Undefined behavior with 2d array of struct C - c

I have a 2d array of structs that I am assigning strings to, here is my struct.
struct node {
char* value;
};
Here is my allocation (I am new to C so I am not sure if it is right) but there will always be 35 columns but there could be millions of rows.( I just had it at 3 for now for testing)
const int rows=3;
struct node ** arrayofnodes[rows][35];
for(int i=0; i<rows; i++) {
array[i] = malloc(test * sizeof array[0]);
for(int j=0; j<35; j++) array[i][j] = malloc(sizeof array[0][0]);
}
I then read in character by character from a csv file and have a temp string, and assign the value of the temp to the position I want by using this below.
//int row and count are defined in my while loop I have for counting commas(or what col I am on) then new lines for the rows
arrayofnodes[row][count]->value=strdup(temp);
printf("%s \n", arrayofnodes[row][count]->value);
printf("%d %d \n",row, count );
When I assign like the way above it seems to work. I added these print statements in to make sure it was assigning the right values.
For example above would print out something like
Red
0 0
And this is correct for that position.
But then after I do all of my assigning. I placed a print statement printf("%s \n", arrayofnodes[0][0]->value); to test if I can retrieve the 1st value as shown above which should be "Red".
In my terminal it outputs "#`??" or "#Pz?" or just any random output. I have tried this for a bunch of different positions besides 0,0, but they all get the same outcome. I guess I am just confused why the print statements work right after I assign them, but not at the end of my code when I call them later.

This is what it looks like you're trying to do. You will need to scan your csv file and compute the number of rows required, then populate the values however you want.
#include <stdio.h>
#include <stdlib.h>
struct node {
char* value;
};
int main() {
const int rows = 3; // you will need to compute this beforehand
const int columns = 35;
struct node** arrayofnodes = malloc(rows * sizeof(struct node*));
for (int i = 0; i < rows; ++i) {
arrayofnodes[i] = malloc(columns * sizeof(struct node));
}
for (int i = 0; i < rows; ++i) {
for (int j = 0; j < columns; ++j) {
arrayofnodes[i][j].value = malloc(...);
strcpy(arrayofnodes[i][j].value, ...); // etc..
}
}
for (int i = 0; i < rows; ++i) {
for (int j = 0; j < columns; ++j) {
free(arrayofnodes[i][j].value);
}
}
for (int i = 0; i < rows; ++i) {
free(arrayofnodes[i]);
}
free(arrayofnodes);
}

You can avoid overcomplicated allocation by using constant size inside the structure:
struct OneRow
{
char Value[35];
}
const int Rows=3;
OneRow *MyArray=NULL;
MyArray = (OneRow*) malloc (Rows*sizeof(OneRow));
You can now access each element (character) or a whole string as
MyArray[rownumber].Value[colnumber] = …
strcpy (MyArray[rownumber].Value, "I'm_shorter_than_35"); //34 chars max + null-term

