Thread: Define an array/vector

Hello, I have tried this for many hours now, but I can't get it to work.


Lets say I want to define an array, with 10 places that are zero, then this work:

Code:

#include <stdlib.h>

main(){


int A[10]={0};


}

But lets say that the size of the array is not know. Call it x, the user will actually give x, then I have to use a code like

Code:

#include <stdlib.h>

main(){
int x;
x=10;

int A[x]={0};

}

testy.c: In function ‘main’:
testy.c:7: error: variable-sized object may not be initialized
testy.c:7: warning: excess elements in array initializer
testy.c:7: warning: (near initialization for ‘A’)

Why do I get this, one must be able to define an array even though you as the programmer don't know the size?


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/////////////////////////////////////////////////////////QUESTION 2////////////////////////////////////////////////////////////////////////////////////////////
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If I define an Array to be to size 10. I am still able to pick out values from A[12], A[13] etc., why is this? Have I dont something wrong? Shouldnt I get an error-message that it is out of the size of the array?



Thank you very much for reading this. I hope you can help me.

> testy.c:7: error: variable-sized object may not be initialized
The message says it all really, you need to initialise it yourself with a loop.

> I am still able to pick out values from A[12], A[13] etc., why is this?
C doesn't provide a safety net against doing dumb things.

Although some compilers can check an array in scope (and tools like Lint do it better), the problem gets much harder when you pass arrays to functions. At this point, all the size information disappears.

> Have I dont something wrong?
Yes, stepping off the end of the array is a bad thing to do.

> Shouldnt I get an error-message that it is out of the size of the array?
Using other tools (eg. valgrind), you can find array overrun problems.

There is something wrong with your slash-key...

The size of the array (A[x]) is not known at compile time, (x is set at runtime) so the compiler got a problem assigning things to it when compiling. Use...

Code:

int main(void) {

    int x = 10, A[x];

    memset(A, 0, sizeof(int) * x);

}

...instead.

Why would you like pick out values outside the array? If you really wanted to do that, why should you get a error message?

Originally Posted by kliro89

But lets say that the size of the array is not know.
Call it x, the user will actually give x, then I have to use a code like

Code:

#include <stdlib.h>

main(){
int x;
x=10;
int A[x]={0};
}

Why do I get this, one must be able to define an array even though you as the programmer don't know the size?

Generally what you need to do is wait till the user enters the x value before sizing your array...

On c-99 compliant compilers:

Code:

int main (void)   // Note: This is the correct form for main!
  { int x;
     
     puts ("Enter the array size please  : ");
     scanf("%d", &x);
  
     int array[x];

     // do stuff

    return 0;
   }

On pre-C99 compilers:

Code:

int main (void)
  {  int x;
      int *array;
      puts ("Enter the array size please : ");
      scanf ("%d",&x);

      array = calloc(x,sizeof(int));

      // do what you need here

      free(array);
     
      return 0;  
   }

It can't allocate memory for the the array until the size is known.

If I define an Array to be to size 10. I am still able to pick out values from A[12], A[13] etc., why is this?
Have I dont something wrong? Shouldnt I get an error-message that it is out of the size of the array?

No you haven't... However; C does not include range checking, so you have to do this by yourself.
It is up to you as a programmer to always stay within array bounds with your code.

Preferably
memset(A,0,sizeof(A));
Sure that will set all bits 0 which is OK for int. but not for pointer,float,...

If I define an Array to be to size 10. I am still able to pick out values from A[12], A[13] etc., why is this? Have I dont something wrong? Shouldnt I get an error-message that it is out of the size of the array?

Yes, You did do something wrong, No, you won't get any error/warning.
It's your responsibility to make sure that you access withing array bound.

> What makes you think that?
Question 7.31
Question 5.17
Whilst you might get away with it on today's common hardware, it would still remain an assumption to believe that it was guaranteed to always work.

Thanks guys, you have been extremely helpful. I just have three follow-up question. What also seemed to work in my program was that if the size x is unknown, I can still write:

Code:

int A[x];

but not

Code:

int A[x]={0};  //this was the problem that you now have helped me with

...1...But when I try to use the first one, I think I get some random numbers in the array. It allways gives that yes?

....2....Someone mentioned a loop, did you mean to use this then?

Code:

#include <stdio.h>


int main(){

int x;
printf("size of array; \n");
scanf("%i", &x);
int A[x];
int i;
for (i=0;i<x;i=i+1)
A[i]=0;

}

...3...This seems to work, I guess it is just as good as doing the calloc or malloc commands?