How to give a variable the size of array using size() function in c++?

Question

When I write the code it shows error

#include <bits/stdc++.h>
using namespace std;
 
int main()
{
    int arr[] = {12, 3, 4, 15};
    int n = arr.size();
    
    cout << "Size of array is " << n;
    return 0;
}




Compilation failed due to following error(s).
main.cpp:7:17: error: request for member ‘size’ in ‘arr’, which is of non-class type ‘int [4]’

    7 |     int n = arr.size();
      |                 ^~~~

it also didn't work for vector array i tried that also can't understand the problem

You can also use sizeof(arr)/sizeof(arr[0]) or the non standard _countof(arr) which exists on some platforms. — Jabberwocky
– Jabberwocky, Commented Mar 18, 2022 at 10:37
"it also didn't work for vector array": what is a vector array? — Jabberwocky
– Jabberwocky, Commented Mar 18, 2022 at 10:40
#include <bits/stdc++.h> -- I know you didn't get this from a reputable C++ book, which you should be using to learn C++. Instead, you probably got code from a dodgy C++ website and used this non-standard header. — PaulMcKenzie
– PaulMcKenzie, Commented Mar 18, 2022 at 13:00

Bathsheba · Accepted Answer · 2022-03-18 10:34:01Z

6

As a rule of thumb, use

std::vector<int> arr = {12, 3, 4, 15};

with

arr.size();

returning the number of elements, unless you have a good reason not to.

Failing that, std::size(arr) will give you the number of elements, but only if arr has not already decayed to a pointer type by virtue of your receiving it as a function parameter. That C++ standard library function does something clever to obviate the normal pointer decay you observe with C-style arrays.

answered Mar 18, 2022 at 10:34

Bathsheba

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Comments

Jarod42 · Accepted Answer · 2022-03-18 12:19:57Z

5

C-array doesn't have methods.

Since C++17, you might use std::size:

const auto n = std::size(arr);

edited Mar 18, 2022 at 12:19

answered Mar 18, 2022 at 10:28

Jarod42

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4 Comments

Bathsheba Over a year ago

int for the type. Really?

Passer By Over a year ago

@Bathsheba Why is that problematic?

Bathsheba Over a year ago

@PasserBy int can be as small as [-32767, 32767] (C++20 gives you an extra negative) and vulnerable to overflow.

Jarod42 Over a year ago

@Bathsheba: I used int as OP. Changed to auto.

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