I am trying to fill a vector with a matrix of values in c++. I'm not very self confident with this procedure (I don't know well about pointers and I don't know if I need it here) however I am trying this
int auxMat[gray.rows][gray.cols];
vector<int> collectionSum;
collectionSum.push_back(auxMat);
When I try to compile I receive an error which says
invalid arguments 'Candidates are: void push_back(const int &)
Can anyone tell me wether it's possible to do, how can I solve it?
I read something about erasing cache memory, changing my eclipse compiler, my c++ version, however I don't think the problem is so big.
You cannot push back a matrix into a vector. What you can do is preallocate memory for your vector (for speeding things up) then use the std::vector<>::assign member function to "copy" from the matrix into the vector:
vector<int> collectionSum(gray.rows * gray.cols); // reserve memory, faster
collectionSum.assign(*auxMat, *auxMat + gray.rows * gray.cols);
This should be pretty fast. Otherwise, you can push back each individual element in a loop.
EDIT
See May I treat a 2D array as a contiguous 1D array? for some technicalities regarding possible undefined behaviour (thanks #juanchopanza for the comment). I believe the code is safe, due to the fact that the storage of the matrix is contiguous.
Because the array auxMat is continuous in memory, you can just copy it directly from memory into your vector. Here, you are telling the vector constructor to copy from the start of auxMat until its end in memory using pointer arithmetic:
std::vector<int> collectionSum(auxMat, auxMat + (gray.rows * gray.cols));
EDIT:
Sorry, I read your question as being a 1D array (int*) rather than a 2D (int**) array. I honestly recommend switching over to a 1D array because often it is faster and easier to work with. Depending on whether your using row-first order or column-first order, you can access the element you want by:
elem = y * width + x; // for row-first order
elem = x * height + y; // for column-first order
For instance:
// Create a 3x3 matrix but represent it continuously as a 1D array
const int A[] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
const unsigned width = 3;
const unsigned height = 3;
for (int y = 0; y < height; ++y)
{
for (int x = 0; x < width; ++x)
{
printf("%d ", A[y * width + x]);
}
printf("\n");
}
Related
I want to write a function addArrays which will, as parameters, take two 2D arrays of type int and of dimensions 3x4, and it's job is to add individual elements of each index from the given arrays and display it in the console.
In main(), I created two 2D arrays arrA and arrB of appropriate sizes with intitialized members and check the functionality of the created function.
My code so far:
#include <iostream>
using namespace std;
void addArrays(int x[3][4], int y[3][4]);
int main()
{
int arrA[3][4] = { {7, 8, 13, 22}, {56, 4, 78, 3}, {22, 13, 46, 5} };
int arrB[3][4] = { {32, 47, 56, 14}, {33, 100, 19, 64}, {4, 18, 157, 84} };
}
void addArrays(int x[3][4], int y[3][4])
{
}
Honestly, I know how to work with 1D arrays, but not displaying the sum of all individual elements. I know I have to use a for loop, but again, I'm confused as to how to pass in a 2D array and use it.
You mention you know how to work with 1D arrays, it's the same for 2D arrays, only with one more dimension.
In a 1D array you use arrA[0] to access the first element of the array, in a 2D array you use arrA[0][0] to access the first element in the first line of the array, arrA[0][1] to access the second element in the first line of the array. To access the first element in the second line you would use arrA[1][0] and so on, you get the idea.
So to loop through all the elements in the array you can use nested loops like so:
void addArrays(int x[3][4], int y[3][4])
{
for( int i = 0; i < 3; i++){ // make sure to use the correct dimensions
for(int j = 0; j < 4; j++){ // 3 lines and 4 columns
// x[i][j] accesses elements in array x
// y[i][j] accesses elements in array y
}
}
}
I think you'll manage to do the math yourself. After that you just need to send data to the standard output, i.e. to print data in the console. For that, as you may know, you use std::cout.
Side notes:
In the function void addArrays(int x[3][4], int y[3][4]){...} you may omit the first dimension of the array i.e. int x[][4] or int (*x)[4] instead of int x[3][4], since the argument will decay to a pointer to array.