Related

C - Segmentation Fault Reading Struct and Dynamic2D Array Member

I just started learning C and I wanted to try creating a test program that works with pointers, structures, and arrays, since I still have a hard time understanding them. I created this test file which is a distilled version of a larger project that I'm working on. The test file has a struct with a dynamic 2D array as a member of the struct:
typedef struct {
int ** array;
int rows, cols;
} Smaller;
However, after running the test file the terminal returns the following error:
zsh: segmentation fault ./a.out
I researched what this error means,
" Segmentation fault is a specific kind of error caused by accessing memory that “does not belong to you.” " (Link)
But I'm still confused on how fix this problem. I'm pretty sure I allocated the correct amount of memory for each row and column. It's even more confusing because the terminal doesn't indicate which line the error is. I would appreciate any help on this issue.
Below is the full code:
#include <stdio.h>
#include <stdlib.h>
typedef struct {
int ** array;
int rows, cols;
} Smaller;
void printArray (Smaller * s);
int main () {
int x, i, j;
Smaller * sand;
// allocate mem for number of rows
sand->array = malloc (3 * sizeof(int *));
//allocate mem for number of columns
sand->array = malloc(4 * sizeof(int));
sand->array = malloc(4 * sizeof(int));
sand->array = malloc(4 * sizeof(int));
// adding a constant value to the 2D array
for (i = 0; i < 3; i ++) {
for (j = 0; j < 4; j ++) {
sand->array[i][j] = 6;
}
}
printArray(sand);
return 0;
}
void printArray (Smaller * sand) {
printf("Welcome to the printArray function! \n");
int i, j;
for (i = 0; i < 3; i ++)
for(j = 0; j < 4; j ++)
printf("array[%d][%d] = %d \n", i, j, sand->array[i][j]);
}
The problem is, as #tromgy pointed out, you are overwriting the base sand->array with the column arrays instead of assigning them to it. A correct code would look like this:
#include <stdlib.h>
#define NUM_ROWS 3
#define NUM_COLS 4
typedef struct {
int ** array;
int rows;
int cols;
} Smaller;
void print_array(Smaller * s);
int main(void) {
Smaller * sand = malloc(sizeof(Smaller));
if (!sand) return -1; /* allocation failed, abort */
sand->rows = NUM_ROWS;
sand->array = malloc(sizeof(int*[NUM_ROWS]));
if (!sand->array) { /* allocation failed, abort */
free(sand); /* free sand first, though */
return -1;
}
for (size_t i = 0; i < NUM_ROWS; ++i) {
sand->array[i] = malloc(sizeof(int[NUM_COLS]));
if (!sand->array[i]) {
/* free the previous rows */
for (size_t j = 0; j < i; ++j) free(sand->array[j]);
free(sand->array);
free(sand);
return -1;
}
}
/* add a constant value to the array */
for (size_t i = 0; i < NUM_ROWS; ++i) {
for (size_t j = 0; j < NUM_COLS; j ++) {
sand->array[i][j] = 6;
}
}
print_array(sand);
/* Ok, now free everything */
for (size_t i = 0; i < NUM_COLS; ++i) {
free(sand->array[i]);
}
free(sand->array);
free(sand);
/* NOW we may exit */
return 0;
}
As you can see, allocating a structure like this is a lot of work, and you have to free whatever you allocate, so it's probably better to extract it out to a function, something like Smaller * smaller_init(size_t nrows, size_t ncols) and void smaller_destroy(Smaller * s) encapsulating all that work.
I will left an example below so you can compare it to the way you wrote it originally...
About your code:
Declare loop variables inside the for command
May be Smaller do not need to be a pointer
Keep dimensions as variables. It is more flexible
You did not set the values for rows and cols in the struct. And in main() do not use fixed values as 3 and 4 as you did
You should set all cells to different values, not the same. You will feel safer when you see reversible values, like 100*row + column in the example... This way you can see if the loops are ok and all elements are being printed. See this output for printArray():
0 1 2 3
100 101 102 103
200 201 202 203
Each line starts with the line number so you can test it a few times before going on.
make your program test itself. In printArray() for example show the dimensions like this:
printArray[3,4]
0 1 2 3
100 101 102 103
200 201 202 203
See the output of the example
always write the code to free the memory, in the reserve order of the allocation, maybe in a separate function that returns NULL in order to invalidate the pointer back in the calling code, like this
Smaller* freeArray(Smaller* A)
{
printf("\nfreeArray()\n");
for (int i = 0; i < A->rows; i++)
{
free(A->array[i]); // delete lines
printf("row %d free()\n", i);
}
free(A->array); // delete cols
printf("pointer to rows free()\n");
free(A); // delete struct
printf("struct free()\n");
return NULL;
}
This way you know that the pointer sand will not be left pointing to an area that has been free()d. Using such a pointer will crash your program so it may be good to write
sand = freeArray(sand);
output of the example code
printArray[3,4]
0 1 2 3
100 101 102 103
200 201 202 203
freeArray()
row 0 free()
row 1 free()
row 2 free()
pointer to rows free()
struct free()
Example code
#include <stdio.h>
#include <stdlib.h>
typedef struct
{
int** array;
int rows, cols;
} Smaller;
void fillArray(Smaller*);
Smaller* freeArray(Smaller*);
Smaller* makeArray(size_t, size_t);
void printArray(Smaller*);
int main(void)
{
int y = 3;
int x = 4;
// sand points to a Smaller
Smaller* sand = makeArray(y, x);
// adding known unique values to cells is easier
fillArray(sand);
printArray(sand); // show values
sand = freeArray(sand); // delete all
return 0;
}
void fillArray(Smaller* A)
{
for (int i = 0; i < A->rows; i++)
for (int j = 0; j < A->cols; j++)
A->array[i][j] = 100 * i + j;
}
Smaller* freeArray(Smaller* A)
{
printf("\nfreeArray()\n");
for (int i = 0; i < A->rows; i++)
{
free(A->array[i]); // delete lines
printf("row %d free()\n", i);
}
free(A->array); // delete cols
printf("pointer to rows free()\n");
free(A); // delete struct
printf("struct free()\n");
return NULL;
}
Smaller* makeArray(size_t y, size_t x)
{
// sand points to a Smaller
Smaller* sand = (Smaller*)malloc(sizeof(Smaller));
sand->rows = y;
sand->cols = x;
// allocate mem for number of rows, that is 'y'
sand->array = malloc(y * sizeof(int*));
// allocate mem for each of the 'x' columns
for (size_t i = 0; i < y; i++)
sand->array[i] = malloc(x * sizeof(int));
return sand;
};
void printArray(Smaller* sand)
{
printf("printArray[%d,%d]\n\n", sand->rows, sand->cols);
for (int i = 0; i < sand->rows; i++)
{
for (int j = 0; j < sand->cols; j++)
printf("%3d ", sand->array[i][j]);
printf("\n");
}
}
About the code
Please SO people do not bother pointing me not to cast the result of malloc(). It is by decision. This common recommendation is a reminiscence of the C-faq of the 90's and now we know that implicit conversions maybe not so good. In fact implicit things may cost you a lot of time: if you malloc() a series of different structs in a program and omit the types if some of them are for example reversed keep in mind that the use of all casts would help you avoid this costly type of mistake...

