C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  negative array index?          C++Talk.NET Forum Index -> C Language (Moderated) View previous topic :: View next topic   Author Message s5n Guest Posted: Mon Feb 19, 2007 10:40 pm    Post subject: negative array index? Hello, all! i've seen the following construct in several pieces of C code, and i can't for the life of me figure out (A) what it is and (B) why it is legal: .... myarray[-1] ... Can someone please explain to me what it means to have a negative array index? Example snippets, taken from the SpiderMonkey source code: stephan@ludo:~/cvs/SpiderApe/src/js> grep -e '\[-1' *.* js.c: value[-1] = '='; .... jsstr.c: argv[-1] = STRING_TO_JSVAL(str); If someone could explain this phenomenon to me i could certainly sleep better at night. :) -- comp.lang.c.moderated - moderation address: clcm (AT) plethora (DOT) net -- you must have an appropriate newsgroups line in your header for your mail to be seen, or the newsgroup name in square brackets in the subject line. Sorry. Back to top Douglas A. Gwyn Guest Posted: Thu Feb 22, 2007 10:11 am    Post subject: Re: negative array index? "s5n" <stephan (AT) s11n (DOT) net> wrote in message news:clcm-20070219-0003 (AT) plethora (DOT) net... Quote: i've seen the following construct in several pieces of C code, and i can't for the life of me figure out (A) what it is and (B) why it is legal: ... myarray[-1] ... Your C textbook should have explained that pointer arithmetic is actually the more fundamental notion, and that a[i] is treated as equivalent to *(a+i). [Also recall that in almost every context (operand of sizeof being the notable exception), the name of an array is automatically converted to a pointer to the first element of that array, not relevant in this instance, but it explains why the usual subscripting works for a declared array.] Now, myarray[-1] wouldn't be valid if "myarray" were actually the name of a declared array, but code using such a construct will actually be using a pointer variable, not an array name. p[-1] merely means *(p-1), i.e., the contents of the object located immediately below where the pointer currently points. The following diagram may help; there is a char array (whose name, if any, we don't care about) containing the sequence of characters spelling "cherrybomb", and p is a pointer to type char that has previously been set up to point to the first 'b'; then p[-1] designates the contents of the preceding char, currently 'y'. c h e r r y b o m b ... . . . . . ^ . . . ... . . . . . p . . . [You need to view that using a constant-width font for it to line up properly.] -- comp.lang.c.moderated - moderation address: clcm (AT) plethora (DOT) net -- you must have an appropriate newsgroups line in your header for your mail to be seen, or the newsgroup name in square brackets in the subject line. Sorry. Back to top Hans-Bernhard Bröker Guest Posted: Thu Feb 22, 2007 10:12 am    Post subject: Re: negative array index? s5n wrote: Quote: i've seen the following construct in several pieces of C code, and i can't for the life of me figure out (A) what it is and (B) why it is legal: ... myarray[-1] ... That would be because, ripped out of context like this, it's (A) meaningless, and (B) there's no reason to assume it might be legal. Quote: Can someone please explain to me what it means to have a negative array index? The clue, if any, would be that the above doesn't necessarily mean that there's an actual array index (-1) involved. As long as myarray is the wrong name for this thing, i.e. it is *not* actually an array, the above can work. E.g. if myarray is actually a pointer to the 5th element of an actual array, the above expression is just a reference to its 4th element. -- comp.lang.c.moderated - moderation address: clcm (AT) plethora (DOT) net -- you must have an appropriate newsgroups line in your header for your mail to be seen, or the newsgroup name in square brackets in the subject line. Sorry. Back to top Markus Moll Guest Posted: Thu Feb 22, 2007 10:12 am    Post subject: Re: negative array index? Hi s5n wrote: Quote: i've seen the following construct in several pieces of C code, and i can't for the life of me figure out (A) what it is and (B) why it is legal: ... myarray[-1] ... Can someone please explain to me what it means to have a negative array index? Well, a[b] means *(a+b), so myarray[-1] means *(myarray-1). Quote: If someone could explain this phenomenon to me i could certainly sleep better at night. Well, if myarray does not point to the beginning of an array, but inside some array, then myarray[-1] just accesses the element one before myarray. Markus -- comp.lang.c.moderated - moderation address: clcm (AT) plethora (DOT) net -- you must have an appropriate newsgroups line in your header for your mail to be seen, or the newsgroup name in square brackets in the subject line. Sorry. Back to top Andrei Voropaev Guest Posted: Thu Feb 22, 2007 10:12 am    Post subject: Re: negative array index? On 2007-02-19, s5n <stephan (AT) s11n (DOT) net> wrote: Quote: i've seen the following construct in several pieces of C code, and i can't for the life of me figure out (A) what it is and (B) why it is legal: ... myarray[-1] ... Read K&R "The C Programming Language". "The expression E1[E2] is identical (by definition) to *((E1)+(E2))" -- Minds, like parachutes, function best when open -- comp.lang.c.moderated - moderation address: clcm (AT) plethora (DOT) net -- you must have an appropriate newsgroups line in your