm

[Xing]

I think I'm going crazy... I'm trying to initialize an array to a random size between 1 and 20. I must be too tired or something.

CODE

#include <iostream>
#include <ctime>

int main(void)
{
    srand((unsigned)time(NULL));

    int iRand_Num = rand()%20+1;
    int iArray[iRand_Num] = {0};

    system("pause");
}

C++ doesn't support VLA's. That's a C99 feature. Follow what Bench is saying. You can further refine his second solution by using smart pointers(like auto_ptr, boost::shared_ptr etc).

This post has been edited by Xing: 21 Aug, 2007 - 06:09 PM

[Bench]

You can further refine his second solution by using smart pointers(like auto_ptr, boost::shared_ptr etc).

No, you can't do that - auto_ptr is not designed to work with new[] because it doesn't use delete[] - it uses plain old delete (in other words its undefined behaviour). I believe the same holds true for boost::shared_ptr, but I'm not 100% sure on that.

The reason auto_ptr doesn't need to work with new[] is that the STL has the vector class, which is intended to be used instead.

This post has been edited by Bench: 22 Aug, 2007 - 02:52 AM

[Xing]

While giving general advice on using smart pointers over raw pointers I by mistakenly extended it to auto_ptr for dynamically allocated arrays. You are right about the point that there is nothing like auto_ptr for dynamically allocated arrays in C++. But boost::shared_ptr, boost::scoped_array can be used for that.