Thread: how to create an user input for 2D array

if i want the user to input (via 'scanf' i guess) a number .
lets say 4.
and then the computer will build a 4x4 board.
and after i will want to add elements to the array.

i tried a way to build an array with variables.
for example

Code:

int myarray[i][j];

but then i got error

Code:

"expected constant expression"

i'm guessing there's a way with maybe allocate memory for a two-star pointer like this :

Code:

 int rows;
int **ptr = malloc(rows*sizeof(int*);

can you help figure this out ?

Update your compiler so it complies with the 1999 C standard (or more recent) and your first way will work. For older compilers it will not.

If you are stuck with an older compiler, there are two ways depending on whether you are adamant that the array has to be two-dimensional.

The first is to malloc() enough memory, but index it as a one-dimensional array;

Code:

    /*  read value for row and columns */
    int *p = malloc(rows*columns*sizeof(*p));

    /*   get values i and j representing element you want to access */

    p[i*rows + j] = 42;    /*  42 = meaning of life, the universe, and everything */

or (if you really want a pointer to pointer and 2D array syntax) ....

Code:

    /*  read value for row and columns */
    int **p = malloc(rows*sizeof(*p));
    for (i = 0; i < rows; ++i)
       p[i] = malloc(columns*sizeof(**p));

    /*   get values i and j representing element you want to access */

    p[i][j] = 42;

It's also necessary to check that ALL of the malloc() calls succeed and, when done with the arrays, to call free(). In the second approach, ALL of the individual malloc() calls need corresponding free() calls.

Originally Posted by Idan Damri

I have the latest visual studio compiler.

Microsoft has cared more about compliance with C++ standards rather than C for at least a decade. No surprises that some of the more recent features of C are not supported.

Originally Posted by grumpy

Update your compiler so it complies with the 1999 C standard (or more recent) and your first way will work. For older compilers it will not.

If you are stuck with an older compiler, there are two ways depending on whether you are adamant that the array has to be two-dimensional.

The first is to malloc() enough memory, but index it as a one-dimensional array;

Code:

    /*  read value for row and columns */
    int *p = malloc(rows*columns*sizeof(*p));

    /*   get values i and j representing element you want to access */

    p[i*rows + j] = 42;    /*  42 = meaning of life, the universe, and everything */

or (if you really want a pointer to pointer and 2D array syntax) ....

Code:

    /*  read value for row and columns */
    int **p = malloc(rows*sizeof(*p));
    for (i = 0; i < rows; ++i)
       p[i] = malloc(columns*sizeof(**p));

    /*   get values i and j representing element you want to access */

    p[i][j] = 42;

It's also necessary to check that ALL of the malloc() calls succeed and, when done with the arrays, to call free(). In the second approach, ALL of the individual malloc() calls need corresponding free() calls.

Why you wrote 42? If the user will want to input an 4x4 2d array?

And i know that in the collage i will learn they use visual studio 2012.
So theres no point for me now to use gcc for c 99. Or any other compiler that supports c99.

42 is just an example value. For the array size, look at the variables named rows and columns.

Originally Posted by Idan Damri

Why you wrote 42? If the user will want to input an 4x4 2d array?

You clearly have not read the work of Douglas Adams.

In any event, the value of 42 has no relationship to the dimensions of the array.

Originally Posted by grumpy

You clearly have not read the work of Douglas Adams.

In any event, the value of 42 has no relationship to the dimensions of the array.

yes.. ok i understood.
i have 2 problems. you may read the code below first.
1) for example if the user input is 4 . the output will be 7-8 rows and 4 columns. and i want it to be identical 2d array (in this case 4x4)
or if the users input '2' so i want 2x2 2d array.
2)the numbers are repeating themselfs. each row is the same. and i wants real random numbers.
and , again, only for example, if the users input is 4 ... an int* in my compiler is 4 bytes. so when i use malloc (4*sizeof(int*) for the rows its 16.
and for the columns also 16. so if my array is 4*4 i need total allocation of 64 bytes ( 16* int which is 4 ) and its allocating only 32 bytes. i could simply allocate rows*colms*sizeof int*. but i want to figure this way below.
this is the code :

Code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>


int main(){
 int rows ;
scanf("%d", &rows);
 int colms = rows;
 int i = 0;
 int j= 0;


int **board = malloc(rows*sizeof(*board));
printf("%d\n",rows*sizeof(*board));     //to check.
board[i] = malloc(colms*sizeof(*board));
printf("%d\n", colms*sizeof(*board)); //to check


for (; i < rows; i++){
for(; j<colms; j++){


srand((unsigned)time(NULL));
board[i][j] = rand() %(rows*colms) ;


printf("%d\n", *board);


 }
}
return 0;
}

You need to indent your code properly, e.g.,

Code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main() {
    int rows;
    scanf("%d", &rows);
    int colms = rows;
    int i = 0;
    int j = 0;

    int **board = malloc(rows * sizeof(*board));
    printf("%d\n", rows * sizeof(&board));   //to check.
    board[i] = malloc(colms * sizeof(*board));
    printf("%d\n", colms * sizeof(*board)); //to check

    for (; i < rows; i++) {
        for(; j < colms; j++) {
            srand((unsigned)time(NULL));

            board[i][j] = rand() %(rows*colms) ;

            printf("%d\n", *board);
        }
    }

    return 0;
}

A few comments:

• You should check that scanf returns 1 before proceeding.
• Your check for number of rows allocated contains a typo error, i.e., sizeof(&board) should be sizeof(*board)
• You need to loop to allocate for each row.
• You should call srand only once, typically near the start of the main function.
• *board is a pointer, so you presumably don't want to print it but rather want to print board[i][j].

Oh, one more thing. This:

Code:

board[i] = malloc(colms * sizeof(*board));

should be:

Code:

board[i] = malloc(colms * sizeof(**board));

of course within a loop. You might find it easier see by writing:

Code:

int **board = malloc(rows * sizeof(board[0]));

and:

Code:

board[i] = malloc(colms * sizeof(board[0][0]));

Originally Posted by Idan Damri

but it all doesnt really matter because **board, or *board, or board[0] are all 4 bytes in size.

It would be sloppy to assume that just because this is the case for now, that it will always be the case.

Originally Posted by Idan Damri

and i tried what you wrote before. it wont change something accept now its print only the columns.

What is your current code?