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Questions tagged [variable-length-array]

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.

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Why aren't variable-length arrays part of the C++ standard?

I haven't used C very much in the last few years. When I read this question today I came across some C syntax which I wasn't familiar with. Apparently in C99 the following syntax is valid: void foo(int n) { int values[n]; //Declare a variable…
Andreas Brinck
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Can't understand this way to calculate the square of a number

I have found a function that calculates square of a number: int p(int n) { int a[n]; //works on C99 and above return (&a)[n] - a; } It returns value of n2. Question is, how does it do that? After a little testing, I found that between…
Emanuel
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C compile error: "Variable-sized object may not be initialized"

Why do I receive the error "Variable-sized object may not be initialized" with the following code? int boardAux[length][length] = {{0}};
helloWorld
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What does "[*]" (star modifier) mean in C?

While trying to implement a C11 parser (for educational purposes), I found that in C11 (p. 470) but also in C99 (p. 412) (thanks Johannes!), the direct declarator is defined as: (6.7.6) direct-declarator: direct-declarator […
elslooo
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Difference between array type and array allocated with malloc

Today I was helping a friend of mine with some C code, and I've found some strange behavior that I couldn't explain him why it was happening. We had TSV file with a list of integers, with an int each line. The first line was the number of lines the…
Jorge Leitao
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Correctly allocating multi-dimensional arrays

The intent of this question is to provide a reference about how to correctly allocate multi-dimensional arrays dynamically in C. This is a topic often misunderstood and poorly explained even in some C programming books. Therefore even seasoned C…
Lundin
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How does the compiler allocate memory without knowing the size at compile time?

I wrote a C program that accepts integer input from the user, that is used as the size of an integer array, and using that value it declares an array of given size, and I am confirming it by checking the size of the array. Code: #include…
Rahul
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Is there any overhead for using variable-length arrays?

Is there some overhead of using variable-length arrays? Could the size of array be passed via command line argument at run time? Why is it introduced, compared to automatic and dynamically allocating an array?
Tim
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C++ replacement for C99 VLAs (goal: preserve performance)

I am porting some C99 code that makes heavy use of variable length arrays (VLA) to C++. I replaced the VLAs (stack allocation) with an array class that allocates memory on the heap. The performance hit was huge, a slowdown of a factor of 3.2 (see…
Szabolcs
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malloced array VS. variable-length-array

There are two ways to allocate memory to an array, of which the size is unknown at the beginning. The most common way is using malloc like this int * array; ... // when we know the size array = malloc(size*sizeof(int)); But it's valid too in C99 to…
linusz
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Does C++ support Variable Length Arrays?

No, wait, bear with me... VLAs were always a GCC extension, but they were adopted by C99: [C99: 6.7.5.2/4]: If the size is not present, the array type is an incomplete type. If the size is * instead of being an expression, the array type is a…
Lightness Races in Orbit
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Enabling VLAs (variable length arrays) in MS Visual C++?

How can I enable the use of VLAs, variable length arrays as defined in C99, in MS Visual C++ or that is not possible at all? Yes I know that the C++ standard is based on C89 and that VLAs are not available in C89 standard and thus aren't available…
Shinnok
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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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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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What's the point of VLA anyway?

I understand what variable length arrays are and how they are implemented. This question is about why they exist. We know that VLAs are only allowed within function blocks (or prototypes) and that they basically cannot be anywhere but on the stack…
Shahbaz
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