Related
In C++, I can statically initialize an array, e.g.:
int a[] = { 1, 2, 3 };
Is there an easy way to initialize a dynamically-allocated array to a set of immediate values?
int *p = new int[3];
p = { 1, 2, 3 }; // syntax error
...or do I absolutely have to copy these values manually?
You can in C++0x:
int* p = new int[3] { 1, 2, 3 };
...
delete[] p;
But I like vectors better:
std::vector<int> v { 1, 2, 3 };
If you don't have a C++0x compiler, boost can help you:
#include <boost/assign/list_of.hpp>
using boost::assign::list_of;
vector<int> v = list_of(1)(2)(3);
You have to assign each element of the dynamic array explicitly (e.g. in a for or while loop)
However the syntax int *p = new int [3](); does initialize all elements to 0 (value initialization $8.5/5)
To avoid endless push_backs, I usually initialize a tr1::array and create a std::vector (or any other container std container) out of the result;
const std::tr1::array<T, 6> values = {T(1), T(2), T(3), T(4), T(5), T(6)};
std::vector <T> vec(values.begin(), values.end());
The only annoyance here is that you have to provide the number of values explicitly.
This can of course be done without using a tr1::array aswell;
const T values[] = {T(1), T(2), T(3), T(4), T(5), T(6)};
std::vector <T> vec(&values[0], &values[sizeof(values)/sizeof(values[0])]);
Althrough you dont have to provide the number of elements explicitly, I prefer the first version.
No, you cannot initialize a dynamically created array in the same way.
Most of the time you'll find yourself using dynamic allocation in situations where static initialization doesn't really make sense anyway. Such as when you have arrays containing thousands of items. So this isn't usually a big deal.
Using helper variable:
const int p_data[] = {1, 2, 3};
int* p = (int*)memcpy(new int[3], p_data, sizeof(p_data));
or, one line
int p_data[] = {1, 2, 3}, *p = (int*)memcpy(new int[3], p_data, sizeof(p_data));
Never heard of such thing possible, that would be nice to have.
Keep in mind that by initializing the array in the code that way
int a[] = { 1, 2, 3 };
..... only gains you easier code writing and NOT performance.
After all, the CPU will do the work of assigning values to the array, either way you do it.
I have classes in the style of Class1 (see code). An enum and a function to get all the values from the enum. The values (FOO_1, FOO_2 etc) differ from Class to Class as well as the number of values (sizeof(Foos)).
I call the function once to get the sizeof the enum, reserve memory and with the second call I want to get all the values to *pFoos (2,1,6 in the sample code).
Is there a better way then using an array with all the values in it (size_t arr[3] ={FOO_1 , FOO_X, FOO_BAR })?
class Class1{
enum Foos{
FOO_1 = 2,
FOO_X = 1,
FOO_BAR = 6
}
};
Class1::GetFoos(size_t* pFoos, size_t* pSize)
{
size_t len = sizeof(Foos);
if (len > *pSize)
{ //Call function once to get the size
*pSize= len ;
return -1;
}
for(size_t i = 0; i< *pSize; i++)
{
//copy all enum values to pFoos
}
};
Disclaimer: shameless plug – I am the author.
Reflective enums are possible in C++. I wrote a header-only library that captures a bunch of "patterns" at compile time and gives you syntax like this:
ENUM(Class1, int, FOO_1 = 2, FOO_X = 1, FOO_BAR = 6)
size_t count = Class1::_size;
for (size_t index = 0; index < Class1::_size; ++index)
do_anything(Class1::_values()[index]);
What it does internally is use the macro to generate an array of the values that you have declared, kind of like in your question, and use a bunch of other tricks to allow you to use initializers naturally. It then provides iterators and other things on top of the array.
Here is a link: https://github.com/aantron/better-enums
EDIT – internals
Here is a pseudocode sketch of what it does internally. The reason I am only giving a "sketch" is because there are a bunch of issues to consider when doing this portably. I will touch on all the most important elements.
