Nice way to create a dynamic 2D matrix in C++ 11 - c++

I already know how to create a dynamic 2D matrix using new and free it using delete. Since C++ 11 is here with many new memory features such as unique_ptr, array container etc.; what is a nice way to create a 2D matrix so that one needs not to free the matrix explicitly using delete operator?

One of the simplest ways is to use a vector of vectors
const int N = 10;
const int M = 10;
vector<vector<int>> matrix2d(N, vector<int>(M, 0)); // 10x10 zero-initialized matrix
matrix2d[0][0] = 42;
You could of course use a single vector and wrap it into an accessor class
vector<int> matrix(N * M, 0) // Ditto as above, but needs stride-aware accessors
I'll post a small example here for completeness' sake
template <typename T>
class Matrix2D {
std::vector<T> data;
unsigned int sizeX, sizeY;
public:
Matrix2D (unsigned int x, unsigned int y)
: sizeX (x), sizeY (y) {
data.resize (sizeX*sizeY);
}
T& operator()(unsigned int x, unsigned int y) {
if (x >= sizeX || y>= sizeY)
throw std::out_of_range("OOB access"); // Throw something more appropriate
return data[sizeX*y + x]; // Stride-aware access
}
};
Live Example
or perhaps combine your way with a smart pointer. Notice that the vector<vector<int>> approach should be used with caution since the vectors are independent from each other and there's nothing to enforce that they should keep their size fixed.

I strongly suggest using array_view from the GSL, which will eventually be part of the standard.
#include <array>
#include <vector>
#include "array_view.h" // must be manually imported until standardization
int main()
{
std::array<int, 10> arr{}; // 10 ints on the stack
std::vector<int> vec{12}; // 12 ints on the heap
auto a = gsl::array_view<int, 2>{{2, 5}, arr}; // 2D, 2x5 matrix
auto b = gsl::array_view<int, 3>{{3, 2, 2}, vec}; // 3D, 3x2x2 matrix
auto c = gsl::array_view<int>{vec}; // 1D, spans from `begin()` to `end()`
a[{0,3}] += b[{0,1,1}] * -c[2]; // access syntax
}
N.B. array_view holds no control over the lifetime of the range it looks at. See here for full details.
Edit:
array_view is dead as it was becoming too complicated in handling multidimensional arrays with zero cost abstraction. You should instead use span from the GSL.
See this for more information about span.

Based on above answers, I have found a simple way to create matrices although not using C++11 features. Here is an illustration.
#include <iostream>
#include <vector>
using namespace std;
typedef vector<vector<int>> Matrix2D;
typedef vector<Matrix2D> Matrix3D;
Matrix2D my_arr;
int main()
{
const size_t N = 9;
for(unsigned s = 4; s <= N; s++)
{
my_arr.resize(s);
for(unsigned i = 0; i < s; i++)
my_arr[i].resize(s,s);
for(unsigned i = 0; i < s; i++)
{
for(unsigned j = 0; j < s; j++)
cout << my_arr[i][j] << " ";
cout << endl;
}
cout << "\n\n";
}
return 0;
}

