Truncate the last few elements of an array

Question

I am new to C and C++, please help me in getting the required solution. I have used memcpy to copy the contents of 'array' to 'arr'. But since the size of 'arr' is 10, it appends 0 to the remaining elements. How can I truncate the 'arr' to size 5.

#include <iostream>

#include <string.h>

using namespace std;

int main()

{
    uint8_t array[5] = {1,2,3,4,5};

    uint8_t arr[10] = {0};

    memcpy( arr, array, 5);

    for (auto e: arr){

        cout << e << " ";

    }

    return 0;    
}

Output for the above code: 1 2 3 4 5 0 0 0 0 0

required output : 1 2 3 4 5

`uint8_t arr[10];` means you have always 10 elements... you might consider some value as sentinel, or use extra size parameter... or use `std::vector`. — Jarod42, Mar 24 '21 at 12:03
You cannot "truncate" a C-style array. Its size is declared when it's created and it will have this size until it is destroyed. — Yksisarvinen, Mar 24 '21 at 12:04
"_But since the size of 'arr' is 10, it appends 0 to the remaining elements._" No, it doesn't. `arr` always have 10 elements. You cannot change the size of the array in C++ after the declaration of said array. — Algirdas Preidžius, Mar 24 '21 at 12:05
If this is not a homework question, then I recommend avoiding C style arrays and instead use `std::vector`. Avoid `using namespace std;` always. Avoid `std::memcpy`, instead use [`std::copy`](https://en.cppreference.com/w/cpp/algorithm/copy). And you'll need `static_cast(e)` on the output, or you might get non-printing ASCII control characters on some platforms. — Eljay, Mar 24 '21 at 12:17

score 3 · Answer 1 · answered Mar 24 '21 at 12:21

3

You cannot truncate the size of the static array after compilation. If you were using some other data structure like vector from C++ STL then the size of that container object might have been variable.

answered Mar 24 '21 at 12:21

Parvinder Singh

1,737
2
12
17

score 0 · Answer 2 · edited Mar 24 '21 at 12:14

0

Instead of

for (auto e: arr){

    cout << e << " ";

}

you can keep array's size in a variable and cout arr that times, like:

int i, arraySize = 5;

for (i=0; i<arraySize; i++) {

    cout << arr[i] << " ";

}

edited Mar 24 '21 at 12:14

Yksisarvinen

18,008
2
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answered Mar 24 '21 at 12:06

Deniz Polat

26
3

In this case, the array size will still be the same right? – Beginner Mar 24 '21 at 12:26
yeah you cannot truncate an already created array in c unfortunately :( – Deniz Polat Mar 24 '21 at 12:33

score -2 · Answer 3 · answered Mar 24 '21 at 12:05

-2

Best Practice is to create a pointer and free the memory or second way is to don't allow kernel to give memory to you create custom memory allocator and assign it to your array

answered Mar 24 '21 at 12:05

YASHRAJ Singh

1
1

I think you mean the right thing, but your description is very unclear. Maybe some code would be helpful. Anyway, pointers and "best practice" don't go well together in modern C++. – Lukas-T Mar 24 '21 at 13:15

Truncate the last few elements of an array

3 Answers3