ENUM(Class1, int, FOO_1 = 2, FOO_X = 1, FOO_BAR = 6)
notionally expands to
struct Class1 {
enum _enumerated { FOO_1 = 2, FOO_X = 1, FOO_BAR = 6 };
// Fairly obvious methods for how to iterate over _values and
// _names go here. Iterators are simply pointers into _values
// and _names below.
static size_t _size = sizeof(_values) / sizeof(int);
int _value;
};
int _values[] = {(fix_t<Class1>)Class1::FOO_1 = 2,
(fix_t<Class1>)Class1::FOO_X = 1,
(fix_t<Class1>)Class1::FOO_BAR = 6};
const char *_names[] = {"FOO_1 = 2", "FOO_X = 1", "FOO_BAR = 6"};
This is done by using variadic macros and stringization. The methods that deal with strings treat not only \0, but also space and equals as terminators, which allows them to ignore the initializers in the stringized constants that you see in _names.
The type fix_t is necessary because having assignments inside an array initializer is not valid C++. What that type does is take on the value of the enum, then ignore the assignment by an overloaded assignment operator, and then return the original value. A sketch:
template <typename Enum>
struct fix_t {
Enum _value;
fix_t(Enum value) : _value(value) { }
const fix_t& operator =(int anything) const { return *this; }
operator Enum() const { return _value; }
};
This makes the _values array possible declare even in the presence of initializers.
Of course, these arrays need to be prefixed so that you can have more than one enum like this. They also need to have the same as "extern inline" linkage for functions, so that they are shared between multiple compilation units.
Until c++ will get reflection you will not get any data from your enum! Simply you can not get "all" values from an enum. A enum is simply a kind of namespace where some constants can be defined and may be enumerated automatically. Not more at all. You have no text representation, no count information, no value to text information!
Is there a better way then using an array with all the values in it (size_t arr[3] ={FOO_1 , FOO_X, FOO_BAR })?
If you're tagging the question as C++ I advise you to give up with the C way of doing things, so the better way to do this in C++ is using a std::vector:
class Class1{
enum Foos{
FOO_1 = 2,
FOO_X = 1,
FOO_BAR = 6
};
public:
std::vector<int> GetFoos()
{
// return all enum values
return {FOO_1, FOO_X, FOO_BAR};
}
};
You can use it this way:
Class1 c1;
auto foos = c1.GetFoos();
std::cout << "I have " << c1.size() << " foos:\n";
for (const auto &foo : foos) std::cout << foo << '\n';
If you don't want to create the vector at runtime, you can create it once declaring it static:
class Alpha{
enum Alphas{
BETA = 0b101010,
GAMMA = 0x20,
EPSILON = 050
};
static const std::vector<int> m_alphas;
public:
const std::vector<int> &GetAlphas()
{
return m_alphas;
}
};
// https://isocpp.org/wiki/faq/ctors#explicit-define-static-data-mems
const std::vector<int> Alpha::m_alphas = {BETA, GAMMA, EPSILON};
Live demo
I know that is a burden to maintain but since there's no way to iterate the values of an enum, all the code that tries to iterate them is a burden as well.
Maybe in the following answer you can find something useful to iterate enums in a better way for your goals:
enum to string in modern C++ and future C++17.
Well I am questioning myself if there is a way to pass a vector directly in a parameter, with that I mean, like this:
int xPOS = 5, yPOS = 6, zPOS = 2;
//^this is actually a struct but
//I simplified the code to this
std::vector <std::vector<int>> NodePoints;
NodePoints.push_back(
std::vector<int> {xPOS,yPOS,zPOS}
);
This code ofcourse gives an error; typename not allowed, and expected a ')'
I would have used a struct, but I have to pass the data to a Abstract Virtual Machine where I need to access the node positions as Array[index][index] like:
public GPS_WhenRouteIsCalculated(...)