Related

Create matrix (2d-array) of size specified by parameter input in C++

I am learning C++ with experiencein mostly Python, R and SQL.
The way arrays (and vectors which differes somehow from 1d-arrays? and matrices which are 2d-arrays?) work in C++ seems quite different as I cannot specify the size of dimension of the array with an argument from the function.
A toy-example of my goal is some thing like this:
Have a function my_2d_array which takes two arguments M and N and returns a matrix or 2d-array of dimension (MxN) with elements indicating the position of that element. E.g. calling my_2d_array(4,3) would return:
[[00, 01, 02],
[10, 11, 12],
[20, 21, 22],
[30, 31, 32]]
The main function should execute my_2d_array and be able to potentially perform calculations with the result or modify it.
This is my attempt (with errors):
int my_2d_array(int N, int M) {
int A[N][M];
for (int i = 0; i < N; i++) {
for (int j = 0; j < M; j++) {
std::string element = std::to_string(i) + std::to_string(j);
A[i][j] = element;
}
}
return A;
}
void main() {
int N, M;
N = 4;
M = 3;
int A[N][M] = my_2d_array(N, M);
// Print the array A
for (int i = 0; i < N; i++) {
for (int j = 0; j < M; j++) {
std::cout << A[i][j] << " ";
}
std::cout << "\n";
}
}
One (1) dimensional attempt of #JustLearning's suggestion:
int my_array(int N) {
std::array<int, N> A;
for (int i = 0; i < N; i++) {
A[i] = i;
}
return A;
}
int main() {
int N = 4;
int A[N] = my_array(N);
// Print the array A
for (int i = 0; i < N; i++) {
std::cout << A[i] << " ";
}
}
You can use a 2d vector like this
vector<vector int> A;
It works the same way as a 2d array
Welcome to C++! Your function my_2d_array has a couple of issues:
the return type is int, however you are attempting to return an array of ints.
the identifier of an array in C++ is actually a pointer to the first element of that array. Therefore, when you return A, you must be aware of how it should be passed to a new variable in the main part of the code. In particular, your code is passing a reference to a temporary variable A, which is not permitted or safe.
In addition, in C++, unless you know what you're doing, main should always return an int:
int main() { ... }
What is not clear from your question is whether you are attempting to implement your own "array" class, or simply want to use arrays already established in the standard. For the latter, std::array is a good place to start. The advantage is that you can return std::arrays from functions like you return ints or doubles.
std::arrays are good if you plan to work with arrays of fixed size, as the size becomes part of the type: std::array<int, 3> my_array;. Then you can fill it in manually or with member functions of the class (see dox linked above).
If for some reason you prefer to work with arrays of dynamical size (sizes that will change during running your program), std::vector is the way to go.
Finally, if you are actually learning C++ by attempting to implement a container MyArray, you should specify that in your question and be a bit more specific in what help you need.
Here's a working example in 1d:
#include <iostream>
#include <array>
template <int N>
std::array<int, N> my_array() {
std::array<int, N> A;
for (int i = 0; i < N; i++) {
A[i] = i;
}
return A;
}
int main() {
const int N = 4;
std::array<int, N> arr = my_array<N>();
// Print the array A
for (int i = 0; i < N; i++) {
std::cout << arr[i] << " ";
}
}
Since the size of a std::array is included it its type, you need to create a function template, which is basically a function that works for different types. (In C++, std::array<int, 3> and std::array<int, 4> are considered different types.)
In order to use this in main, the index is promoted to a const int, as plain ints can vary during run time, and therefore are not suitable for defining types. (In C++ jargon, look up constant expressions).
