I have been writing a java code for duplication inside an array.
Here is the code:
private int[] id;
public void union(int p, int q)
{
int pid = id[p];
int qid = id[q];
for(int i=0; i<id.length; i++)
if(id[i]==pid)id[i]=qid;
}
But my guide said had I used id[i]==id[p] instead of id[i]==pid it would have given me an error. And went on calling it an "insidious bug". But I don't see the problem with it. It should give the same result as while I am calling that function, it should take the values I mentioned as parameters and there shouldn't be a problem.
Anybody throw some light on this please?
The problem I see with your code is what if p and q are out of range? You'd get an array out of bounds exception. You should first check to see if p and q are within the array's length.
Before trying to access your array with p and q, you should somehow try to check them first.
if(p >= 0 && p < id.length)
if(q >= 0 && q < id.length)
An insidious bug will not be visible at user end but it'll continue causing problems from inside. In your code when your iteration reaches i=p, it'll update the value of id[p] and on next iteration the if condition will check for wrong value of id[p]. You won't be able to detect the error making it insidious.
Related
I am trying to understand how the concepts of parameter passing by Value Result and parameter passing by Name would apply to the following program:
// GLOBAL variables
int num[10] = {0,0,0,0,0,0,0,0,0,0}; // Subscripts start at 0 as in C++
int index = 1;
void somefun(int alpha, int beta)
{
alpha = 7;
num[index] = 33;
index = index - 6;
num[alpha] = 44;
beta = 55;
}
void main()
{
somefun( index, num[index + 4] ); // Function Call
}
This program is pseudo-code, not specific to any language, and the array subscripts start at 0.
If someone could help me understand how this program would work with parameter passing by Value Result and parameter passing by Name, I would greatly appreciate it. Thank you.
Unfortunately, I was not able to try anything on my own thus far.
I have a problem to solve with Android, but it's really confusing.
Using the function below:
function accumulate(combiner, nullValue, list){
if(list.length == 0){
return nullValue;
}
var first = list.removeFirst();
return combiner(first, accumulate(combiner, nullValue, list));
}
Develop the function sumOfSquares which returns the sum of squares of a list (Example: 1² + 2² + 3²...)
sumOfSquares([1,2,3,4,5])
returns the number 55.
In this case, the function accumulate must be used. The variable "combiner" is a "pointer to a function". The implementation of the function "combiner" is part of the solution.
I have no problem with the basics, doing the sum of squares, etc, but the part "pointer to a function" really confused me.
If anyone can tell me which is the way to get to the answer, I will be thankful :)
I have done until the code below:
public class MainActivity extends Activity{
protected void onCreate(...){
....
List<Integer> list = new ArrayList<Integer>();
//Fill the list with values
long value = accumulate(sumOfSquares(list), 0, list);
//Show the value
}
private int sumOfSquares(List<Integer> list){
int sum = 0;
for(int i = 0; i < list.size(); i++){
sum += Math.pow(list.get(i), 2);
}
return sum;
}
private long accumulate(int combiner, long nullValue, List<Integer> list){
if(list.size() == 0){
return nullValue;
}
int first = list.get(0);
list.remove(0);
return combiner(first, accumulate(combiner, nullValue, list));
}
private long combiner(int first, int rest){
return first + rest;
}
}
In some languages, the notion of a pointer to a function makes sense, and you could write the code pretty much as you've given it in the example. Not in Java, though, which is what underlies Android. (Android is a bit of a weird choice for this, by the way...)
What you want to do in Java (without giving you the whole solution) is to define a
private int combiner(int first, int rest);
method that takes the first element of the list and the solution to the smaller problem defined by the rest of the list, and produces the answer from these two bits. In other words, if first is the first element, and rest is the sum of the squares of everything except the first element, what is the sum of the squares of the whole list (in terms of first and rest)?
Now your accumulate method does almost exactly what you've written out. It just removes the first element, recursively calls itself on the rest of the list, and returns the value of combining the first element with the result of the recursive call.
The nullValue is there to give you the sum of the squares of an empty list.
If you want to look up more of the details of the theory, you're basically doing functional programming but in an imperative language :)
If I assign a Vector like
var vec1:Vector.<Number> = new Vector.<Number>(3);
vec1 = (1,2);
the result of vec1.length is 3. Is there any built-in method to return the number of the elements actually present in the vector?
I'm an ActionScript noob so any help would be appreciated.
Well you can solve your problem by creating an empty vector instead of a defining the vector size at the time of its declaration and then you gradually add and remove elements to the vector. In this way you will always get the total number of elements inside the vector when you call vector.length
For example:
var vec1:Vector.<Number> = new Vector.<Number>();
vec1.push(5);
vec1.push(6,7);
vec1.pop();
Then vec1.length would give you 2.
Its been awhile, heres the reference: http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/Vector.html
I believe the .length is the normal way to check its length. If you want to check for "empty elements" you will need to loop through it with a loop.
I don't remmember the exact syntax, but to loop, do something like:
int count = 0;
for (int i = 0; i < vec.length; i++) {
if (vec[i] == ... );
count++;
}
I have noticed a weird behavior of the variables in for loops. It's not really a problem, but it disturbs me a lot.
Actually I've created two loops this way:
for (var i:uint; i<19; i++) SomeFunction (i);
for (var i:uint; i<26; i++) SomeOtherFunction (i);
What I received was a compilation warning:
Warning: Duplicate variable definition.
This warning really surprised me. Nothing like that ever happened to me in other languages.
It seems that the i variable gets into the scope that is higher in the hierarchy and becomes available out of the loop's block. I've also tried to embrace the loop block in a curly brace, but it didn't change anything.
