Sizeof List and traversal

Question

I have this array in main.

int main()
{
    int list[] = {1,2,3};

    print_list(list);
}

According to the compiler warning the list is NULL terminated.

When I try to print the list I will print an extra element because sizeof(list) seems to be +1.

void print_list(const int *list)
{ 
    int i;
    for(i=0; i < sizeof(list); i++)
        printf(list[i]);
}

Should I run the for loop with sizeof(list) - 1 for any case that looks like this? Or should I check inside the loop for if(list[i] == NULL?

If my function was not making a pointer out of the list I could use: sizeof(list)/sizeof(list[0])

The function is turning the array into a pointer. So how can I get the accurate size?

It is true my question is very simple and trivial to some. I do believe I have shown enough effort though, please help.

[EDIT] Why isn't the size of an array parameter the same as within main? That is what I know already, it still doesn't answer my question so not a duplicate.

In function `print_list()`, `list` is just a pointer. So you get the sizeof the pointer. — P.P, Dec 11 '13 at 22:48
possible duplicate of [Sizeof an array in the C programming language?](http://stackoverflow.com/questions/1975128/sizeof-an-array-in-the-c-programming-language) — P.P, Dec 11 '13 at 22:49
Yeah I am aware of what is going on with the pointer. So how do I get the accurate size? — , Dec 11 '13 at 22:57
Note that even if it were an array, your code would be wrong. `sizeof list` would be 12 on most machines, not 3. You have to do `sizeof array / sizeof *array` to get the number of elements in an array (unless it's an array of `char`). — Barmar, Dec 11 '13 at 23:00
The answer to your question is simple: you **can't**. You have to pass the size as a parameter, or put an end marker in the array. — Barmar, Dec 11 '13 at 23:06
The comment "The function is turning the array into a pointer" is a red flag. C arrays *are* pointers into memory, there is nothing to turn it into. "list" is just a name, not a C structure, so the function is not "making a pointer out of the list" - the variable "list" was always just a pointer, except in how your code used it. There is no way to get the size of the entire array from the pointer to the array. — Erik Johnson, Dec 11 '13 at 23:44
@BlueMoon The other question is specifically a 'why' question, whereas this is a 'how' question. — mydoghasworms, Dec 12 '13 at 05:34

score 3 · Accepted Answer · edited May 23 '17 at 12:29

3

sizeof returns the size in bytes of its parameter. sizeof(int*) will return, in most architectures, 4. It is impossible to get the size of the array using a pointer. Read this

Use this code:

main:

int main()
{
    int list[] = {1,2,3};

    print_list(list, sizeof(list) / sizeof(int));
}

print_list:

void print_list(const int *list, int size)
{ 
    int i;
    for(i=0; i < size; i++)
        printf(list[i]);
}

edited May 23 '17 at 12:29

Community

1
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answered Dec 11 '13 at 23:04

Pacha

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I personally like `#define LENGTHOF(a) (sizeof(a) / sizeof((a)[0]))` as it makes the rest of the code more legible when it is used frequently. [Ideone example](http://ideone.com/YXkGGV) – altendky Jan 07 '14 at 18:59

score 1 · Answer 2 · answered Dec 11 '13 at 22:48

1

sizeof list reports the size of the variable list, which is a const int *, in modern computers it will probably be something like 4 or 8.

When you pass the array as a parameter to a function, it isn't an array anymore, the function receives a pointer.

In this link you can find the answer to your question. I recommend you to read this whole page (it is short), just to learn the difference between pointers and arrays.

answered Dec 11 '13 at 22:48

Marco

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Might be 16 or 32 in the future?! – Ed Heal Dec 11 '13 at 22:49
Probably not, unless we ever design computers that can use the Internet as part of their main memory. But never say never -- Bill Gates supposedly opined that 640Kb would be enough for personal computers (although he claims he never said this, and no citation can be found). – Barmar Dec 11 '13 at 23:05

Sizeof List and traversal

2 Answers2