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Nov 13th, 2008
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Reverse a pointer to a set of integers

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Hi all,

Just a simple question. How do i reverse a pointer to a set of values given that i know the size of the pointer?

Eg
int *temp = malloc(ptr_size);
// values of *ptr let say is 1......100,
// if i use the algorithm below, it gets me 1..50...1 and not 100...1

ptr_size2 = ptr_size;
temp = ptr;
for (i=0;i<ptr_size;i++) {
temp[i]=ptr[ptr_size2--];
}

I also tried using temp[ptr_size] but still gives me the same result
Any idea?
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Re: Reverse a pointer to a set of integers

where you set the "ptr_size" value. Meybe it is equal to 50
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Re: Reverse a pointer to a set of integers

Problem is you are going both ways!
You are incrementing i iterator and at same time decrementing ptr_size2.
Write something like:
  1. ptr_size2 = ptr_size;
  2. temp = ptr;
  3. while (ptr_size2>=0) {
  4. temp[ptr_size-ptr_size2]=ptr[ptr_size2--];
  5. }
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Re: Reverse a pointer to a set of integers

Thx for the quick response.
However after using ur codes, my compiler gave this warning:
operation on 'ptr_size2' may be undefined
and won't compile.
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Re: Reverse a pointer to a set of integers

I only gave you an idea... Probably I messed up some interators, so in some point you call temp[negative]...
Check for this, it shouldn't be hard
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Re: Reverse a pointer to a set of integers

Sorry, I still don't have any clue what is wrong. I also don't understand what is wrong with "incrementing i iterator and at same time decrementing ptr_size2"? Both are different integers, so incrementing iterator i won't affect ptr_size2 and vice versa.
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Re: Reverse a pointer to a set of integers

I apologise, my bad (still early in the morning here )
I just realised that you decrease ptr_size2, not ptr_size, so it shouldn't be problem there.
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Re: Reverse a pointer to a set of integers

This is likely what happens in my loop
.
.
temp[49] = ptr[51]
temp[50] = ptr[50]
temp[51] = ptr[49]

.
.
ptr[49] is actually referring to temp[49] since they are shared pointees. So it's almost the same like

temp[51] = temp[49]
temp[52] = temp[48]
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Re: Reverse a pointer to a set of integers

Oh, yes... Your temp is actually same memory space as ptr. So:
  1. temp[0] = ptr[99]
  2. ...
  3. temp[99]=ptr[0]

But ptr[0] is temp[0], which is actually ptr[99]

You have to make a REAL copy of ptr to temp, where:
temp != ptr , but
temp[i] == ptr[i]
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Re: Reverse a pointer to a set of integers

Yes, i already tried that, as mentioned in my first post.
int temp[ptr_size];

temp[i] = ptr[i];
ptr[i]=temp[ptr_size2--];

It still gives me the same ptr result that is 1...50...1
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