Finally, note that both the return type and the type of the variable that receives the value returned by the function must be std::array, not int as you tried in your 1d code.
Following your comment, I can see why you are confused in your attempts to use a matrix in code.
There are many types of containers in C++. Many of them you can find in the standard library (std::vector, std::list, std::set, ...), others you can create yourself or use other libraries. Plain arrays (like int a[5]) are a somewhat unique case because they come from C and are part of the language itself.
A plain array lives on the stack (not very important but you might want to read up on stack vs heap allocations), and refers to a contiguous region of memory.
If you declare some array a like int a[5], you get a region of 5 integers one after the other, and you can point to the first one by just writing a. You can access each of them using a[i] or, equivalently, *(a+i).
If you declare a like int a[5][3], you now get a region of 15 integers, but you can access them slightly differently, like a[i][j], which is equivalent to *(a+i*3+j).
The important thing to you here is that the sizes (5 and 3) must be compile-time constants, and you cannot change them at runtime.
The same is true for std::array: you could declare a like std::array<std::array<int, 3, 5> a and get a similar region of 15 integers, that you can access the same way, but with some convenience (for example you can return that type, whereas you cannot return a plain array type, only a pointer, losing the size information in the process).
My advice is not to think of these arrays as having dimensionality, but as simple containers that give you some memory to work with however you choose. You can very well declare a like std::array<int, 15> a and access elements in a 2D way by indexing like this: a[i*3+j]. Memory-wise, it's the same.
Now, if you want the ability to set the sizes at runtime, you can use std::vector in a similar way. Either you declare a like std::vector<std::vector<int>> a(5, std::vector<int>(3)) and deal with the nested vectors (that initialization creates 5 std::vector<int> of size 3 each), or you declare a as a single vector like std::vector<int> a(15) and index it like a[i*3+j]. You can even make your own class that wraps a vector and helps with the indexing.
Either way, it's rare in C++ to need a plain array, and you should generally use some kind of container, with std::vector being a good choice for a lot of things.
Here is an example of how your code would look like using vectors:
#include <vector>
#include <string>
#include <iostream>
std::vector<std::string> my_2d_array(int N, int M) {
std::vector<std::string> A(N*M);
for (int i = 0; i < N; i++) {
for (int j = 0; j < M; j++) {
std::string element = std::to_string(i) + std::to_string(j);
A[i*M+j] = element;
}
}
return A;
}
int main() {
int N, M;
N = 4;
M = 3;
std::vector<std::string> A = my_2d_array(N, M);
// Print the array A
for (int i = 0; i < N; i++) {
for (int j = 0; j < M; j++) {
std::cout << A[i*M+j] << " ";
}
std::cout << "\n";
}
}
And here is a very crude example of a Matrix class used to wrap the vectors:
#include <vector>
#include <string>
#include <iostream>
template<typename T>
class Matrix {
public:
Matrix(int rowCount, int columnCount) : v(rowCount*columnCount), columnCount(columnCount) {}
T& operator()(int row, int column) {
return v[row*columnCount + column];
}
private:
std::vector<T> v;
int columnCount;
};
Matrix<std::string> my_2d_array(int N, int M) {
Matrix<std::string> A(N, M);
for (int i = 0; i < N; i++) {
for (int j = 0; j < M; j++) {
std::string element = std::to_string(i) + std::to_string(j);
A(i, j) = element;
}
}
return A;
}
int main() {
int N, M;
N = 4;
M = 3;
Matrix<std::string> A = my_2d_array(N, M);
// Print the array A
for (int i = 0; i < N; i++) {
for (int j = 0; j < M; j++) {
std::cout << A(i, j) << " ";
}
std::cout << "\n";
}
}