{
for(new i = 0; i < amount_of_nodes; ++i)
{
printf("Point(%d)=NodeID(%d), Position(X;Y;Z):{%f;%f;%f}",i,node_id_array[i],NodePosition[i][0],NodePosition[i][1],NodePosition[i][2]);
}
return 1;
}
Ofcourse I could do it like this:
std::vector <std::vector<int>> NodePoints;//global
std::vector<int> x;//local
x.push_back(xPOS);
x.push_back(yPOS);
x.push_back(zPOS);
NodePoints.push_back(x);
or this:
std::vector <std::vector<int>> NodePoints;//global
std::vector<int> x;//global
x.push_back(xPOS);
x.push_back(yPOS);
x.push_back(zPOS);
NodePoints.push_back(x);
x.clear()
but then I'm wondering which of the two would be faster/more efficient/better?
Or is there a way to get my initial code working (first snippet)?
Use C++11, or something from boost for this (also you can use simple v.push_back({1,2,3}), vector will be constructed from initializer_list).
http://liveworkspace.org/code/m4kRJ$0
You can use boost::assign as well, if you have no C++11.
#include <vector>
#include <boost/assign/list_of.hpp>
using namespace boost::assign;
int main()
{
std::vector<std::vector<int>> v;
v.push_back(list_of(1)(2)(3));
}
http://liveworkspace.org/code/m4kRJ$5
and of course you can use old variant
int ptr[1,2,3];
v.push_back(std::vector<int>(ptr, ptr + sizeof(ptr) / sizeof(*ptr));
If you don't have access to either Boost or C++11 then you could consider quite a simple solution based around a class. By wrapping a vector to store your three points within a class with some simple access controls, you can create the flexibility you need. First create the class:
class NodePoint
{
public:
NodePoint( int a, int b, int c )
{
dim_.push_back( a );
dim_.push_back( b );
dim_.push_back( c );
}
int& operator[]( size_t i ){ return dim_[i]; }
private:
vector<int> dim_;
};
The important thing here is to encapsulate the vector as an aggregate of the object. The NodePoint can only be initialised by providing the three points. I've also provided operator[] to allow indexed access to the object. It can be used as follows:
NodePoint a(5, 6, 2);
cout << a[0] << " " << a[1] << " " << a[2] << endl;
Which prints:
5 6 2
Note that this will of course throw if an attempt is made to access an out of bounds index point but that's still better than a fixed array which would most likely seg fault. I don't see this as a perfect solution but it should get you reasonably safely to where you want to be.
If your main goal is to avoid unnecessary copies of vector<> then here how you should deal with it.
C++03
Insert an empty vector into the nested vector (e.g. Nodepoints) and then use std::swap() or std::vector::swap() upon it.
NodePoints.push_back(std::vector<int>()); // add an empty vector
std::swap(x, NodePoints.back()); // swaps contents of `x` and last element of `NodePoints`
So after the swap(), the contents of x will be transferred to NodePoints.back() without any copying.
C++11
Use std::move() to avoid extra copies
NodePoints.push_back(std::move(x)); // #include<utility>
Here is the explanation of std::move and here is an example.
Both of the above solutions have somewhat similar effect.
This question already has answers here:
Initialization of all elements of an array to one default value in C++?
(12 answers)
Closed 4 months ago.
I'm trying to initialize an int array with everything set at -1.
I tried the following, but it doesn't work. It only sets the first value at -1.
int directory[100] = {-1};
Why doesn't it work right?
I'm surprised at all the answers suggesting vector. They aren't even the same thing!
Use std::fill, from <algorithm>:
int directory[100];
std::fill(directory, directory + 100, -1);
Not concerned with the question directly, but you might want a nice helper function when it comes to arrays:
template <typename T, size_t N>
T* end(T (&pX)[N])
{
return pX + N;
}
Giving:
int directory[100];
std::fill(directory, end(directory), -1);
So you don't need to list the size twice.
I would suggest using std::array. For three reasons:
1. array provides runtime safety against index-out-of-bound in subscripting (i.e. operator[]) operations,
2. array automatically carries the size without requiring to pass it separately
3. And most importantly, array provides the fill() method that is required for
this problem
#include <array>
#include <assert.h>
typedef std::array< int, 100 > DirectoryArray;
void test_fill( DirectoryArray const & x, int expected_value ) {
for( size_t i = 0; i < x.size(); ++i ) {
assert( x[ i ] == expected_value );
}
}
int main() {
DirectoryArray directory;
directory.fill( -1 );
test_fill( directory, -1 );
return 0;
}
Using array requires use of "-std=c++0x" for compiling (applies to the above code).
