I will go straight to what I'm asking for, I also see some similar question but is not what I'm looking for...so it seems I have to ask with a new forum.
I'm preparing myself for a future examination, where is not required the pointer, but I would like to get some extra information and abilities.
Here's the code followed by the question...
I'm using Fedora 33, I know is different from some IDE on Windows (ex: Visual Studio or Dev C++)
/* It's just a simple test, if this work I will get myself into a more complicated one, as you could read in the
* forum, I'm getting ready ( just a recheck of my abilities ) for an universitary examinaton. */
#include <stdio.h>
#include <stdlib.h>
#define N 5
void casual_generation(int** mat);
void prompt_print(int** mat);
int main()
{
int **mat[N][N];
casual_generation(**mat);
prompt_print(**mat);
}
void casual_generation(int** mat)
{
int i=0,j=0;
for(i=0;i<N;i++)
for(j=0;j<N;j++)
mat[i][j] = rand() % 50;
}
void prompt_print(int** mat)
{
int i=0,j=0;
for(i=0;i<N;i++)
{
for(j=0;j<N;j++)
printf("%d ", mat[i][j]);
printf("\n");
}
}
Somebody else on the forum used malloc, struct or other stuff, as you can see in this picture, when I try to execute him it says "Segmentation fault (core dumped)"
screen error
Where is my error?
And if you want, can you also send me the version with the passed value pointer?
Thanks for whoever will give me an answer, and time dedicated.
This declaration
int **mat[N][N];
does not make a sense. It means that you have a matrix elements of which are pointers of the type int **. But you need a matrix elements of which are integer numbers of the type int. That is you need a declaration like this
int mat[N][N];
So now you have a two-dimensional array (or matrix) of integers.
As you are going to pass this two-dimensional array to functions then used as an argument expression it is converted to pointer to its first element of the type int ( * )[N].
Correspondingly the functions that accepts such an array should be declared like
void casual_generation( int mat[][N], size_t n );
void prompt_print( int mat[][N], size_t n );
or (that is fully equivalent) like
void casual_generation( int ( * mat )[N], size_t n );
void prompt_print( int ( *mat )[N], size_t n );
because the compiler adjusts function parameters having array types to pointers to array element types.
Now for example the first function can be defined the following way
void casual_generation( int ( * mat )[N], size_t n )
{
for ( size_t i = 0; i < n; i++ )
{
for ( size_t j = 0; j < N; j++ )
{
mat[i][j] = rand() % 50;
}
}
}
And the function can be called like
casual_generation( mat, N );
A similar way can be defined the function prompt_print.
Using the second parameter makes the function more general. For example it can be called for two-dimensional arrays with different numbers of rows.
Now I will explain why you are getting a segmentation fault in your original code.
You have this declaration
int **mat[N][N];
a two dimensional array of pointers of the type int **.
Then you are using the expression **mat as an argument of function calls like this
casual_generation(**mat);
Then you are applying the dereference operator like *mat the array designator is converted to pointer to its first element (row) having the type int ** ( * )[N]. So dereferencing this pointer you get the first row of your array int **[N]. Applying the second time the dereferenced operator to this expression that has an array type the used expression is again is converted to pointer to its first element of the type int **( * ). That is it points to the first element of the first row of the original two-dimensional array. Dereferencing this pointer you get the first element of the type int **. This uninitialized pointer with indeterminate value the function accepts as its argument.
Thus dereferencing this first uninitialized element of the original matrix within the function
mat[i][j] = rand() % 50;
^^^
you get a segmentation fault. The reason of the fault is the incorrect matrix and the corresponding function parameter as it was shown above in tbe beginning of the answer.
Where is my error?
The "Segmentation fault" error happens because you define the variable mat as a pointer, but don't allocate any memory for it to point to.
int **mat[N][N];
You meant to do
Int mat[N][N];
and
casual_generation(mat);
prompt_print(mat);
By passing **mat you are passing mat[0][0] that is an int, but you want to pass the whole matrix which is a pointer to pointers to int (i.e. int **)
And you may want to introduce srand() in your code.
