Are negative array indexes allowed in C?

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I was just reading some code and found that the person was using arr[-2] to access the 2nd element before the arr, like so:

|a|b|c|d|e|f|g|
       ^------------ arr[0]
         ^---------- arr[1]
   ^---------------- arr[-2]

Is that allowed?

I know that arr[x] is the same as *(arr + x). So arr[-2] is *(arr - 2), which seems OK. What do you think?

That is correct. From C99 §6.5.2.1/2:

The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))).

There's no magic. It's a 1-1 equivalence. As always when dereferencing a pointer (*), you need to be sure it's pointing to a valid address.

Are negative array indexes allowed in C?, That is correct. From C99 �6.5.2.1/2: The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))). There's no magic. An array in most programming languages is just a contiguous area of addressable storage and the language, say C, Rexx, NetRexx/JAVA keeps track of each element's 'address' starting from zero. When

This is only valid if arr is a pointer that points to the second element in an array or a later element. Otherwise, it is not valid, because you would be accessing memory outside the bounds of the array. So, for example, this would be wrong:

int arr[10];

int x = arr[-2]; // invalid; out of range

But this would be okay:

int arr[10];
int* p = &arr[2];

int x = p[-2]; // valid:  accesses arr[0]

It is, however, unusual to use a negative subscript.

Why do negative array indices make sense?, The array indexing operation a[i] gains its meaning from the following features of C. The syntax a[i] is equivalent to *(a + i) . Thus it is valid to say 5[a] to get at the� Hi guys, today we will discuss how to declare an array, how to enter the values in an array, and how to access the negative array index in C++. Before moving to the negative index, let us discuss something about arrays. Now, let us see how to define an array. The syntax of array declaration is: data_type variable_name [size]; Example- int arr[5

Sounds fine to me. It would be a rare case that you would legitimately need it however.

Negative Indexing Multi Arrays in C/++, When we query the array (i.e. access elements), we are performing pointer arithmetic to extract the value from a single 6 unit memory block. int arr� the first element. I want to use a negative index to derefence the pointer, like so: a[-1]. Is this valid C99? If not, I can just say *(a-1), right? a[ -1 ] and *(a-1) are identical, the second being what the compiler converts the first to (already with C89). And yes, you can access elements of the array also with negative indexes for 'a' as long as

What probably was that arr was pointing to the middle of the array, hence making arr[-2] pointing to something in the original array without going out of bounds.

negative index of array - general, hi guys can you explain what the meaning of negative index. i read someone code and In C/C++, the element a[i] just means go to the i th element starting from the base But otherwise, that code is, allow me to use some compiler warning� As we know, indexes are used in arrays in all the programming languages. We can access the elements of an array by going through their indexes. But no programming language allows us to use a negative index value such as -4. Python programming language supports negative indexing of arrays, something which is not available in arrays in most other programming languages.

I'm not sure how reliable this is, but I just read the following caveat about negative array indices on 64-bit systems (LP64 presumably): http://www.devx.com/tips/Tip/41349

The author seems to be saying that 32 bit int array indices with 64 bit addressing can result in bad address calculations unless the array index is explicitly promoted to 64 bits (e.g. via a ptrdiff_t cast). I have actually seen a bug of his nature with the PowerPC version of gcc 4.1.0, but I don't know if it's a compiler bug (i.e. should work according to C99 standard) or correct behaviour (i.e. index needs a cast to 64 bits for correct behaviour) ?

Negative indices in C, Negative indices in C. Hello Friends,. Anyone please explain to me what happens when i supply negative index in C array. I got garbage value when i tried it. To explain how negative indexes work, you first have to learn (or remember) that for any array or pointer a and index i, the expression a[i] is equal to *(a + i). That means that you can have a pointer to the middle element of an array, and use it with a positive or negative index, and it's simple arithmetic.

Negative array indexing, Below, gorgonzola demonstrates how beautifully C++ lets you build array-like classes that allow various forms of negative array indexing. As with everything� As for negative indexes, they compile, but if the resulting location is outside the array (it is in your case), the behavior of the program is undefined. It can do anything at all, including giving you any sort of output, crashing, mysteriously skipping chunks of code, etc. Many classes of errors in C++ (and in C) behave that way.

Arrays in C/C++, An array in C or C++ is a collection of items stored at contiguous memory locations and elements can be accessed randomly using indices of an array. They are� In C++, ordinary pointers (and therefore also arrays) are random access iterators. And random access iterators permit values to be accessed via the offset dereference operator (more commonly known as the subscript or [] operator). As others have pointed out, the negative index can be legal.

Are negative index in array allowed? - C++ Forum, Are negative indexes allowed in c++?. 1 2 3 4 5 6 7 8 9 10 11 Are negative table indexes allowed in C? advertisements I was just reading some code and found that the person was using arr[-2] to access the 2nd element before the arr , like so:

Comments
  • Note also that you don't have to dereference the pointer to get UB. Merely computing somearray-2 is undefined unless the result is in the range from the start of somearray to 1 past its end.
  • In older books the [] were referenced as a syntax sugar for pointer arithmetic. Favorite way to confuse beginners is to write 1[arr] - instead of arr[1] - and watch them guessing what that supposed to mean.
  • What happens on 64 bit systems (LP64) when you have a 32 bit int index which is negative ? Should the index get promoted to a 64 bit signed int prior to the address calculation ?
  • @Paul, from §6.5.6/8 (Additive operators), "When an expression that has integer type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the pointer operand points to an element of an array object, and the array is large enough, the result points to an element offset from the original element such that the difference of the subscripts of the resulting and original array elements equals the integer expression." So I think it will be promoted, and ((E1)+(E2)) will be a (64-bit) pointer with the expected value.
  • @Matthew: thanks for that - it sounds like it should work as one might reasonably expect.
  • I wouldn't go so far as to say it's invalid, just potentially messy
  • @Matt: The code in the first example yields undefined behavior.
  • It is invalid. By the C standard, it explicitly has undefined behavior. On the other hand, if int arr[10]; were part of a structure with other elements before it, arr[-2] could potentially be well-defined, and you could determine if it is based on offsetof, etc.
  • Found it in K&R Section 5.3, near the end: If one is sure that the elements exist, it is also possible to index backwards in an array; p[-1], p[-2], and so on are syntactically legal, and refer to the elements that immediately precede p[0]. Of course, it is illegal to refer to objects that are not within the array bounds. Still, your example is better in help me understand it. Thanks!
  • Sorry for the thread necromancy, but I just love how K&R are ambiguous as to what "illegal" means. The last sentence makes it sound like out-of-bounds accesses throw a compilation error. That book is poison for beginners.
  • It's not that rare - it's very useful in e.g. image processing with neighbourhood operators.
  • I just needed to use this because I am creating a memory pool with a stack and heap [ structure / design ] . The stack growing towards higher memory addresses, the heap growing towards lower memory addresses. Meeting in the middle.
  • This sounds like a compiler bug.
  • While this code may answer the question, providing additional context regarding why and/or how this code answers the question improves its long-term value.
  • Python groovy... have them. A simple use-case is one can access last element of an array without knowing the array size, a very real requirement in many Project situations. Also many DSLs benefit from this.