How to find out the length of an array?

Question

std::string array[] = { "one", "two", "three" };

How do I find out the length of the array in code?

juanchopanza · Accepted Answer · 2013-04-07T11:16:16.463

6

You can use std::begin and std::end, if you have C++11 support.:

int len = std::end(array)-std::begin(array); 
// or std::distance(std::begin(array, std::end(array));

Alternatively, you write your own template function:

template< class T, size_t N >
size_t size( const T (&)[N] )
{
  return N;
}

size_t len = size(array);

This would work in C++03. If you were to use it in C++11, it would be worth making it a constexpr.

edited Apr 07 '13 at 11:16

answered Apr 07 '13 at 09:58

juanchopanza

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Or `std::extent::value`. – Joseph Mansfield Apr 07 '13 at 09:59
@sftrabbit I hadn't even thought of that one. Nice! – juanchopanza Apr 07 '13 at 10:01
1

@sftrabbit Make **that** an answer and get my +1 – Daniel Frey Apr 07 '13 at 10:02
Your `len` is missing a `constexpr` :) – Daniel Frey Apr 07 '13 at 10:06
@DanielFrey good point, but I wanted to provide a C++03 option. I will add a note about that. – juanchopanza Apr 07 '13 at 10:10
I'd also argue that the function should be named `size`, rather than `len`, to relate it to the member functions of the containers in the same way `begin()` and `end()` were. – James Kanze Apr 07 '13 at 10:40
@JamesKanze You're right, that is what I usually call it. I have been doing too much python this week. – juanchopanza Apr 07 '13 at 11:15

bash.d · Answer 2 · 2013-04-07T10:15:50.190

4

Use the sizeof()-operator like in

int size = sizeof(array) / sizeof(array[0]);

or better, use std::vector because it offers std::vector::size().

int myints[] = {16,2,77,29};
std::vector<int> fifth (myints, myints + sizeof(myints) / sizeof(int) );

Here is the doc. Consider the range-based example.

edited Apr 07 '13 at 10:15

answered Apr 07 '13 at 10:00

bash.d

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Is it possible to define an `std::vector` with initializaiton (like in my example with ordinary array)? – NPS Apr 07 '13 at 10:04
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C++11 offeres initializers. Otherwise you can create an array as you did and assign it to the std::vector. Ill post the doc in a minute – bash.d Apr 07 '13 at 10:09
This C level expression should not be recommended without explaining [its type unsafety and how that can be remedied](http://stackoverflow.com/questions/4810664/how-do-i-use-arrays-in-c/7439261#7439261). – Cheers and hth. - Alf Apr 07 '13 at 10:12

score 3 · Answer 3 · answered Apr 07 '13 at 10:06

3

C++11 provides std::extent which gives you the number of elements along the Nth dimension of an array. By default, N is 0, so it gives you the length of the array:

std::extent<decltype(array)>::value

answered Apr 07 '13 at 10:06

Joseph Mansfield

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Does it do all the calculations at compile time (i.e. after compilation it's just a number, needing no extra calculations)? – NPS Apr 07 '13 at 10:13
@NPS: yes, you can see that from the fact that it's a type – Cheers and hth. - Alf Apr 07 '13 at 10:15
Any way to make it short (like macro but I'd rather not use macros)? – NPS Apr 07 '13 at 10:31

score 2 · Answer 4 · answered Apr 07 '13 at 10:00

2

Like this:

int size = sizeof(array)/sizeof(array[0])

answered Apr 07 '13 at 10:00

Peter R

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I knew this one but I thought it might not work with `std::string` (due to variable text lengths). – NPS Apr 07 '13 at 10:02
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This C level expression should not be recommended without explaining [its type unsafety and how that can be remedied](http://stackoverflow.com/questions/4810664/how-do-i-use-arrays-in-c/7439261#7439261). – Cheers and hth. - Alf Apr 07 '13 at 10:18
@NPS `sizeof(std::string)` is always the same ; it does not have anything to do with how many characters are stored in the string – M.M Sep 27 '14 at 03:44

How to find out the length of an array?

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