Since it seems that you are not to change the values of the passed arrays, using const is recommend. You would have void addArrays(const int (*x)[4], const int (*y)[4]);
As you are using C++, you can take advantage of the possibility of using references, something like void addArrays(const int (&x)[3][4], const int (&y)[3][4]){/*same code*/}, the benefit being you must pass a correct object to the function otherwise the program will fail to compile whereas in the previous versions if you pass, for example, NULL, i.e. addArrays(arrA, NULL); the program will compile fine but will result in undefined behavior when you run it. References are safer and you should use them when possible.
It's more or less consensual among more experienced C++ programmers that the usage of using namespace std; is not a good practice, you can read more about it, and find alternatives following the link.
I will start this for you and try to give you an idea of the general structure, but since you have not shown your attempt at the problem I won't fill things in for you.
The basic idea here when looping through 2D arrays (of size MxN) is that you can really just think about it in terms of looping through M arrays of length N.
void loopThroughArray(int arr[M][N])
{
// Loop over M arrays
for (int m = 0; m < M; ++m) {
// For each m'th array, loop over its N contents
for (int n = 0; n < N; ++n) {
// Doing work
arr[m][n] = 1234;
}
}
}
I'm making a picture editing program, and I'm stuck in allocating memory.
I have no idea what is going on.
Ok.. So when I do this:
std::vector<unsigned char> h;
for (int a = 0; a < 10000 * 10000 * 3; a++) {
h.push_back(0);
}
this is fine(sorry I had to), but when I do this:
std::vector<std::vector<std::vector<unsigned char>>> h;
for (uint32_t a = 0; a < 10000; a++) {
h.push_back({});
for (uint32_t b = 0; b < 10000; b++) {
h.at(a).push_back({});
for (uint32_t c = 0; c < 3; c++) {
h.at(a).at(b).push_back(0xff);
}
}
}
my memory usage explodes, and I get error: Microsoft C++ exception: std::bad_alloc at memory location 0x009CF51C
I'm working with .bmp.
Currently, code is in testing mode so it's basically a giant mess...
I'm 15, so don't expect much of me.
I was searching for solutions, but all I found was like how to handle large integers and so on...
If you can give me maybe another solution, but I want my code to be as beginner friendly as it can get.
This is due to overhead of vector<char>. Each such object with 3 elements takes not 3 bytes, but probably 4 (due to reallocation policy), plus 3 pointers which probably take 3*8=24 bytes. Overall your structure takes 9.3 times the memory it could have.
If you replace the inner vector with an array, it will start working, since array does not have this overhead:
std::vector<std::vector<std::array<unsigned char, 3>>> h;
for (uint32_t a = 0; a < 10000; a++) {
h.emplace_back();
for (uint32_t b = 0; b < 10000; b++) {
h.at(a).emplace_back();
for (auto &c : h.at(a).at(b)) {
c = 0xff;
}
}
}
Another alternative is to put the smaller dimension first.
My guess would be that the memory is being heavily fragmented by the constant vector reallocation, resulting in madness. For data this large, I would suggest simply storing a 1-dimensional pre-allocated vector:
std::vector h(10000 * 10000 * 3);
And then come up with an array accessing scheme that takes the X/Y arguments and turns them into an index in your 1d array, eg.:
int get_index(int x, int y, int width) {
return ((y * width) + x) * 3;
}
If the image size is always fixed, you can also use std::array (see multi-dimensional arrays), since the size is defined at compile-time and it won't suffer the same memory issues as the dynamically allocated vectors.
I don't know if this will help your problem, but you could try allocating the memory for the vec of vecs of vecs all at the beginning, with the constructor.
std::vector<std::vector<std::vector<unsigned char>>> h(10000, std::vector<std::vector<unsigned char>>(10000, std::vector<unsigned char>(3,0xff)));
BTW, you're getting a good start writing C++ at 15! I didn't start studying computer science till I was in my 20s. It really is a very marketable career path, and there are a lot of intellectually stimulating, challenging things to learn. Best of luck!
