What does var[x] do?

Question

Regarding c variables, I want to know what the x does in:

int var[x]

What are `var` and `x`? Is `var` an array? and `x` some sort of integer? — UnholySheep, Feb 08 '17 at 19:18
We don't know what `x` is, so I did not mention VLA in my first comment. For all we know, it is `#define x 42` although it is conventional to use uppercase. — Weather Vane, Feb 08 '17 at 19:22
I'm voting to close this question as off-topic because this information is available in the first pages of any text on `c` — KevinDTimm, Feb 08 '17 at 20:12

score 3 · Accepted Answer · edited May 23 '17 at 12:24

3

This is a declaration of a Variable Length Array (VLA).

The value of expression x (most likely, a variable) is treated as the number of array elements. It must have a positive value at the time the expression is evaluated, otherwise the declaration of the VLA produces undefined behavior.

edited May 23 '17 at 12:24

Community

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answered Feb 08 '17 at 19:20

Sergey Kalinichenko

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AFAIK the variable length array was in C99 standard, but made optional in C11. So the VLA must have seemed like the best thing ever, but for reasons I have not followed fell from favour. Perhaps MS was wise never to have implemented VLA (tongue in cheek). – Weather Vane Feb 08 '17 at 19:36
@WeatherVane I agree, VLAs introduce more problems than they solve, because you never know if the declaration is going to crash for `x` being too large for the stack. – Sergey Kalinichenko Feb 08 '17 at 19:38
_This is a declaration of a Variable Length Array (VLA)._ Not in this case: `enum values{n, x, y, z};` :) – David Ranieri Feb 08 '17 at 19:47
@KeineLust Absent OP's explanation, it's impossible to know. Given that OP's asking, it's unlikely that `x` is constant, because declarations with constants are a lot more common. – Sergey Kalinichenko Feb 08 '17 at 19:49
_because you never know if the declaration is going to crash for x being too large for the stack._, good point, what about `int *a[n]; a = malloc(sizeof(*a) * 1000000);`, where `n` is `4`, this will not overflow the stack because `a` is just an array of 4 pointers, even if we reserve a lot of space in the next operation, isn't it? – David Ranieri Feb 08 '17 at 19:55
@KeineLust Even simpler - `int *var = malloc(sizeof(*var)*x)`. The only downside is having to call `free`, but that's not as scary as a possibility to overflow the stack. – Sergey Kalinichenko Feb 08 '17 at 20:00
@dasblinkenlight, I'm not making myself clear. This will overflow the stack: `int n = 100000000; int a[4][n];` but this not: `int n = 4; int (*a)[n]; a = malloc(sizeof(*a) * 1000000);`, is my assumption correct? – David Ranieri Feb 08 '17 at 20:10
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@KeineLust `int *a[n]` is still a large VLA. If you want an array of Nx4, use `int (*a)[4] = malloc(n*sizeof(*a));` with parentheses around `*a`. – Sergey Kalinichenko Feb 08 '17 at 20:23

score 0 · Answer 2 · answered Feb 08 '17 at 19:19

The int defines the type of the variable, the [] defines the variable as an array, the number in the [] says the number of elements in the array. Int var[4] sets var as an array of 4 ints So, after that, var[0] is the first element, var[1] the second and var[3] the last one (index starts in 0 as you can see)

What does var[x] do?

2 Answers2