If that is not available or if that is not an option, then the other options like std::fill() (as suggested by GMan) or hand coding the a fill() method may be opted.
If you had a smaller number of elements you could specify them one after the other. Array initialization works by specifying each element, not by specifying a single value that applies for each element.
int x[3] = {-1, -1, -1 };
You could also use a vector and use the constructor to initialize all of the values. You can later access the raw array buffer by specifying &v.front()
std::vector directory(100, -1);
There is a C way to do it also using memset or various other similar functions. memset works for each char in your specified buffer though so it will work fine for values like 0 but may not work depending on how negative numbers are stored for -1.
You can also use STL to initialize your array by using fill_n. For a general purpose action to each element you could use for_each.
fill_n(directory, 100, -1);
Or if you really want you can go the lame way, you can do a for loop with 100 iterations and doing directory[i] = -1;
If you really need arrays, you can use boosts array class. It's assign member does the job:
boost::array<int,N> array; // boost arrays are of fixed size!
array.assign(-1);
It does work right. Your expectation of the initialiser is incorrect. If you really wish to take this approach, you'll need 100 comma-separated -1s in the initialiser. But then what happens when you increase the size of the array?
use vector of int instead a array.
vector<int> directory(100,-1); // 100 ints with value 1
It is working right. That's how list initializers work.
I believe 6.7.8.10 of the C99 standard covers this:
If an object that has automatic
storage duration is not initialized
explicitly, its value is
indeterminate. If an object that has
static storage duration is not
initialized explicitly, then:
if it has pointer type, it is initialized to a null pointer;
if it has arithmetic type, it is initialized to (positive or unsigned)
zero;
if it is an aggregate, every member is initialized (recursively) according
to these rules;
if it is a union, the first named member is initialized (recursively)
according to these rules.
If you need to make all the elements in an array the same non-zero value, you'll have to use a loop or memset.
Also note that, unless you really know what you're doing, vectors are preferred over arrays in C++:
Here's what you need to realize about containers vs. arrays:
Container classes make programmers more productive. So if you insist on using arrays while those around are willing to use container classes, you'll probably be less productive than they are (even if you're smarter and more experienced than they are!).
Container classes let programmers write more robust code. So if you insist on using arrays while those around are willing to use container classes, your code will probably have more bugs than their code (even if you're smarter and more experienced).
And if you're so smart and so experienced that you can use arrays as fast and as safe as they can use container classes, someone else will probably end up maintaining your code and they'll probably introduce bugs. Or worse, you'll be the only one who can maintain your code so management will yank you from development and move you into a full-time maintenance role — just what you always wanted!
There's a lot more to the linked question; give it a read.
u simply use for loop as done below:-
for (int i=0; i<100; i++)
{
a[i]= -1;
}
as a result as u want u can get
A[100]={-1,-1,-1..........(100 times)}
I had the same question and I found how to do, the documentation give the following example :
std::array<int, 3> a1{ {1, 2, 3} }; // double-braces required in C++11 (not in C++14)
So I just tried :
std::array<int, 3> a1{ {1} }; // double-braces required in C++11 (not in C++14)
And it works all elements have 1 as value. It does not work with the = operator. It is maybe a C++11 issue.
Can't do what you're trying to do with a raw array (unless you explicitly list out all 100 -1s in the initializer list), you can do it with a vector:
vector<int> directory(100, -1);
Additionally, you can create the array and set the values to -1 using one of the other methods mentioned.
Just use this loop.
for(int i =0 ; i < 100 ; i++) directory[i] =0;
the almighty memset() will do the job for array and std containers in C/C++/C++11/C++14
The reason that int directory[100] = {-1} doesn't work is because of what happens with array initialization.
