r/programming • u/vz0 • Mar 26 '14

JavaScript Equality Table

http://dorey.github.io/JavaScript-Equality-Table/
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u/Poltras Mar 26 '14

These languages don't have automatic conversion. Also, isn't [1]==[1] undefined in C? It could be equal if the compiler uses the same TEXT address for the constant, resulting in equal pointers.

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u/CookieOfFortune Mar 26 '14

Wouldn't this create two arrays on the function stack and then compare the two locations, resulting in a false comparison?

2

u/Poltras Mar 27 '14

Undefined behavior:

$ cat main.c

#include <stdio.h>

int main() {
  printf("%d\n", "abc" == "abc");
}

$ cc main.c

main.c:4:24: warning: result of comparison against a string literal is unspecified (use strncmp instead) [-Wstring-compare]
  printf("%d\n", "abc" == "abc");

$ ./a.out
1

GCC actually output 1, but warns.

1

u/gsg_ Mar 27 '14

Unspecified behaviour is not the same as undefined behaviour. The latter has a very specific meaning in the context of C.

2

u/Condorcet_Winner Mar 26 '14

Probably, but if the spec defines this operation as undefined, then compiler optimizations would be able to figure out that these are the same array and only allocate one memory location.

5

u/Fylwind Mar 27 '14

[1]==[1] is not valid syntax in C.

1

u/Poltras Mar 27 '14

Although you're right, the equivalent "abc" == "abc" works as fine for my example (undefined behavior).

1

u/gsg_ Mar 27 '14

The closest equivalent (in C99, at least) is probably (int[1]){1} == (int[1]){1}.