How to increment value in a struct matrix in c?

I've calloc'd a correlation matrix in a struct but am unable to increment/set values in that matrix. The struct is
typedef struct matrixStruct{
char** word;
int numberOfWords;
int** matrix;
} matrix;
This is how i've allocated the structure using calloc. I've used calloc because i'd like all the values in the matrix to be 0.
//Allocate rows of matrix
wordStore->matrix = calloc(2000,sizeof(int*));
//Allocate columns of matrix
for(int j = 0; j< 2000; j++)
{
wordStore->matrix[j] = calloc(2000,sizeof(int*));
}
Assume i have an array which has values of
int reference [] = {20,400,5,1899};
And this is how i'm trying to assign/increment values within the matrix, but it doesn't seem to work.
for(int k = 0; k<lenReference; k++)
{
for(int l = 0; l<lenReference;l++)
{
wordStore->matrix[k][l] += 1;
if(k == l){
wordStore->matrix[k][l] = 0;
}
}
}
This is the print loop that i'm using, and when i run the file it has an error that says "subscripted value is not an array, pointer, or vector", but i thought that this was the correct way to print a 2d array. What is the issue with this print?
for(int i = 0; i<2000; i++)
{
for (int j = 0; j<2000; j++)
{
printf("%08d ", words.matrix[i][j]);
}
printf("\n");
}
Any help would be much appreciated!