How to make index in an vector as a pair of (x,y) coordinates?

#include<bits/stdc++.h>
using namespace std;
int main() {
int i,j;
vector<int> v(100); // defined a vector of size 100
for(i=1;i<=50;i++) {
v[i]=i; // storing the values as we do in 1-d array
}
for(int i=1;i<=50;i++) {
cout<<"index="<<i<<" "<<v[i]<<"\n"; // It will give output similar
// to 1-d array
}
return 0;
}
So this is the case of one dimensional vector where index of vector is integer and the value is also integer. The above code is running fine.
But i want to take the index of a vector as pair (i,j) and the value as an integer.
See the below code for more clarification .
#include<bits/stdc++.h>
using namespace std;
int main() {
int i,j;
vector<pair<int,int>> ve(make_pair(100,100));
//defined a vector of size of indices (100,100)
for(i=1;i<=50;i++) {
for(j=0;j<=50;j++) {
ve[make_pair(i,j)]=2; // Storing value of 2 in all the
// (i,j) indices
}
}
for(int i=1;i<=50;i++) {
for(j=0;j<=50;j++) {
cout<<ve[make_pair(i,j)]<<" ";
// Output should be 2 in all the possible pairs of (i,j)
}
}
return 0;
}
But the above code is not working :(.
Please tell me how i can fix this problem.
Vectors, like arrays, work only with integer indexes. For a vector vec, the index must be in range 0 <= index < vec.size(), otherwise it either does not compile (not convertible to unsigned int) or the behavior is undefined (out of bounds).
You have written
vector<pair<int,int>> ve
Which means that you create a vector that contains pairs, not that it is indexed by pairs.
Now, if you want a two dimensional vector, i.e. a matrix, you should check Boost matrix. You can also implement it yourself, but making it general purpose will take some effort. The basic idea is to convert the pair to an single integer value. The simplest implementation is:
template <class T>
void init_2d(std::vector<T> &vec, std::pair<unsigned, unsigned> coordinates)
{
vec.resize(coordinates.first * coordinates.second);
}
inline unsigned flatten(std::pair<unsigned, unsigned> coordinates,
unsigned num_columns)
{
return coordinates.first * num_columns + coordinates.second;
}
template <class T>
T & get_2d(std::vector<T> & vec,
std::pair<unsigned, unsigned> coordinates,
unsigned num_columns)
{
return vec.at(flatten(coordinates, num_columns));
}
template <class T>
const T & get_2d(const std::vector<T> & vec,
std::pair<unsigned, unsigned> coordinates,
unsigned num_columns)
{
return vec.at(flatten(coordinates, num_columns));
}
and then use it in your code:
int main() {
std::vector<int> ve;
auto dimensions = std::make_pair(100,100);
init_2d(ve, dimensions);
for(int i=1;i<=50;i++) {
for(int j=0;j<=50;j++)
get_2d(ve, {i,j}, dimensions.second) =j;
}
for(int i=1;i<=50;i++) {
for(int j=0;j<=50;j++)
std::cout << get_2d(ve, {i,j}, dimensions.second) <<" ";
std::cout << '\n';
}
return 0;
}
But, instead of reimplementing existing code, you should prefer to use boost matrix. If you are trying to learn how a matrix can be implemented (which is a very good idea), then go on and try to convert the above functions+vector into a class, and put the dimensions pair into that class. Having a class, for a matrix, is better than to use separate functions. It is easier to maintain invariants for a class than for separate functions.
Note: you can use std::map<std::pair<int, int>> instead, but it will be more difficult to iterate over it and it will be much slower. If it makes your code clearer, using std::map is a good idea, but it is not clear cut whether std::map<pair<...>> is easier to work with than with std::vector + _2d functions.