All array elements that are not initialized explicitly are initialized implicitly the same way as objects that have static storage duration.
ints which are implicitly initialized are:
initialized to unsigned zero
All array elements that are not initialized explicitly are initialized implicitly the same way as objects that have static storage duration.
C++11 introduced begin and end which are specialized for arrays!
This means that given an array (not just a pointer), like your directory you can use fill as has been suggested in several answers:
fill(begin(directory), end(directory), -1)
Let's say that you write code like this, but then decide to reuse the functionality after having forgotten how you implemented it, but you decided to change the size of directory to 60. If you'd written code using begin and end then you're done.
If on the other hand you'd done this: fill(directory, directory + 100, -1) then you'd better remember to change that 100 to a 60 as well or you'll get undefined behavior.
If you are allowed to use std::array, you can do the following:
#include <iostream>
#include <algorithm>
#include <array>
using namespace std;
template <class Elem, Elem pattern, size_t S, size_t L>
struct S_internal {
template <Elem... values>
static array<Elem, S> init_array() {
return S_internal<Elem, pattern, S, L - 1>::init_array<values..., pattern>();
}
};
template <class Elem, Elem pattern, size_t S>
struct S_internal<Elem, pattern, S, 0> {
template <Elem... values>
static array<Elem, S> init_array() {
static_assert(S == sizeof...(values), "");
return array<Elem, S> {{values...}};
}
};
template <class Elem, Elem pattern, size_t S>
struct init_array
{
static array<Elem, S> get() {
return S_internal<Elem, pattern, S, S>::init_array<>();
}
};
void main()
{
array<int, 5> ss = init_array<int, 77, 5>::get();
copy(cbegin(ss), cend(ss), ostream_iterator<int>(cout, " "));
}
The output is:
77 77 77 77 77
Just use the fill_n() method.
Example
int n;
cin>>n;
int arr[n];
int value = 9;
fill_n(arr, n, value); // 9 9 9 9 9...
Learn More about fill_n()
or
you can use the fill() method.
Example
int n;
cin>>n;
int arr[n];
int value = 9;
fill(arr, arr+n, value); // 9 9 9 9 9...
Learn More about fill() method.
Note: Both these methods are available in algorithm library (#include<algorithm>). Don't forget to include it.
Starting with C++11 you could also use a range based loop:
int directory[10];
for (auto& value: directory) value = -1;
How do I reliably get the size of a C-style array? The method often recommended seems to be to use sizeof, but it doesn't work in the foo function, where x is passed in:
#include <iostream>
void foo(int x[]) {
std::cerr << (sizeof(x) / sizeof(int)); // 2
}
int main(){
int x[] = {1,2,3,4,5};
std::cerr << (sizeof(x) / sizeof(int)); // 5
foo(x);
return 0;
}
Answers to this question recommend sizeof but they don't say that it (apparently?) doesn't work if you pass the array around. So, do I have to use a sentinel instead? (I don't think the users of my foo function can always be trusted to put a sentinel at the end. Of course, I could use std::vector, but then I don't get the nice shorthand syntax {1,2,3,4,5}.)
In C array parameters in C are really just pointers so sizeof() won't work. You either need to pass in the size as another parameter or use a sentinel - whichever is most appropriate for your design.
Some other options:
Some other info:
for C++, instead of passing a raw array pointer, you might want to have the parameter use something that wraps the array in a class template that keeps track of the array size and provides methods to copy data into the array in a safe manner. Something like STLSoft's array_proxy template or Boost's boost::array might help. I've used an array_proxy template to nice effect before. Inside the function using the parameter, you get std::vector like operations, but the caller of the function can be using a simple C array. There's no copying of the array - the array_proxy template takes care of packaging the array pointer and the array's size nearly automatically.
a macro to use in C for determining the number of elements in an array (for when sizeof() might help - ie., you're not dealing with a simple pointer): Is there a standard function in C that would return the length of an array?
A common idiom mentioned in GNU Libstdc++ documentation is the lengthof function:
template<typename T, unsigned int sz>
inline unsigned int lengthof(T (&)[sz]) { return sz; }
You can use it as
int x[] = {1,2,3,4,5};
std::cerr << lengthof(x) << std::endl;
Warning: this will work only when the array has not decayed into a pointer.
