Array pointer arithmetic question

Question

Is there a way to figure out where in an array a pointer is?

Lets say we have done this:

int nNums[10] = {'11','51','23', ... };   // Some random sequence
int* pInt = &nNums[4];                     // Some index in the sequence.

...

pInt++;      // Assuming we have lost track of the index by this stage.

...

Is there a way to determine what element index in the array pInt is 'pointing' to without walking the array again?

int* pInt = nNums[4]; needs to be int* pInt = &nNums[4];. — James McNellis
– James McNellis, Commented Mar 31, 2010 at 17:13

James McNellis · Accepted Answer · 2010-03-31 17:10:36Z

17

Yes:

ptrdiff_t index = pInt - nNums;

When pointers to elements of an array are subtracted, it is the same as subtracting the subscripts.

The type ptrdiff_t is defined in <stddef.h> (in C++ it should be std::ptrdiff_t and <cstddef> should be used).

edited Mar 31, 2010 at 17:10

answered Mar 31, 2010 at 17:05

James McNellis

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Comments

SamB · Accepted Answer · 2010-03-31 17:06:03Z

1

Yeah. You take the value of:

pInt - nNums

answered Mar 31, 2010 at 17:06

SamB

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Comments

KTC · Accepted Answer · 2010-03-31 17:06:14Z

1

ptrdiff_t delta = pInt - nNums;

answered Mar 31, 2010 at 17:06

KTC

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Comments

miked · Accepted Answer · 2010-03-31 17:09:46Z

1

If I'm understanding your question (it's not too clear)

Then

int offset=pInt-nNums;

will give you how far from the beginning of nNums pIint is. If by

int* pInt=nNum[4];

you really meant

int* pInt=nNums+4;

then in

int offset=pInt-nNums

offset will be 4, so you could do

int value=nNums[offset]

which would be the same as

int value=*pInt

answered Mar 31, 2010 at 17:09

miked

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Comments

N 1.1 · Accepted Answer · 2010-03-31 17:45:39Z

1

pInt - nNums

Also,

int* pInt = nNums[4] is probably not what you want. It will point to memory, address of which would be nNums[4]

Change it to

int* pInt = &nNums[4];

edited Mar 31, 2010 at 17:45

answered Mar 31, 2010 at 17:10

N 1.1

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2 Comments

KTC Over a year ago

Um, NOT the last line. You're assigning the value in nNums[4] to an uninitialized variable.

KTC Over a year ago

@JonH, now the address of nNums[4] (type int*) is being assigned to a pointer that is dereferenced (type int). Type mismatch....

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