access violation (segmentation fault)

**MarlonDean** · 05-15-2008

Why does this code cause my program to crash with the error "access violation (segmentation fault)"? and what is it?

Code:

for (counter=0;counter<UsedSpace;counter++)
{
   if (Numbers1[counter]<Numbers2[counter])
     {
     sort[counter]=Numbers1[counter];
     }
     else sort[counter]=Numbers2[counter];
}

thanks in advance

**laserlight** · 05-15-2008

Perhaps you are accessing an array out of bounds.

**anon** · 05-15-2008

You are providing a bit too little info to answer that.
Most probably you have out-of-bounds accesses. For example, is sort resized to UsedSpace.

Anyway, if this is meant for merging two vectors, you'll need to figure out the algorithm first (for example, on paper).

Something like this (X marks past end of array, if any of the pointers reaches that, you'll need to figure out something in addition):

Code:

1 2 5 X
^
a

3 4 7 X
^
b



a points to smaller. Pick *a and advance a.

1 2 5 X
  ^
  a

3 4 7 X
^
b

1

a still points to smaller. Pick *a and advance it again.

1 2 5 X
    ^
    a

3 4 7 X
^
b

1 2

Now b points to a smaller value. Pick *b and advance it.

1 2 5 X
    ^
    a

3 4 7 X
  ^
  b

1 2 3

etc.

**MarlonDean** · 05-15-2008

Ok thanks guys, guess its back to the drawing board. Again thank you

**Elysia** · 05-15-2008

Here's another tip for you:
Use std::vector if you aren't, then use the .at member function.
If you try to access out-of-bounds, it will throw an exception.

**anon** · 05-15-2008

You are also probably better off adding items to Sort (which is initially empty) using push_back(), rather than indices.

**MarkZWEERS** · 05-16-2008

But then you might want to precede the 'push_back' by a 'reserve' for efficiency...

**laserlight** · 05-16-2008

But then you might want to precede the 'push_back' by a 'reserve' for efficiency...

Take a look at the last paragraph of Stroustrup's answer to the FAQ: Why are the standard containers so slow?

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