header for your mail to be seen, or the newsgroup name in square brackets in the subject line. Sorry. Back to top Clark S. Cox III Guest Posted: Thu Feb 22, 2007 10:12 am    Post subject: Re: negative array index? s5n wrote: Quote: Hello, all! i've seen the following construct in several pieces of C code, and i can't for the life of me figure out (A) what it is and (B) why it is legal: .... myarray[-1] ... Can someone please explain to me what it means to have a negative array index? It means the same as a positive index: a[b] is equivalent to *(a + b), so a[-1] is equivalent to *(a - 1) If you have a pointer that points into an array (i.e. not at the first element), a negative index is perfectly legal: int arr[10] = ...; int p = arr +5; p[-5] == arr[0]; p[-4] == arr[1]; p[-3] == arr[2]; p[-2] == arr[3]; p[-1] == arr[4]; p[0] == arr[5]; p[1] == arr[6]; p[2] == arr[7]; p[3] == arr[8]; p[4] == arr[9]; -- Clark S. Cox III clarkcox3 (AT) gmail (DOT) com -- comp.lang.c.moderated - moderation address: clcm (AT) plethora (DOT) net -- you must have an appropriate newsgroups line in your header for your mail to be seen, or the newsgroup name in square brackets in the subject line. Sorry. Back to top Kenneth Brody Guest Posted: Thu Feb 22, 2007 10:12 am    Post subject: Re: negative array index? s5n wrote: Quote: Hello, all! i've seen the following construct in several pieces of C code, and i can't for the life of me figure out (A) what it is and (B) why it is legal: ... myarray[-1] ... Can someone please explain to me what it means to have a negative array index? Example snippets, taken from the SpiderMonkey source code: stephan@ludo:~/cvs/SpiderApe/src/js> grep -e '\[-1' *.* js.c: value[-1] = '='; ... jsstr.c: argv[-1] = STRING_TO_JSVAL(str); If someone could explain this phenomenon to me i could certainly sleep better at night. You can't (legally) take a negative subscript of an array. However, it may be that "value" and "argv" in the above examples aren't really arrays, but pointers. And, if the code guarantees that they point into an array at something other than array[0], they are valid. In that case, you can think of the above as: *(value-1) = '='; or *(argv-1) = STRING_TO_JSVAL(str); That is, "the previous array entry is assigned the value of...". Perhaps these references are in a loop, in which the pointer has been incremented prior to the lines with the "[-1]" reference? -- +-------------------------+--------------------+-----------------------+ | Kenneth J. Brody | www.hvcomputer.com | #include | | kenbrody/at\spamcop.net | www.fptech.com | <std_disclaimer.h> | +-------------------------+--------------------+-----------------------+ Don't e-mail me at: <mailto:ThisIsASpamTrap (AT) gmail (DOT) com> -- comp.lang.c.moderated - moderation address: clcm (AT) plethora (DOT) net -- you must have an appropriate newsgroups line in your header for your mail to be seen, or the newsgroup name in square brackets in the subject line. Sorry. Back to top Jack Klein Guest Posted: Thu Feb 22, 2007 10:12 am    Post subject: Re: negative array index? On 19 Feb 2007 16:40:37 GMT, "s5n" <stephan (AT) s11n (DOT) net> wrote in comp.lang.c.moderated: Quote: Hello, all! i've seen the following construct in several pieces of C code, and i can't for the life of me figure out (A) what it is and (B) why it is legal: ... myarray[-1] ... Can someone please explain to me what it means to have a negative array index? Example snippets, taken from the SpiderMonkey source code: stephan@ludo:~/cvs/SpiderApe/src/js> grep -e '\[-1' *.* js.c: value[-1] = '='; ... jsstr.c: argv[-1] = STRING_TO_JSVAL(str); If someone could explain this phenomenon to me i could certainly sleep better at night. :) The code might be valid, and it might not be. Can't say for sure without seeing where "value", "myarray", and "argv" come from. This is perfectly legal C (if included inside a function, of course): char hello [] = "hello"; char *cp = hello + 4; cp [-4] = 'j'; The result is that the character array 'hello' will contain the string "jello" So if you have a pointer into an array object, beyond the first element, you can use a negative subscript so long as it does not try to access memory beyond the beginning of the array. -- Jack Klein Home: http://JK-Technology.Com FAQs for comp.lang.c http://c-faq.com/ comp.lang.c++ http://www.parashift.com/c++-faq-lite/ alt.comp.lang.learn.c-c++ http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html -- comp.lang.c.moderated - moderation address: clcm (AT) plethora (DOT) net -- you must have an appropriate newsgroups line in your header for your mail to be seen, or the newsgroup name in square brackets in the subject line. Sorry. Back to top s5n Guest Posted: Sun Feb 25, 2007 10:11 am    Post subject: Re: negative array index? On Feb 19, 5:40 pm, "s5n" <step...@s11n.net> wrote: Quote: Can someone please explain to me what it means to have a negative array index? A general response to all who have answered my question: Thanks very much for the explanation. More digging around in the SpiderMonkey sources reveals that they do indeed over-allocate an array and then tell client code that the array starts somewhere else, keeping a few elements for themselves for bookkeeping. -- comp.lang.c.moderated - moderation address: clcm (AT) plethora (DOT) net -- you must have an appropriate newsgroups line in your header for your mail to be seen, or the newsgroup name in square brackets in the subject line. Sorry. 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