I need to store an array on heap since I got a seg fault when running the program, due to it being too large. Normally this would be easy, but in this case it is a multidimensional array (3D specifically) and it's a variable length array too.
I tried to fit this answer for a 2D array (which I'm pretty sure works because I found it on another answer on SO) into one for a 3D array
int **ary = new int*[sizeY];
for(int i = 0; i < sizeY; ++i) {
ary[i] = new int[sizeX];
}
by doing this:
double **isoarray = new double*[nBinsX];
for(int xi = 0; xi < nBinsX; ++xi){
isoarray[xi] = new double[nBinsY];
for(int yi = 0; yi < nBinsY; ++yi){
isoarray[xi][yi] = new double[nShuffles];
}
}
Where I should mention that the array is meant to have dimensions nBinsX x nBinsY x nShuffles, but it isn't working, nor did I really think it would to be honest. Can anyone offer a suggestion on how I would do this? Thanks in advance!
Rather than heap-allocating arrays of pointers to more heap-allocated arrays and so on, you should make a single giant allocation and do appropriate indexing. This is a common technique:
double *isoarray = new double[nBinsX * nBinsY * nShuffles];
If you want to make a nice C++ solution out of it, store that pointer in a class which has an indexing method something like this:
double& at(x, y, shuffle) {
return isoarray[x * nBinsY * nShuffles + y * nShuffles + shuffle];
}
This way you have a single contiguous allocation which is better for performance when allocating, when using, and when deallocating. You can play with the indexing in terms of which dimension comes "first" to achieve even better performance depending on which way you usually traverse the data.
EDIT: I will improve this question. I will clarify it right in a little days.
first, I am writing a litlle bmp image analyzer. I have the following problem: The image is stored on plain bytes, without format as an array.
The image is 24 bits, and requires 3 bytes per pixel. I have tried with a solution that I have found on this stackoverflow page, but I can not adapt it for structures.
I have tried but it references invalid areas and bytes. Here's my complete code if you want to see it in TinyPaste (just for a better highlighting): The code in TinyPaste
EDIT 1: This code is in C++, I want to translate it to pure C for portability reasons. This is just the example from I taken the idea of convert a linear array to bidimensional. I have tried to adapt it to pure C for structs but I fail.
This snippet was taken from a stackoverflow question that made me think about this
//The resulting array
unsigned int** array2d;
// Linear memory allocation
unsigned int* temp = new unsigned int[sizeX * sizeY];
// These are the important steps:
// Allocate the pointers inside the array,
// which will be used to index the linear memory
array2d = new unsigned int*[sizeY];
// Let the pointers inside the array point to the correct memory addresses
for (int i = 0; i < sizeY; ++i)
{
array2d[i] = (temp + i * sizeX);
}
// Fill the array with ascending numbers
for (int y = 0; y < sizeY; ++y)
{
for (int x = 0; x < sizeX; ++x)
{
array2d[y][x] = x + y * sizeX;
}
}
I adapt it to reference structs, but it fails. I have tried multiplying by three in this line:
array2d[i] = (temp + i * sizeX /* multiply by 3*/);
But it still without work. I have also done the related castings from char to the struct bmp_pixel(char r, char g, char b).
Can somebody tell me how to adapt it to pure C for structs?? Thanks.
Can someone please point out what I am doing wrong in the following code?
int* a = NULL;
int* b = NULL;
a = new int[map->mapSize.width];
b = new int[map->mapSize.height];
layer->tileGids = new int[a][b];
Here's what the code uses:
typedef struct _size {
int width, height;
} size;
class Map {
size mapSize;
}
class Layer {
int * tileGids;
}
EDIT: Compiler-Errors (in line 6 of the first bit of code):
error: expression in new-declarator must have integral or enumeration type|
error: 'b' cannot appear in a constant-expression|
Solution:
I have decided to accept lightalchemist's answer. In essence, what works for me is use a vector instead of the array. Vector manages the memory for you and hence is a lot easier to deal with.
You can't pass a pointer for initializing the size of an array. Others have now mentioned this.