How about this?..
template <int N>
void foo(int (&x)[N]) {
std::cerr << N;
}
You can either pass the size around, use a sentinel or even better use std::vector. Even though std::vector lacks initializer lists it is still easy to construct a vector with a set of elements (although not quite as nice)
static const int arr[] = {1,2,3,4,5};
vector<int> vec (arr, arr + sizeof(arr) / sizeof(arr[0]) );
The std::vector class also makes making mistakes far harder, which is worth its weight in gold. Another bonus is that all C++ should be familiar with it and most C++ applications should be using a std::vector rather than a raw C array.
As a quick note, C++0x adds Initializer lists
std::vector<int> v = {1, 2, 3, 4};
You can also use Boost.Assign to do the same thing although the syntax is a bit more convoluted.
std::vector<int> v = boost::assign::list_of(1)(2)(3)(4);
or
std::vector<int> v;
v += 1, 2, 3, 4;
c provides no native support for this. Once an array is passed out of its declared scope, its size is lost.
You can pass the size with the array. You can even bundle them into a structure if you always to to keep the size, though you'll have some bookkeepping overhead with that.
I also agree that Corwin's method above is very good.
template <int N>
void foo(int (&x)[N])
{
std::cerr << N;
}
I don't think anybody gave a really good reason why this is not a good idea.
In java, for example, we can write things like:
int numbers [] = {1, 2, 3, 4};
for(int i = 0; i < numbers.length(); i++)
{
System.out.println(numbers[i]+"\n");
}
In C++ it would be nice instead of saying
int numbers [] = {1, 2, 3, 4};
int size = sizeof(numbers)/sizeof(int);
for(int i = 0; i < size; i++)
{
cout << numbers[i] << endl;
}
We could take it a step further and go
template <int N>
int size(int (&X)[N])
{
return N;
}
Or if that causes problems I suppose you could write explicitly:
template < int N >
int size(int (&X)[N])
{
int value = (sizeof(X)/sizeof(X[0]));
return value;
}
Then we just have to go in main:
int numbers [] = {1, 2, 3, 4};
for(int i = 0; i < size(numbers); i++)
{
cout << numbers[i] << endl;
}
makes sense to me :-)
An array expression will have its type implicitly converted from "N-element array of T" to "pointer to T" and its value will be the address of the first element in the array, unless the array expression is the operand of either the sizeof or address-of (&) operators, or if the array expression is a string literal being used to initialize another array in a declaration. In short, you can't pass an array to a function as an array; what the function receives is a pointer value, not an array value.
You have to pass the array size as a separate parameter.
Since you're using C++, use vectors (or some other suitable STL container) instead of C-style arrays. Yes, you lose the handy shorthand syntax, but the tradeoff is more than worth it. Seriously.
Now, you can use C++11's extent and rank.
By example:
#include <iostream>
#include <type_traits>
int main()
{
int a[][3] = {{1, 2, 3}, {4, 5, 6}};
std::cout << "\nRank: : " << std::rank<decltype(a)>::value;
std::cout << "\nSize: [_here_][]: " << std::extent<decltype(a), 0>::value;
std::cout << "\nSize: [][_here_]: " << std::extent<decltype(a), 1>::value;
std::cout << "\nSize: [][]_here_: " << std::extent<decltype(a), 2>::value;
}
prints:
Rank: : 2
Size: [_here_][]: 2
Size: [][_here_]: 3
Size: [][]_here_: 0
You need to pass the size along with the array, just like it is done in many library functions, for instance strncpy(), strncmp() etc. Sorry, this is just the way it works in C:-).
Alternatively you could roll out your own structure like:
struct array {
int* data;
int size;
};
and pass it around your code.
Of course you can still use std::list or std::vector if you want to be more C++ -ish.
Since c++11, there is a very convenient way:
static const int array[] = { 1, 2, 3, 6 };
int size = (int)std::distance(std::begin(array), std::end(array))+1;