Allocating memory to a 2D array in C

Why can't I allocate each index of a 2D array individually.
Is there any correct and proper way to allocate memory if you want to use a dynamically allocated array in C?
I am getting a segmentation error for this code
here is my code:
typedef struct
{
char ***sheet;
int columns;
int rows;
} SPREADSHEET; // to access data sheet[COL][ROW]
SPREADSHEET *createSpreadSheet(int columns, int rows) {
if (columns <= 0 || rows <= 0){ // Disallow 0 or negative rows/columns
return NULL;
}
SPREADSHEET *sp = malloc(sizeof(SPREADSHEET));
sp->columns = columns;
sp->rows = rows;
for(int i=0; i<rows; i++){
for(int j=0; j<columns; j++){
printf("i: %i, j: %i\n", i, j);
sp->sheet[i][j] = malloc(sizeof(char(*))+2);
}
}
return sp;
}
I think that what you are looking for is this:
typedef struct
{
int columns;
int rows;
char sheet[];
} SPREADSHEET;
SPREADSHEET *sp = malloc( sizeof(SPREADSHEET) + sizeof(char[columns][rows]) );
sp->columns = columns;
sp->rows = rows;
for(size_t i=0; i<columns; i++)
{
for(size_t j=0; j<rows; j++)
{
sp->sheet[i*rows + j] = something;
}
}
...
free(sp);
Where sheet is a so-called flexible array member. It has to be declared last in the struct, and it's guaranteed to behave deterministically when you allocate trailing memory after the struct itself. The variable in itself does not add to the size of the struct.
Unfortunately C only allows single row flexible array members, so you have to use a "mangled 2D array" if you wish to use it as in your case. It means we calculate the array index manually in run-time, in order to access the array items.
Alternatively, you can access the items through a temporary array pointer, which is also well-defined:
char (*arr)[rows] = (void*) sp->sheet;
for(size_t i=0; i<columns; i++)
{
for(size_t j=0; j<rows; j++)
{
arr[i][j] = something;
}
}

2D arrays using arrays of pointers or pointers to pointers in C?

I'm writing a C for which I need to create a 2D array. I've found a solution to my problem using double pointers (pointers to pointers) in the following way:
#include <stdio.h>
#include <stdlib.h>
int d = 3;
#define DIM_MAX 9
void changeArray(int d, int *array[d]);
int main()
{
//alocate array of 'd' colummns and 'd' row using malloc using array of pointers
int **array = malloc(d*sizeof(int *));
for(int count = 0; count < d; count++)
{
array[count] = malloc(d*sizeof(int *));
}
/* Call changeArray function */
changeArray(d, array);
for(int i = 0; i < d; i++)
{
for(int j = 0; j < d; j++)
{
printf("%d ", array[i][j]);
}
printf("\n");
}
for(int count = 0; count < d; count++)
{
free(array[count]);
}
return 0;
}
void changeArray(int n, int *array[d])
{
for(int i =0; i < n; i++)
{
for(int j = 0; j < n; j++)
{
array[i][j] = i*j;
}
}
return;
}
The code above works pretty well (it seems), but I've read in the web that using pointer to pointer is not the correct way to create 2D arrays. So I've come up with the following code, which also works:
#include <stdio.h>
#include <stdlib.h>
#define DIM_MAX 9
int d = 3;
void changeArray(int d, int *array[d]);
int main()
{
//alocate array of 'd' colummns and 'd' row using malloc using array of pointers
int *array[DIM_MAX] = {0};
for(int count = 0; count < d; count++)
{
array[count] = (int *)malloc(d*sizeof(int *));
}
/* Call changeArray function */
changeArray(d, array);
for(int i = 0; i < d; i++)
{
for(int j = 0; j < d; j++)
{
printf("%d ", array[i][j]);
}
printf("\n");
}
for(int count = 0; count < d; count++)
{
free(array[count]);
}
return 0;
}
void changeArray(int n, int *array[d])
{
for(int i =0; i < n; i++)
{
for(int j = 0; j < n; j++)
{
array[i][j] = i*j;
}
}
return;
}
What is the difference in using any of the two ways above to write this code?
[Not an answer, but an alternative approach to achieve the desired result, namely defining a user-defined 2D array.]
Assuming the compiler in use supports VLAs you could do this:
#include <stddef.h> /* for size_t */
void init_a(size_t x, size_t y, int a[x][y]); /* Order matters here!
1st give the dimensions, then the array. */
{
for (size_t i = 0; i < x; ++i)
{
for (size_t j = 0; j < y; ++j)
{
a[i][j] = (int) (i * j); /* or whatever values you need ... */
}
}
}
int main(void)
{
size_t x, y;
/* Read in x and y from where ever ... */
{
int a[x][y]; /* Define array of user specified size. */
init_a(x, y, a); /* "Initialise" the array's elements. */
...
}
}
It is actually pretty simple. All you have to do is this:
int i[][];
You are overthinking it. Same as a normal array, but has two indexes.
Let's say you want to create a "table" of 4 x 4. You will need to malloc space for 4 pointers, first. Each of those index points will contain a pointer which references the location in memory where your [sub] array begins (in this case, let's say the first pointer points to the location in memory where your first of four arrays is). Now this array needs to be malloc for 4 "spaces" (in this case, let's assume of type INT). (so array[0] = the first array) If you wanted to set the values 1, 2, 3, 4 within that array, you'd be specifying array[0][0], array[0][1], array[0][2], array[0][3]. This would then be repeated for the other 3 arrays that create this table.
Hope this helps!