I would suggest you to use map instead.
Operator [] for map takes key_type as parameter which is a container which means you can use pair object as index (called key in map), however Operator [] for vetor takes size_type as parameter which is an unsigned integer.
Your code could look like the following:
map< pair<int, int>, int > notVector;
for(i=0;i<=50;i++)
for(j=0;j<=50;j++)
notVector[make_pair(i,j)]=2; // Storing value of 2 in all the (i,j) indices
for(i=0;i<=50;i++)
for(j=0;j<=50;j++)
cout<<notVector[make_pair(i,j)]<<" ";
another solution: create your own container by aggregating a standard one.
Extremely simplified example:
#include <vector>
#include <iostream>
struct xy
{
std::size_t x, y;
};
constexpr std::size_t linear_extent(xy _)
{
return _.y * _.x;
}
constexpr std::size_t linear_position(xy _, xy extent)
{
return _.y * extent.x + _.x;
}
template<class T>
struct vector_2d
{
vector_2d(xy size, T x = T())
: extent_(size)
, storage_(linear_extent(extent_), x)
{
}
T& operator[](xy const& _)
{
return storage_[linear_position(_, extent_)];
}
T const& operator[](xy const& _) const
{
return storage_[linear_position(_, extent_)];
}
constexpr auto extent() const { return extent_; }
xy extent_;
std::vector<T> storage_;
};
template<class T>
std::ostream& operator<<(std::ostream& os, vector_2d<T> const& v)
{
const char* sep = " [";
os << "[";
auto extent = v.extent();
for(auto y = std::size_t(0) ; y < extent.y ; ++y)
{
os << sep;
const char* sep2 = " ";
for (auto x = size_t(0) ; x < extent.x ; ++x)
{
std::cout << sep2 << v[{x, y}];
sep2 = ", ";
}
os << " ]";
sep = "\n [";
}
os << " ]";
return os;
}
int main()
{
auto v = vector_2d<int>({5, 5});
v[{1, 3}] = 8;
std::cout << v << std::endl;
}
Keep it simple
All you need is 1 (One!) C++ line: ...+ Update: Optional macro (the macro is to answer #eneski comment):
std::vector<std::vector<int>> ve(100, std::vector<int>(100, 0)); // Initialize to 0
// Turns [wr(pair)] syntax to [pair.first][pair.second] syntax:
#define wr(pr) (pr).first][(pr).second // Wrapper macro
And than use:
ve[wr(std::make_pair(i, j))] = 35; // For example
int val = ve[wr(std::make_pair(i, j))];
// Or:
ve[i][j] = 70; // For example
val = ve[i][j];
There is no real need to use the wr wrapper macro. Use ve[i][j] and if you have somewhere in your code a std::pair p, use: ve[p.first][p.second] instead of ve[p] - Both are the same. Also, it is nonsense to take (i, j) and to make_pair on-the-fly just to use them again as [i][j] indexes. If, despite all, one insists on the syntax, than use the wr wrapper macro.
--
But, in a case of a fixed sequential range, a 2D std::array is a better choice (& you can add the wrapper macro as well, if you insist):
#include <array>
int main()
{
int i = 7, j = 5;
std::array<std::array<int, 100>, 100> ar; // 100 X 100
ar[0].fill(0); ar.fill(ar[0]); // Initialize to 0
ar[i][j] = 35; // For example
return 0;
}
Are you simply attempting to access 1d vector content as if it was a 2d vector? Perhaps you could use a helper function to convert 2d index into 1d index, for example:
#include <vector>
#include <iostream>
#include <cassert>
template<size_t ROWS, size_t COLS>
size_t convertIndex(size_t row, size_t col)
{
assert(row < ROWS && col < COLS);
return row * COLS + col;
}
int main()
{
std::vector<int> v =
{
0,1,2,3,4,
5,6,7,8,9
};
std::cout << v[convertIndex<2, 5>(1, 3)];
}
This outputs 8

