Creating a multi dimensional array at c

Question

Hello, need help creating a multi dimensional array i am new at C, any help is appreciated. this is the code

#include <stdio.h>
char init_board(int row, int col);
int main(int argc, char** argv) {
 int row = argv[3];
 int col = argv[4];
 char** board = init_board(row, col);
 for (int i = 0; i < row; i++) {
 for (int j = 0; j < col; j++) {
 printf("%d", board[i][j]);
 }
 }
 return 0;
}
char init_board(int row, int col) {
 int count = 0;
 int count2 = 0;
 char** out;
 while (count < row) {
 while (count2 < col) {
 out[count][count2] = ".";
 count2++;
 }
 count++;
 count2 = 0;
 }
 return out;
}

any idea how i can fix this? what i am doing wrong? when i compile i receive following warnings and when i run it says segmentation fault

s@ss:~/s216/arc/ass1/ass1$ gcc ass1.c -std=c99 -o test
ass1.c: In function ‘main’:
ass1.c:6:12: warning: initialization makes integer from pointer without a cast [enabled by default]
 int row = argv[3];
 ^
ass1.c:7:12: warning: initialization makes integer from pointer without a cast [enabled by default]
 int col = argv[4];
 ^
ass1.c:8:17: warning: initialization makes pointer from integer without a cast [enabled by default]
 char** board = init_board(row, col);
 ^
ass1.c: In function ‘init_board’:
ass1.c:27:23: warning: assignment makes integer from pointer without a cast [enabled by default]
 out[count][count2] = ".";
 ^
ass1.c:33:2: warning: return makes integer from pointer without a cast [enabled by default]
 return out;
 ^
s@ss:~/s216/arc/ass1/ass1$ ./test x x 5 5
Segmentation fault
s@ss:~/s216/arc/ass1/ass1$

• kaylum: int row = argv[3]; Left hand side is an int, right hand side is a char *. That are not compatible types - exactly as the warning tells you. You need to convert argv[3] to an int with something like atoi or better still strtol.
• iRove: Maybe this is completely irrelevant, but if everything is in bold, then nothing really stands out. Formatting is meant to draw attention to a specific part of the post, but if you draw attention everything, you may as well have drawn attention to nothing.

Answer 1

Type of argv[3] and argv[4] is char*. Hence, the lines

int row = argv[3];
int col = argv[4];

are wrong. You are trying to initialize two int objects with char*.

If you run the program using:

program 10 20 30 40

then,

The value of argv[0] will be "program".
The value of argv[1] will be "10".
The value of argv[2] will be "20".
The value of argv[3] will be "30".
The value of argv[4] will be "40".

Yes, they seem like they are numbers in the command line but they are strings in a C program. You can use atoi to extract the integral numbers from the strings.

int row = atoi(argv[3]);
int col = atoi(argv[4]);

Make sure to use

#include <stdlib.h>

to get the declaration of the function.

Answer 2

When you write int row = argv[3]; you are making an int and initialising it to the value of argv[3]. However, argv is of type char **, which is a pointer to a pointer, or effectively an array of type char *. This is an invalid assignment, you need to make a char buffer to read the arguments in, then use atoi(argv[i]) to get the integer value.

Answer 3

the initially there was many errors with the code, i belive the following code is what it should have been..

#include <stdio.h>
#include <stdlib.h>
char **init_board(int row, int col);
int main(int argc, char** argv) {
 int row = atoi(argv[3]);
 int col = atoi(argv[4]);
 char** board = init_board(row, col);
 for (int i = 0; i < row; i++) {
 for (int j = 0; j < col; j++) {
 board[i][j] = 'x';
 printf("%c", board[i][j]);
 }
 }
 return 0;
}
char **init_board(int row, int col) {
 char** out = malloc(sizeof(char*) * row);
 for (int i = 0; i < row; i++) 
 out[i] = malloc(sizeof(char) * col);
 return out;
}

• Anh Pham: Usually it's better to explain a solution instead of just posting some rows of anonymous code. You can read How do I write a good answer, and also Explaining entirely code-based answers