Just to make things clear:
mat is of type int ** and it's the whole matrix (or if you want it's a pointer to the first row)
*mat is of type int * and it's the first row of the matrix (or if you want it's a pointer to the first element of the first row)
**mat is of type int and it's the first element of the first row of the matrix
int **mat[N][N];
Here, you defined a double pointer to a 2D array. You only need to use one of those - a double pointer or a 2D array, like so:
int mat[N][N];
However, the bigger problem comes from trying to interchange 2D arrays and double pointers. This isn't possible in C since the 2D array is laid out flat in memory.
You need to create an array mat_ptr of pointers yourself and then pass that to casual_generation and prompt_print.
Finally, these casual_generation and prompt_print functions expect to be given a pointer, so you shouldn't dereference the pointer with ** before calling the function.
The final working code is:
int main()
{
int mat[N][N];
int *mat_ptr[N];
for (int i = 0; i < N; i++)
mat_ptr[i] = mat[i];
casual_generation(mat_ptr);
prompt_print(mat_ptr);
}
As you can find a detailed explanation why the code crashed in the other answer I will only propose a quite elegant solution that uses a neat though little known feature from C99 called Variable Length Arrays (aka VLA).
#include <stdio.h>
#include <stdlib.h>
void casual_generation(int n, int mat[n][n]);
void prompt_print(int n, int mat[n][n]);
int main()
{
const int N = 5;
int mat[N][N];
casual_generation(N, mat);
prompt_print(N, mat);
}
void casual_generation(int n, int mat[n][n])
{
for(int i=0;i<n;i++)
for(int j=0;j<n;j++)
mat[i][j] = rand() % 50;
}
void prompt_print(int n, int mat[n][n])
{
for(int i=0;i<n;i++)
{
for(int j=0;j<n;j++)
printf("%d ", mat[i][j]);
printf("\n");
}
}
It compiles in pedantic mode with no warnings and it works like a charm.
VLA were introduced to C to simplify numerical computation over multidimensional arrays.
Related
Having looked around I've built a function that accepts a matrix and performs whatever it is I need on it, as follows:
float energycalc(float J, int **m, int row, int col){
...
}
Within the main the size of the array is defined and filled, however I cannot passs this to the function itself:
int matrix[row][col];
...
E=energycalc(J, matrix, row, col);
This results in a warning during compilation
"project.c:149: warning: passing argument 2 of ‘energycalc’ from
incompatible pointer type project.c:53: note: expected ‘int **’ but
argument is of type ‘int (*)[(long unsigned int)(col +
-0x00000000000000001)]’
and leads to a segmentation fault.
Any help is greatly appreciated, thank you.
The function should be declared like
float energycalc( float J, int row, int col, int ( *m )[col] );
if your compiler supports variable length arrays.
Otherwise if in declaration
int matrix[row][col];
col is some constant then the function can be declared the following way
float energycalc(float J, int m[][col], int row );
provided that constant col is defined before the function.
Your function declaration
float energycalc(float J, int **m, int row, int col);
is suitable when you have an array declared like
int * matrix[row];
and each element of the array is dynamically allocated like for example
for ( int i = 0; i < row; i++ ) matrix[i] = malloc( col * sizeof( int ) );
If the array is declared as
int matrix[row][col];
then change the function declaration to
float energycalc(float J, int m[][col], int row, int col){
The name of a two dimensional array of type T does not decay to T**.
If col is a local variable, then you need to call the function with the col parameter before matrix. This is done so that col is visible in the function.
Passing two dimensional array to a function in C is often confusing for newbies.
The reason is that they assume arrays are pointers and having lack of understanding how arrays decays to pointer.
Always remember that when passed as an argument arrays converted to the pointer to its first element.