Best way to convert 2D vector to C type (for SGX enclave)

I'm trying to write an SGX enclave that takes in a 2D vector of int (representing a grayscale image), but edger8r accepts only C types.
I am aware of this and this, but could someone provide an example on how to transform a type like:
vector< vector <int> > ImagePixels;
into a C type so that I can pass it to an SGX enclave?
std::vector has a data() method that will give you a plain pointer to its data. However, you'll need to do something with that second layer of vectors, so you'll need to create a temporary array that will store a list of plain pointers from the inner vector.
#include <algorithm>
#include <iostream>
#include <vector>
extern "C" int my_c_function(int **double_pointer, int rows, int cols) {
for (int i = 0; i < rows; ++i) {
for (int j = 0; j < cols; ++j) {
printf("%d ", double_pointer[i][j]);
}
printf("\n");
}
}
int main() {
std::vector<std::vector<int>> ImagePixels{{1, 2, 3}, {4, 5, 6}};
// transform a vector of vectors into a vector of pointers
std::vector<int*> ptrs(ImagePixels.size());
std::transform(begin(ImagePixels), end(ImagePixels), begin(ptrs),
[](std::vector<int> &inner_vec) {
return inner_vec.data();
});
my_c_function(ptrs.data(), ImagePixels.size(), ImagePixels[0].size());
}
Live example: https://onlinegdb.com/BJvb3yG3f
However, if you have control of the entire code, I would suggest using a flat structure, where you just store the whole picture into a falt std::vector<int> row-by-row. You could even do a simple wrapper around std::vector to make it easier to manipulate such a structure:
class image {
public:
image(int rows = 0; int cols = 0)
: rows_{rows}, cols_{cols}, data_(rows * cols) {}
int &operator ()(int row, int col) {
return data_[row * cols_ + col];
}
int *data() { return data_.data(); };
// needs a few more utilities and constant versions of the methods
private:
std::vector<int> data_;
int rows_;
int cols_;
};
Then you can do:
image im(3, 2);
// set pixels
im(0, 0) = 1;
im(0, 1) = 2;
int *d = im.data(); // a plain pointer to an array of data

