A variable length array is an array in C99 and other languages whose size is unknown at compile time; instead, it's determined at runtime.
Questions tagged [variable-length-array]
413 questions
32
votes
8 answers
Convert Python sequence to NumPy array, filling missing values
The implicit conversion of a Python sequence of variable-length lists into a NumPy array cause the array to be of type object.
v = [[1], [1, 2]]
np.array(v)
>>> array([[1], [1, 2]], dtype=object)
Trying to force another type will cause an…
Marco Ancona
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28
votes
3 answers
Variable-length std::array like
As my usually used C++ compilers allow variable-length arrays (e.g. arrays depending on runtime size), I wonder if there is something like std::array with variable size? Of course std::vector is of variable size, but it allocates on heap, and…
dronus
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25
votes
3 answers
Is the operand of `sizeof` evaluated with a VLA?
An argument in the comments section of this answer prompted me to ask this question.
In the following code, bar points to a variable length array, so the sizeof is determined at runtime instead of compile time.
int foo = 100;
double…
PC Luddite
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24
votes
3 answers
Variable length array in the middle of struct - why this C code is valid for gcc
There is some strange code using VLA (Variable Length Arrays) which is treated as Valid C (C99, C11) by gcc 4.6:
$ cat a.c
int main(int argc,char**argv)
{
struct args_t{
int a;
int params[argc]; // << Wat?
…
osgx
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23
votes
5 answers
How can I initialize an array without knowing its size?
I have a situation, where I have to apply a criteria on an input array and reuturn another array as output which will have smaller size based upon the filtering criteria.
Now problem is I do not know the size of filtered results, so I can not…
Space Rocker
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23
votes
3 answers
Variable two dimensional array printing "subscript of pointer to incomplete type" when accessed
I am declaring a two dimensional array as such:
char arr[10][10];
arr[0][0] = 'X';
Now I print in debugger;
(lldb) po arr[0][0]
'X'
Awesome!! No problem.
Now I am declaring a two dimensional array as such:
int col = 10;
int row = 10;
char…
etayluz
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21
votes
4 answers
Dynamic arrays in C without malloc?
I've always wondered how I could get away with this:
int main(int argc, char **argv) {
printf("%p %s %d\n", &argv[1], argv[1], strlen(argv[1]));
char copy[strlen(argv[1]) + 1];
strcpy(copy, argv[1]);
printf("%p %s %d\n", ©, copy,…
lt0511
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21
votes
1 answer
VLAs and side-effect in sizeof's operand
I know that sizeof never evaluates its operand, except in the specific case where said operand is a VLA. Or, I thought I knew.
void g(int n) {
printf("g(%d)\n", n);
}
int main(void) {
int i = 12;
char arr[i]; // VLA
(void)sizeof…
Quentin
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21
votes
3 answers
Initializing variable length array
On initializing a Variable length array compiler gives an error message:
[Error] variable-sized object may not be initialized
Code snippet:
int n;
printf("Enter size of magic square: ");
scanf("%d",&n);
int board[n][n] = {0};
How should…
haccks
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18
votes
1 answer
What is the type of a pointer to a variable-length array in C?
Here's a short C program that prompts the user for a number, creates a variable-length array of ints of that size, and then uses pointer arithmetic to step over the elements that were allocated:
#include
int main() {
/* Read a size…
templatetypedef
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17
votes
1 answer
Is it safe to use variable-length arrays?
I have a concern about variable-length arrays. When I want to allocate an array dynamically, I'll get null, if it is not possible to allocate enough memory and I can respond to this properly in my program. With a variable length array I don't get…
gruszczy
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15
votes
3 answers
Why is it allowed to declare an automatic array with size depending on user input?
I'm using MinGW to compile for C++11 and I found out that this doesn't throw an error:
int S;
cin>>S;
char array[S];
While this does ("storage size of 'array' isn't known"):
char array[];
To me, the size is also unknown in the first case, as it…
Floella
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14
votes
2 answers
Why is there a number 22 in GCC's implementation of a VLA (variable-length array)?
int read_val();
long read_and_process(int n) {
long vals[n];
for (int i = 0; i < n; i++)
vals[i] = read_val();
return vals[n-1];
}
The assembly language code compiled by x86-64 GCC 5.4 is:
read_and_process(int):
pushq …
lastans7
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14
votes
2 answers
Variable Length Array overhead in C++?
Looking at this question: Why does a C/C++ compiler need know the size of an array at compile time ? it came to me that compiler implementers should have had some times to get their feet wet now (it's part of C99 standard, that's 10 years ago) and…
Matthieu M.
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14
votes
5 answers
Can we have a struct element of type Variable length array?
Can we declare a structure element of variable length?
The condition is as follows:
typedef struct
{
uint8_t No_Of_Employees;
uint8_t Employee_Names[No_Of_Employees][15];
}st_employees;
user12345
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