In function call
E = energycalc(J, matrix, row, col);
matrix is converted to pointer to its first element which is matrix[0]. It means that passing matrix is equivalent to passing &matrix[0]. Note that the type of &matrix[0] is int(*)[col] (pointer to an array of col int) and hence is of matrix. This suggest that the second parameter of function energycalc must be of type int(*)[col]. Change the function declaration to
float energycalc(int col, int (*m)[col], int row, float J);
and call your function as
E = energycalc(col, matrix, row, J);
I think you can do something like this in C
float function_name(int mat_width, int mat_height, float matrix[mat_width][mat_height]) {
... function body...
}
A pointer-to-pointer is not an array, nor does it point to a two-dimensional array. It has nothing to do with arrays, period, so just forget you ever heard about pointer-to-pointers and arrays together.
(The confusion likely comes from the hordes of confused would-be programming teachers/authors telling everyone that they should use pointer-to-pointer when allocating dynamic 2D arrays. Which is just plain bad advice, since it will not allocate a real array allocated in adjacent memory cells, but rather a fragmented look-up table exploded in pieces all over the heap. See this for reference about how to actually allocate such arrays.)
Assuming that your compiler isn't completely ancient, simply use a variable length array (VLA), which is a feature introduced in the C language with the C99 standard.
#include <stdio.h>
void print_matrix (size_t row, size_t col, int matrix[row][col]);
int main (void)
{
int matrix[2][3] =
{
{1,2,3},
{4,5,6}
};
print_matrix(2, 3, matrix);
return 0;
}
void print_matrix (size_t row, size_t col, int matrix[row][col])
{
for(size_t r=0; r<row; r++)
{
for(size_t c=0; c<col; c++)
{
printf("%d ", matrix[r][c]);
}
printf("\n");
}
}
I am trying to compute the power of the matrix A using multiplications.
I am having problems with the ArrayPower function. It does not function as i think it should.The MultiArray function however seems to work fine. Can anyone help me ?
#include <stdio.h>
int** MultiArray(int a[2][2],int b[2][2]);
int** ArrayPower(int a[2][2],int e);
int main(void)
{
int fa[2][2];
fa[0][0]=0;
fa[0][1]=1;
fa[1][0]=1;
fa[1][1]=1;
int **multifa=malloc(sizeof(int)*2);
for (int i=0;i<2;i++) {
multifa[i]=malloc(sizeof(int)*2);
}
multifa=ArrayPower(fa,2);
printf("%d %d\n",multifa[0][0],multifa[0][1]);
printf("%d %d\n",multifa[1][0],multifa[1][1]);
return 0;
}
int** MultiArray(int a[2][2], int b[2][2]) {
//multi a *b
//memory allocation
int i,rows=2,cols=2;
int **c=malloc(rows*sizeof(int));
for (i=0;i<rows;i++) {
c[i]=malloc(cols*sizeof(int));
}
c[0][0]=a[0][0]*b[0][0]+a[0][1]*b[1][0];
c[0][1]=a[0][0]*b[0][1]+a[0][1]*b[1][1];
c[1][0]=a[1][0]*b[0][0]+a[1][1]*b[1][0];
c[1][1]=a[1][0]*b[0][1]+a[1][1]*b[1][1];
return c;
}
int** ArrayPower(int a[2][2],int e) {
//memory allocation
int i,rows=2,cols=2;
int **c=malloc(rows*sizeof(int));
for (i=0;i<rows;i++) {
c[i]=malloc(cols*sizeof(int));
}
c[0][0]=a[0][0];
c[0][1]=a[0][1];
c[1][0]=a[1][0];
c[1][1]=a[1][1];
for (i=1;i<e;i++) {
c=MultiArray(a,c);
}
return c;
}
MultiArray is declared as taking a second parameter of type int [2][2], but it is called with an argument of c, which as type int **. These are not compatible types.