Computing the scalar product of two vectors in C++

I am trying to write a program with a function double_product(vector<double> a, vector<double> b) that computes the scalar product of two vectors. The scalar product is
$a_{0}b_{0}+a_{1}b_{1}+...+a_{n-1}b_{n-1}$.
Here is what I have. It is a mess, but I am trying!
#include <iostream>
#include <vector>
using namespace std;
class Scalar_product
{
public:
Scalar_product(vector<double> a, vector<double> b);
};
double scalar_product(vector<double> a, vector<double> b)
{
double product = 0;
for (int i = 0; i <= a.size()-1; i++)
for (int i = 0; i <= b.size()-1; i++)
product = product + (a[i])*(b[i]);
return product;
}
int main() {
cout << product << endl;
return 0;
}
Unless you need to do this on your own (e.g., writing it is homework), you should really use the standard algorithm that's already written to do exactly what you want:
#include <iostream>
#include <numeric>
#include <vector>
int main() {
std::vector<double> a {1, 2, 3};
std::vector<double> b {4, 5, 6};
std::cout << "The scalar product is: "
<< std::inner_product(std::begin(a), std::end(a), std::begin(b), 0.0);
return 0;
}
Note that while begin(a) and end(a) are new in C++11, std::inner_product has been available since C++98. If you are using C++ 98 (or 03), it's pretty easy to write your own equivalent of begin and end to work with arrays though:
template <class T, size_t N>
T *begin(T (&array)[N]) {
return array;
}
template <class T, size_t N>
T *end(T (&array)[N]) {
return array + N;
}
Using these, a C++ 98 version of the previous code could look something like this:
int main() {
double a[] = {1, 2, 3};
double b[] = {4, 5, 6};
std::cout << "The scalar product is: "
<< std::inner_product(begin(a), end(a), begin(b), 0.0);
return 0;
}
Note that the begin and end above will only work for arrays, where the begin and end in C++11 (and later) will also work for normal collection types that define a .begin() and .end() (though it's trivial to add overloads to handle those as well, of course):
template <class Coll>
typename Coll::iterator begin(Coll const& c) { return c.begin(); }
template <class Coll>
typename Coll::iterator end(Coll const& c) { return c.end(); }
You can delete the class you have defined. You don't need it.
In your scalar_product function:
double scalar_product(vector<double> a, vector<double> b)
{
double product = 0;
for (int i = 0; i <= a.size()-1; i++)
for (int i = 0; i <= b.size()-1; i++)
product = product + (a[i])*(b[i]);
return product;
}
It's almost there. You don't need 2 loops. Just one.
double scalar_product(vector<double> a, vector<double> b)
{
if( a.size() != b.size() ) // error check
{
puts( "Error a's size not equal to b's size" ) ;
return -1 ; // not defined
}
// compute
double product = 0;
for (int i = 0; i <= a.size()-1; i++)
product += (a[i])*(b[i]); // += means add to product
return product;
}
Now to call this function, you need to create 2 vector objects in your main(), fill them with values, (the same number of values of course!) and then call scalar_product( first_vector_that_you_create, second_vector_object );
While you have been presented many solutions that work, let me spin up another variation to introduce a couple of concepts that should help you writing better code:
class are only needed to pack data together
a function should check its preconditions as soon as possible, those should be documented
a function should have postconditions, those should be documented
code reuse is the cornerstone of maintenable programs
With that in mind:
// Takes two vectors of the same size and computes their scalar product
// Returns a positive value
double scalar_product(std::vector<double> const& a, std::vector<double> const& b)
{
if (a.size() != b.size()) { throw std::runtime_error("different sizes"); }
return std::inner_product(a.begin(), a.end(), b.begin(), 0.0);
} // scalar_product
You could decide to use the inner_product algorithm directly but let's face it:
it requires four arguments, not two
it does not check for its arguments being of the same size
so it's better to wrap it.
Note: I used const& to indicate to the compiler not to copy the vectors.
You seem to want to make a class specifically for vectors. The class I made in my example is tailored to 3 dimensional vectors, but you can change it to another if desired. The class holds i,j,k but also can conduct a scalar products based on other MathVectors. The other vector is passed in via a C++ reference. It is hard to deduce what the question was, but I think this might answer it.
#include <iostream>
using namespace std;
class MathVector
{
private:
double i,j,k;
public:
MathVector(double i,double j,double k)
{
this->i=i;
this->j=j;
this->k=k;
}
double getI(){return i;}
double getJ(){return j;}
double getK(){return k;}
double scalar(MathVector &other)
{
return (i*other.getI())+(j*other.getJ())+(k*other.getK());
}
};
int main(int argc, char **argv)
{
MathVector a(1,2,5), b(2,4,1);
cout << a.scalar(b) << endl;
return 0;
}
Here is the code that you should have. I see you have used class in your code, which you do not really need here. Let me know if the question required you to use class.
As you are new and this code might scare you. So, I will try to explain this as I go. Look for comments in the code to understand what is being done and ask if you do not understand.
//Scalar.cpp
#include <stdlib.h>
#include <iostream>
#include <vector>
using namespace std;
/**
This function returns the scalar product of two vectors "a" and "b"
*/
double scalar_product(vector<double> a, vector<double> b)
{
//In C++, you should declare every variable before you use it. So, you declare product and initialize it to 0.
double product = 0;
//Here you check whether the two vectors are of equal size. If they are not then the vectors cannot be multiplied for scalar product.
if(a.size()!=b.size()){
cout << "Vectors are not of the same size and hence the scalar product cannot be calculated" << endl;
return -1; //Note: This -1 is not the answer, but just a number indicating that the product is not possible. Some pair of vectors might actually have a -1, but in that case you will not see the error above.
}
//you loop through the vectors. As bobo also pointed you do not need two loops.
for (int i = 0; i < a.size(); i++)
{
product = product + a[i]*b[i];
}
//finally you return the product
return product;
}
//This is your main function that will be executed before anything else.
int main() {
//you declare two vectors "veca" and "vecb" of length 2 each
vector<double> veca(2);
vector<double> vecb(2);
//put some random values into the vectors
veca[0] = 1.5;
veca[1] = .7;
vecb[0] = 1.0;
vecb[1] = .7;
//This is important! You called the function you just defined above with the two parameters as "veca" and "vecb". I hope this cout is simple!
cout << scalar_product(veca,vecb) << endl;
}
If you are using an IDE then just compile and run. If you are using command-line on a Unix-based system with g++ compiler, this is what you will do (where Scalar.cpp is the file containing code):
g++ Scalar.cpp -o scalar
To run it simply type
./scalar
You should get 1.99 as the output of the above program.

