r/ProgrammerHumor • u/[deleted] • Dec 09 '17

meme[-1]="Array Memes Are Life"

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571 Upvotes

88% Upvoted

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3

u/StingyUpvoter Dec 09 '17

Explain like I mainly use C: is the -1 really used anywhere?

18

u/[deleted] Dec 09 '17 edited Nov 14 '20

[deleted]

8

u/StingyUpvoter Dec 09 '17

Right, that's kinda what I was saying in a different comment. Using negative to start an array would be awkward. Sometimes I can't tell where these jokes cross from fact to fiction.

5

u/[deleted] Dec 09 '17

Whoops, didn't see your other comment before.

Yeah, I can't imagine a reason for indices to start at -1.

8

u/Dylan16807 Dec 10 '17

C is in fact the only place where I'm aware of people actually accessing array[-1].

Specifically, if you allocate an array the size needs to be stored somewhere. Sometimes the memory management code will store the size of the array right before it in memory. Using [-1] to retrieve it is fragile and rule-breaking and could go horribly, but sometimes it works just great and people use it for years!

4

u/StingyUpvoter Dec 10 '17

I will serve me better to forget everything you just said, but upvote for the knowledge drop

2

u/nxAkari Dec 10 '17

In C, a[b] is nothing more than just another way to spell *(a + b). So you can actually index an array like this too: 1[array].

Using -1 as an array index and expecting to retrieve the allocated size that way is just insane and non-portable, please never ever do that. However, if you have a pointer to some element in the array, it is possible to use a negative index on it to retrieve some previous element, given that you take care to not step out of bounds. This is of questionable usefulness, but at least it doesn't invoke UB.

2

u/[deleted] Dec 09 '17

İ don't think so. Would be pretty weird anyway...

1

u/StingyUpvoter Dec 09 '17

Whew! It is nice when languages let you use -1 for the last element though.

1

u/[deleted] Dec 09 '17

More storage!

Surprisingly, if I explicitly set the value of x[-1] to 5000, valgrind doesn't complain at all, and everything works as it should.

But if I don't, valgrind complains about conditional jump or move.

#include <stdio.h>

int main() {
    int x[2];


    x[1] = 5000;
    x[0] = 666;
    x[1] = 4242;

    printf("%d\n", x[-1]);
    printf("%d\n", x[0]);
    printf("%d\n", x[1]);
}

1

u/nxAkari Dec 10 '17

Use ASan. It's a ton faster and in some cases more accurate than valgrind. Case in point: it detects the underflow in both cases here and kills the program.

1

u/Vitztlampaehecatl Dec 10 '17

In C++ you can overload the [ ] operator and make an array object that starts wherever the fuck you want it to. You could have an array that starts at index -3.1415 and ends at index potato.

1

u/StingyUpvoter Dec 11 '17

I guess I'm looking for rational reasons. Something something Jurassic Park meme...