In a parameter, the type int [2][2] is automatically converted to a pointer to an array of two int, the type int (*)[2]. This is a pointer to a place where there are two int objects (and, because we know it is the first element of an array of two arrays of two int objects, we know there are two more int objects beyond the first two).
The definition of c with int **c means that c is a pointer to a pointer to an int. A pointer to a pointer and a pointer to an array are different and are not compatible.
One way to fix this is to define c with int (*c)[2] = malloc(2 * sizeof *c);. It is then unnecessary to have the loop after the definition that allocates more space; the single allocation allocates the entire array.
The return type of MultiArray should be changed similarly, as well as the code within it and elsewhere in the program. Alternatively, the second parameter of MultiArray can be changed from int b[2][2] to int **b. (This latter is an easier edit but produces an inferior program, since it uses more pointers and allocations than necessary.)
You should always compile your code with warnings enabled. That would have alerted you to the incorrect call.
I just want to print out a static array (2D array) in C using functions. I use gcc as my compiler. When I try to run my code it gives me a seg fault and I dont have any idea why:
#include <stdio.h>
void print_out_an_array(int n, int m, int tab[n][m])
{
int i,j;
for(i=0; i<n; i++)
for(j=0; j<m; j++)
printf("tab[%d][%d] = %d\n", i, j, tab[i][j]);
}
int main(int argc, char **argv)
{
int tab[2][4] = {{1,2,3,4}, {5,6,7,8}};
print_out_an_array(tab, 2, 4);
return 0;
}
your function call and function definition doesnt match
your function call
print_out_an_array(tab, 2, 4);
but in function definition your first argument is int
void print_out_an_array(int n, int m, int tab[n][m])
make the arguments same, like:
change function call to
``print_out_an_array(2, 4, tab);`
update:
check this code it works
and also read this for reference C, passing 2 dimensional array
In your function definition, first parameter is int type but you are calling your function with first argument as int **. Change your function call to
print_out_an_array(2, 4, tab);
About the question in your comment:
Ok, but how about this code: http://ideone.com/Z4mHkb why it gives me an error?
Function parameters **tab and tab[n][m] are not equivalent. Compiler, on seeing tab [m][n] as function parameter, interprets it as
void fun(int (*)[m]tab, int n, int m)
i.e , it interprets tab as a pointer to an array of m integers. While on seeing int **tab, it simply interprets tab as a pointer to a pointer to integer (or an array of pointers ( int *tab[]) to int ).
Instead of answering the original solution whose solution is obvious, I answer the other one from your comment.
Your array is a
int[][N]
while you pass a
int ** to your function.
These are completely different from each other:
An int[][N] has all values from all dimensions beneath each other.
An int **, however, points to one or more pointers, in turn pointing to the real values.
At int[][], you can omit one level of indirection and can turn it into a
int (*)[N]
i. e. a pointer to an array. This array must be of determined size, which isn't fulfilled in your case as well.
Below program (a toy program to pass around arrays to a function) doesn't compile.
Please explain me, why is the compiler unable to compile(either because of technical reason or because of standard reason?)
I will also look at some book explaining pointers/multi dimensional arrays(as I am shaky on these), but any off-the-shelf pointers here should be useful.
void print2(int ** array,int n, int m);
main()
{
int array[][4]={{1,2,3,4},{5,6,7,8}};
int array2[][2]={{1,2},{3,4},{5,6},{7,8}};
print2(array,2,4);
}
void print2(int ** array,int n,int m)
{
int i,j;
for(i=0;i<n;i++)
{
for(j=0;j<m;j++)
printf("%d ",array[i][j]);
printf("\n");
}
}
This (as usual) is explained in the c faq. In a nutshell, an array decays to a pointer only once (after it decayed, the resulting pointer won't further decay).
An array of arrays (i.e. a
two-dimensional array in C) decays
into a pointer to an array, not a
pointer to a pointer.