Pushing an array into a vector

I've a 2d array, say A[2][3]={{1,2,3},{4,5,6}}; and I want to push it into a 2D vector(vector of vectors). I know you can use two for loops to push the elements one by on on to the first vector and then push that into the another vector which makes it 2d vector but I was wondering if there is any way in C++ to do this in a single loop. For example I want to do something like this:
myvector.pushback(A[1]+3); // where 3 is the size or number of columns in the array.
I understand this is not a correct code but I put this just for understanding purpose. Thanks
The new C++0x standard defines initializer_lists which allows you to:
vector<vector<int>> myvector = {{1,2,3},{4,5,6}};
gcc 4.3+ and some other compilers have partial C++0x support.
for gcc 4.3+ you could enable c++0x support by adding the flag -std=c++0x
Its not the best way to have your static data represented like that. However, if your compiler vendor supports C++ tr1 then you could do:
#include <tr1/array> // or #include <array>
...
typedef vector<vector<int> > vector2d;
vector2d myvector;
// initialize the vectors
myvector.push_back(vector<int>());
myvector.push_back(vector<int>());
typedef std::array<std::array<int, 3>, 2> array2d;
array2d array = {{1,2,3},{4,5,6}};
array2d::const_iterator ai = array.begin(), ae = array.end();
for (vector2d::iterator i = myvector.begin(), e = myvector.end()
; i != e && ai != ae
; i++, a++)
{
// reserve vector space
i->reserve(array.size());
// copy array content to vector
std::copy(ai.begin(), ai->end(), i->begin());
}
You can use vector::assign (pointers to array elements are valid iterators):
int a[2][3] = {{1, 2, 3}, {4, 5, 6}};
std::vector<std::vector<int> > v(2);
for (size_t i = 0; i < 2; ++i)
v[i].assign(a[i], a[i] + 3);
This is a little tricky, but you could use template recursion to help you in having the assignment done almost completely at compile-time. I understand that's not exactly what you are looking for, but I think it's worthwhile :-)
Here's the code:
#include <vector>
using namespace std;
typedef vector<vector<int> > vector2d;
template<size_t K, size_t M, size_t N>
struct v_copy {
static void copy(vector2d& v, int(&a)[M][N])
{
v[K - 1].assign(a[K - 1], a[K - 1] + N);
v_copy<K - 1, M, N>::copy(v, a);
}
};
template<size_t M, size_t N>
struct v_copy<1, M, N> {
static void copy(vector2d& v, int(&a)[M][N])
{
v[0].assign(a[0], a[0] + N);
}
};
template<size_t M, size_t N>
void copy_2d(vector2d& v, int(&a)[M][N])
{
v_copy<M, M, N>::copy(v, a);
}
int main()
{
int A[2][3] = {{0, 1, 2}, {10, 11, 12}};
vector2d vector(2);
copy_2d(vector, A);
}
it needed a struct because in C++ you can't do partial specialization of functions. BTW , compiling it with gcc version 4.5.0, this code produces the same assembly as
vector[1].assign(A[1], A[1] + 3);
vector[0].assign(A[0], A[0] + 3);
It should not be very hard to have it compile with different types of 2-dimensions arrays.
If you want to push the data into vector of vectors, you have to write something like this:
vector<int> inner;
vector< vector<int> >outer;
...
outer.pushback(inner);
I think there is no way to do it in a single loop.
If you want to use just one vector (something similar like you written), then you can do it in a single loop:
int A[2][3]={{1,2,3},{4,5,6}};
int* p = A[0];
std::vector<int> inner;
std::vector< std::vector<int> >outer;
for(int i = 0; i < 6; ++i)
{
inner.push_back(*p++);
}
It's kind of cheating, but you could take advantage of the vector constructor to do one of the loops for you:
#include <vector>
int main() {
const int XMAX = 2, YMAX = 3;
int A[XMAX][YMAX] = {{1,2,3}, {4,5,6}};
std::vector<std::vector<int> > v;
for (size_t x = 0; x < XMAX; ++x) {
v.push_back(std::vector<int>(&A[x][0], &A[x][YMAX]));
}
}
You can resize vectors and then use copy.
int A[2][3]={{1,2,3},{4,5,6}};
std::vector< std::vector<int> > vec;
vec.resize(2);
for (int i=0; i<2; i++)
{
vec[i].resize(3);
std::copy(A[i], A[i]+3, vec[i].begin());
}
Is it practical? Definetly not.
Hm... I can produce a partial answer but not a full one.
int elementCount = 6; // I wonder if this can be done somehow with sizeof(A) * sizeof(A[0])
int* end = A + elementCount;
for(int* current = A; current < end; ++current) {
myvector.pushback(*current);
}
No. The only thing you can do is leverage existing loop functions so that you only have to write one or zero of your own loops.