How to allocate a 2D array of pointers to a struct [duplicate]

This question already has answers here:
How do I correctly set up, access, and free a multidimensional array in C?
(5 answers)
Closed 6 years ago.
This is were I got so far,but I don't know if it's right.
This function receives the dimensions of the 2D array (nxn),and allocates it.
flightInfo is the name of the struct.
Will this work?
thanks in advanced
after allocating the array(ignore the method ,since we are not allowed to use the method you proposed) I would like to initialize the struct (I built a function to do it but it didn't work),I tried to do it right after the allocation and kept getting the" Unhandled exception" warning, does it has to do
with the syntax, am I forgetting a '*'?
void flightMatrix()
{
FILE * fpf;
int checkScan,Origin,Dest;
float time,cost;
char flightName[3];
flightInfo *** matrix;
if(!(fpf=fopen("flights.txt","r")))exit(1);
while((checkScan=fscanf(fpf,"%*10c%3d%3d%3c%5f%7f%*",&Origin,&Dest,flightName,&time,&cost))!=EOF)
{
matrix=allocateMatrix(Dest);
matrix[Origin-1][Dest-1]->o=Origin;
}
}
flightInfo*** allocateMatrix(int n)
{ int i,j;
flightInfo*** matrix;
matrix=(flightInfo***)malloc(sizeof(flightInfo **)*n);
for(i=0;i<n;i++)
matrix[i]=(flightInfo **)malloc(sizeof(flightInfo*)*n);
for (int i = 0; i < n; ++i)
{
for (int j = 0; j < n; ++j)
matrix[i][j] = NULL;
}
return matrix;
}
[http://i.stack.imgur.com/MFC7V.png]
this is what happens when I try to initialize
Technically speaking, this won't create 2D array. The result will be array of pointers, where each one points to different array of pointers to a struct.
The difference is that, memory will be fragmented, so every element will point to some memory location, instead of single continuous memory block.
The common approach for this is to create flatten 2D array:
flightInfo** allocateMatrix(int n)
{
flightInfo** matrix = malloc(n*n * sizeof(*matrix));
for (int i = 0; i < n; i++)
for (int j = 0; j < n; j++)
matrix[i*n + j] = NULL;
return matrix;
}
If you are forced to use two indices, then you could place matrix as function argument:
void allocateMatrix(int n, flightInfo* (**matrix)[n])
{
*matrix = malloc(n * sizeof(**matrix));
for (int i = 0; i < n; ++i)
for (int j = 0; j < n; ++j)
(*matrix)[i][j] = NULL;
}
The second asterisk is required, because pointers are passed by value, otherwise you would end up with modified local copy of the pointer, that does nothing to matrix from main function.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct flightInfo {
char airport[30];
int altitude;
} flightInfo;
void allocateMatrix(int n, flightInfo* (**matrix)[n])
{
*matrix = malloc(n * sizeof(**matrix));
for (int i = 0; i < n; ++i)
for (int j = 0; j < n; ++j)
(*matrix)[i][j] = NULL;
}
int main()
{
int n = 10;
flightInfo* (*matrix)[n];
allocateMatrix(n, &matrix);
matrix[0][0] = malloc(sizeof(flightInfo));
strcpy(matrix[0][0]->airport, "Heathrow");
matrix[0][0]->altitude = 10000;
printf("%s, %d\n", matrix[0][0]->airport, matrix[0][0]->altitude);
}
The another way would be to encapsulate the array within a struct.

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