Why does it happen? Is it normal? Is it possible to avoid it? For now I've just set different names for both of the variables, but that's not a real solution I think. I'd really like to use the i-named variable in most of my for-loops.
yes, the loop increment variable is in the scope of the loops parent, not inside the loop itself. This is intentional, for examples like this:
public function getPositionOfValue ( value:String ) : int
{
for ( var i:int = 0; i < someArray; i++ )
{
if (someArray[i] == value )
{
break;
}
}
return i;
}
this allows you to access the value of i once the loop is over. There are lots of cases where this is very useful.
What you should do in the cases where you have multiple loops inside the same scope is var the i outside of the loops:
public function getPositionOfValue ( value:String ) : int
{
var i:int;
for ( i = 0; i < 15; i++ )
{
//do something
}
for ( i = 0; i < 29; i++ )
{
//do something else
}
return i;
}
then you get rid of your warning. The other thing to consider is to name your loop increment variables something more descriptive.
Update: Two other things to consider:
1) you shouldn't use uints except for things like colors and places where Flex expects a uint. They are slower than int's to use. Source]1 Update: it looks like this may no longer be the case in newer versions of the flash player: source
2) when you var a loop increment variable inside of a loop declaration, you want to make sure you set it to the proper initialization value, usually 0. You can get some hard to track down bugs if you dont.
As mentioned here, as3 has global and local scope and that's about it.
It does not do block-level scoping (or for-level either). With hoisting, you can even write to variables before you define them. That's the bit that would do my head in :-)
Early versions of Visual C had this bug, leading to all sorts of wonderful funky macro workarounds but this is not a bug in as3, it's working as designed. You can either restrict your code to having the declaration in the first for only or move the declaration outside all the for statements.
Either way, it's a matter of accepting that the language works one way, even though you may think that's a bad way :-)
Declare the variable i outside the loops to avoid this. As long as you reset it (i=0) you can still use it in all loops.
var i : uint;
for (i=0; i<19; i++) SomeFunction(i);
for (i=0; i<26; i++) SomeOtherFunction(i);
My code is as follows:
void Scene::copy(Scene const & source)
{
maxnum=source.maxnum;
imagelist = new Image*[maxnum];
for(int i=0; i<maxnum; i++)
{
if(source.imagelist[i] != NULL)
{
imagelist[i] = new Image;
imagelist[i]->xcoord = source.imagelist[i]->xcoord;
imagelist[i]->ycoord = source.imagelist[i]->ycoord;
(*imagelist[i])=(*source.imagelist[i]);
}
else
{
imagelist[i] = NULL;
}
}
}
A little background: The Scene class has a private int called maxnum and an dynamically allocated Array of Image pointers upon construction. These pointers point to images. The copy constructor attempts to make a deep copy of all of the images in the array. Somehow I'm getting a Segfault, but I don't see how I would be accessing an array out of bounds.
Anyone see something wrong?
I'm new to C++, so its probably something obvious.
Thanks,
I would suggest that maxnum (and maybe imagelist) become a private data member and implement const getMaxnum() and setMaxnum() methods. But I doubt that is the cause of any segfault the way you described this.
I would try removing that const before your reference and implement const public methods to extract data. It probably compiles since it is just a reference. Also, I would try switching to a pointer instead of pass by reference.
Alternatively, you can create a separate Scene class object and pass the Image type data as an array pointer. And I don't think you can declare Image *imagelist[value];.
void Scene::copy(Image *sourceimagelist, int sourcemaxnum) {
maxnum=sourcemaxnum;
imagelist=new Image[maxnum];
//...
imagelist[i].xcoord = sourceimagelist[i].xcoord;
imagelist[i].ycoord = sourceimagelist[i].ycoord;
//...
}
//...
Scene a,b;
//...
b.Copy(a.imagelist,a.maxnum);
If the source Image had maxnum set higher than the actual number of items in its imagelist, then the loop would run past the end of the source.imagelist array. Maybe maxnum is getting initialized to the value one while the array starts out empty (or maxnum might not be getting initalized at all), or maybe if you have a Scene::remove_image() function, it might have removed an imagelist entry without decrementing maxnum. I'd suggest using an std::vector rather than a raw array. The vector will keep track of its own size, so your for loop would be:
for(int i=0; i<source.imagelist.size(); i++)
and it would only access as many items as the source vector held. Another possible explanation for the crash is that one of your pointers in source.imagelist belongs to an Image that was deleted, but the pointer was never set to NULL and is now a dangling pointer.
delete source.imagelist[4];
...
... // If source.imagelist[4] wasn't set to NULL or removed from the array,
... // then we'll have trouble later.
...
for(int i=0; i<maxnum; i++)
{
if (source.imagelist[i] != NULL) // This evaluates to true even when i == 4
{
// When i == 4, we're reading the xcoord member from an Image
// object that no longer exists.
imagelist[i]->xcoord = source.imagelist[i]->xcoord;
That last line will access memory that it shouldn't. Maybe the object still happens to exist in memory because it hasn't gotten overwritten yet, or maybe it has been overwritten and you'll retrieve an invalid xcoord value. If you're lucky, though, then your program will simply crash. If you're dealing directly with new and delete, make sure that you set a pointer to NULL after you delete it so that you don't have a dangling pointer. That doesn't prevent this problem if you're holding a copy of the pointer somewhere, though, in which case the second copy isn't going to get set to NULL when you delete-and-NULL the first copy. If you later try to access the second copy of the pointer, you'll have no way of knowing that it's no longer pointing to a valid object.
It's much safer to use a smart pointer class and let that deal with memory management for you. There's a smart pointer in the standard C++ library called std::auto_ptr, but it has strange semantics and can't be used in C++ containers, such as std::vector. If you have the Boost libraries installed, though, then I'd suggest replacing your raw pointers with a boost::shared_ptr.