This post (it's not mine) seems like it might help you: http://eli.thegreenplace.net/2003/07/23/allocating-multi-dimensional-arrays-in-c/
You should also consider doing the allocation in the class Layer's constructor and then deleting the memory in it's destructor (i.e. RAII - resource acquisition is initialization). This is considered good style.
Finally, you might consider using continuous memory and a custom indexing scheme, which you could easily use Layer to encapsulate. This of course depends upon how big things will get. The bigger they get the better the case for continuous memory becomes.
This should give you a flavor.
#include <iostream>
#include <cstdlib>
int main()
{
const size_t ROWS = 5;
const size_t COLS = 2;
const size_t size = ROWS*COLS;
int* arr = new int[size];
int i = 0;
for ( size_t r = 0 ; r < ROWS; ++r )
{
for (size_t c = 0; c < COLS; ++c )
{
arr[r*COLS+c] = i++;
}
}
for ( int j = 0; j < i; ++j)
{
std::cout << arr[j] << std::endl;
}
delete [] arr;
}
Firstly, your variables "a" and "b" are pointers. Your code:
layer->tileGids = new int[a][b]
is the root cause of the problem.
I'm trying to guess your intention here and I think what you are trying to do is make layer.tileGids a 2 dimension array to reference a "grid" of size (mapSize.Width, mapSize.height) so that you can refer to each "cell" in the grid using layer.tileGids[x][y].
If you are indeed trying to create a 2 dimension array, there are 2 methods to do it.
Method 1:
class Layer {
int ** tileGids; // NOTE the "**" to indicate tileGids is a pointer to pointer i.e. 2D array.
}
To initialize it:
int width = map->mapSize.width;
int height = map->mapSize.height;
layer.tileGids = new int*[width]; // NOTE the "int*" to indicate tileGids is a new array of pointers to int.
for (int i = 0; i < width; i++) // Initialize each element in layer.tileGids[] to be a pointer to int.
{
layer.tileGids[i] = new int[height];
}
Now you can access the items in layer.tileGids using:
int value = layer.tileGids[x][y] // where 0 <= x < width and 0 <= y < height
To deallocate this data structure, similar to how you allocate it, you need to deallocate each dynamically allocated array in each "row":
for (int i = 0; i < width; i++)
{
delete [] layer.tileGids[i]; // Deallocate each row.
}
delete [] layer.tileGids; // Deallocate "array" to the pointers itself.
Method 2:
Now another easier, less messy method (avoid pointers) is to use the C++ vector class. You need to make the following changes:
#include <vector>
class Layer {
vector<vector<int> > tileGids; // Note the space at "<int> >".
}
To initialize:
int width = map->mapSize.width;
int height = map->mapSize.height;
layer.tileGids = vector<vector<int> >(width, vector<int>(height, 0)); // Initialize all entries to 0.
To access the elements:
int value = layer.tileGids[x][y]; // Where 0 <= x < width and 0 <= y < height
Note that for the second method using vectors, you do not have to do any memory cleanup as is required in the first method because the vector will automatically take care of it. However, because a vector can grow dynamically i.e. you can add items to it, you lose the safety of having a fixed size array i.e. someone could accidentally increase the size of your grid if you use the vector method but if he tries to do that when you intialized it using the first method above an error will occur and you will immediately know that something is wrong.
Can someone please point out what I am doing wrong in the following code?
A lot. You're allocating two single arrays (a "row array" and a "column array", not what you need), and then you try to do something strange.
Generally you can't (strictly speaking) dynamically allocate a 2D array in C++ (because the type system would still need the type, along with the dimensions, to be known at compile time). You can emulate it with an array of arrays or so, but the best way is to allocate an 1D array:
int width=5;
std::vector<int> tab(width*height);
and then access the element by calculating the coordinates manually:
// access the element (1,2)
tab[1 + 2*width] = 10;
This way you're essentially interpreting a 1D array as a 2D array (with performance equal to static 2D arrays).
Then it's best to wrap the indexing with a class for convenience; boost::multi_array also has this done for you already.
a and b are int* here:
layer->tileGids = new int[a][b];
Perhaps you meant to say this?
layer->tileGids = new int[*a][*b];