Easiest way to solve it:
int **array; /* and then malloc */
In C99, as a simple rule for functions that receive "variable length arrays" declare the bounds first:
void print2(int n, int m, int array[n][m]);
and then your function should just work as you'd expect.
Edit:
Generally you should have a look into the order in which the dimension are specified. (and me to :)
In your code the double pointer is not suitable to access a 2D array because it does not know its hop size i.e. number of columns. A 2D array is a contiguous allotted memory.
The following typecast and 2D array pointer would solve the problem.
# define COLUMN_SIZE 4
void print2(int ** array,int n,int m)
{
// Create a pointer to a 2D array
int (*ptr)[COLUMN_SIZE];
ptr = int(*)[COLUMN_SIZE]array;
int i,j;
for(i = 0; i < n; i++)
{
for(j = 0; j < m; j++)
printf("%d ", ptr[i][j]);
printf("\n");
}
}
I just started programming so pointers and arrays confuse me.
This program just assigns random numbers from 0 - 9 into array and prints them out
(#include <stdio.h> #include <stdlib.h> #include <time.h>)
int function(int *num[]){
int i;
for(i=0; i<10; i++){
srand((unsigned)time(NULL));
*num[i] = rand()%10;
printf("%d", *num[i]);
}
return 0;
}
int main(){
int num[10];
function(&num); // incompatable pointer type (how do i fix this?)
return 0;
}
Thankyou
Change your code to read something like this:
int function(int num[]){
int i;
for(i=0; i<10; i++){
srand((unsigned)time(NULL));
num[i] = rand()%10;
printf("%d", num[i]);
}
return 0;
}
int main(){
int num[10];
function(num);
return 0;
}
In main(), you are allocating an array of 10 integers. The call to function(num) passes the address of this array (technically, the address of the first element of the array) to the function() function. In a parameter declaration, int num[] is almost exactly equivalent to int *num (I'll let you ponder on that one). Finally, when you access the elements of num[] from within the function, you don't need any extra *. By using num[i], you are accessing the i'th element of the array pointed to by num.
It may help to remember that in C, the following are exactly the same:
num[i]
*(num+i)
You don't need to pass in a pointer to your array. Just pass in the array. I have fixed your pointer code below.
Also, you should not reset the seed (srand) through every iteration through the for loop.
int function(int num[]){
int i;
srand((unsigned(time(NULL));
for(i=0; i<10; i++){
num[i] = rand()%10;
printf("%d", num[i]);
}
return 0;
}
int main(){
int num[10];
function(num);
return 0;
}
It all depends on what you want to achieve in the end.
(1) If your function function is intended to be used with arrays of run-time size, then you have to pass a pointer to the first element of the array. That is achieved by the following equivalent declarations
int function(int *num)
or
int function(int num[])
Inside the function you have to access the passed array as
num[i] = rand() % 10;
And you call your function as
function(num);
Of course, when the array size is a run-time value, it makes sense to pass that size to the function as well, meaning that your function should have the following interface
int function(int num[], size_t n)
And implement it for an array of size n, instead of hardcoding 10 directly into your implementation.
(2) If your function function is intended to be used with arrays of fixed compile-time size (10 in this case), then the better approach would be to pass a pointer to the entire array. It is achieved by the following declaration
int function(int (*num)[10])
Inside the function you have to access the passed array as
(*num)[i] = rand() % 10;
And you call your function as
function(&num);
So it is either (1) or (2).
What you currently have in your code looks like a mix of these two approaches. More precisely, you code looks like an attempt to implement the second approach, but it is missing some important parentheses :)
Most answers given to you so far suggest using the first approach. However, seeing that the array size is actually a compile-time constant in your case, I would suggest sticking with the second approach.
Use either int *num or int num[], don't use both together as you have right now (int *num[]). An array is passed to a function as a pointer to the first element, i.e. int *num, which is equivalent to int num[].