Getting the values in an array into a pointer

[kbklpl21]

Hi salem c,

I noticed in line 168 and 169 there're delete [] hold_value; delete [] chosen;
But these two lines appear again in line 409 and 410, do these have any side effects? Or actually only needs once?

Also, in first_2_opt function, since it is a void function, do we still need line 333 (delete [] x?

pointer = nullptr; Is this also considered an initialisation of the pointer? To an empty address.

[salem c]

> I noticed in line 168 and 169 there're delete [] hold_value; delete [] chosen;
Having the same name for a local variable in two different functions doesn't make it the same variable. Each instance has it's own separate existence, and operations on one have nothing to do with operations on the other.

The ones in main are never used for anything to begin with, and could be safely removed from the code, along with a few other variables in main which serve only to clutter.

> Also, in first_2_opt function, since it is a void function, do we still need line 333 (delete [] x?
...
> x = generate_random_vector(n);
This generates a new[] vector and returns it to first_2_opt_symmetric.
You have exactly 3 options:
- you delete[] it when you're done.
- you return x to the caller function for them to take responsibility for deleting.
- you save x somewhere safe so you can delete it later on - say when the program ends.

If you just let the function exit without doing one of those things, then you have a memory leak.

Every 'new' needs a 'delete' and every 'new[]' needs a 'delete[]'.
So every time you call either new, you need to be thinking at the same time, "OK, where does the corresponding delete go in this code?". Your code wouldn't work if you neglected putting all the { } in the right places.


> pointer = nullptr; Is this also considered an initialisation of the pointer? To an empty address.
Yes.

[kbklpl21]

Hi salem c,

I see. So actually there's a few of the variable declarations and delete [] that I can remove from the main function right?

Is pointer = nullptr more preferable than *pointer = new []? Or they have different functions?

[salem c]

Yes there is code cleanup you can do.

> Is pointer = nullptr more preferable than *pointer = new []? Or they have different functions?

int *ptr;
...
ptr[0] = 0;

This is an uninitialised pointer, and its a crap-shoot as to what happens when you try to dereference the pointer.

int *ptr = nullptr;
...
ptr[0] = 0;

This is an initialised null pointer, and it's pretty much guaranteed on any sane machine that it will segfault when you try to dereference the pointer. Doing this is far more preferable than the randomised chaos of using an uninitialised pointer by mistake.

int *ptr = new int[1];
...
ptr[0] = 0;

You can assign and use ptr[0] all you want until the cows come home.

[kbklpl21]

Hi salem c,

I forgot to ask you about this:

    for ( i = 0 ; i < n ; i++ ) {
      delete [] d[i];
      delete [] f[i];
    }

Does this mean deleting the memory of every element?

[kbklpl21]

Hi salem c,

I've tried the code, I get the error of nullptr not declared in the scope, is it because my C++ is too old?

[keskiverto]

The compiler, not "C++". Which compiler do you have?

If you have GCC, but not the latest, then it uses old C++ standard by default.
Already quite old GCC does support (some) newer standards.

[kbklpl21]

Hi salem c,

Sorry, my compiler is TDM-GCC 4.9.2

[salem c]

You could try
g++ -std=c++11 prog.cpp

But if your compiler is really old, just replace all the nullptr with NULL

[kbklpl21]

Hi salem c,

How do I do that? Since I'm using Dev C++, so it comes with the old compiler. Any suggestions for other IDE's'? I tried Visual Studio but errors keep occurring in my code.

[kbklpl21]

*Update

I've changed all the nullptr to NULL, but got segmentation fault in the main function when at line p = generate_random_vector(long int n);

[salem c]

Put the latest on pastebin and I'll have a look in about 90 mins.

code::blocks is a more modern successor to dev-c++, is reasonably well maintained, and comes with more up to date compilers.

[kbklpl21]

Hi salem c,

https://pastebin.com/eRihvaTu

Sorry for the trouble.

[salem c]

You left line 233 commented out!

[kbklpl21]

Hi salem c,

Sorry for being so careless, I thought that line was extra so I commented out and I forgot I did that. Really sorry for causing you unnecessary trouble. The code works fine now, I'm sorry again and thanks.