Allocating memory dynamically to a 2-d array of strings

Question

int ReadNames(char ***Names, int *r,int *c)    
{
int i, j, k;
char name[100];
printf("Number of Rows: ");
scanf("%d", r);

printf("Number of Columns: ");
scanf("%d", c);

Names=(char ***)malloc(sizeof(char **)*(*r));
for(i=0;i<(*r);i++)
*(Names+i)=(char **)malloc(sizeof(char *)*(*c));

for(i=0;i<(*r);i++)
for(j=0;j<(*c);j++)
{
fflush(stdin);
gets(name);
strcpy(*(*(Names+i)+j),name);
}
return 1;
}

I am trying to allocate the memory to a 2-D array of strings. Later on i want to sort them row wise and column wise, but while allocating the memory , the program is not responding. Is there something i am doing in my code.

in main function readname is called as

    ReadNames(&p,&r,&c)

where r and c are the no. of rows and columns.

Answer 1

You need:

 *Names = (char **)malloc(sizeof(char **) * (*r)); 

and consequential changes.

You're passing a triple-pointer in order to be able to return a double-pointer. What you're doing is losing the information about where to store the double-pointer.

There is some truth to the struck out comment; there is also a miscue. A 2D array of strings means that you have three levels of pointer in the basic data. And you need a fourth level of pointer to pass into the function.

Also, using gets() is a recipe for disaster. Don't ever (as in never , as in never ever ) use the the gets() function. Not even in toy programs. It gets you into bad habits. The first Internet worm propagated through a program that used gets() (Google search 'morris internet worm').

On Unix and other POSIX-based systems, using fflush(stdin) leads to undefined behaviour. On Windows, the behaviour is defined by Microsoft. If you're running on Windows, then you're OK; if not, you're not.

---

And I thought Three-Star Programming was bad!

This probably isn't the way I'd do it, but it is a pretty direct translation of what you wrote into something that works, along with a main() program that tests it and frees all the allocated memory. It assumes strdup() is available; if not, it is trivial to write it.

Sample output:

 Number of Rows: 2 Number of Columns: 3 R0C0: Row 1, Column 1. R0C1: Ambidextrous Armless Individual. R0C2: Data for the third column of the first row. R1C0: Row 2, Column 1. R1C1: Row 2, Column 2. R1C2: Given that the number of rows is 2 and the number of columns is 3, this should be the last input! Rows = 2, cols = 3. [0,0] = <<Row 1, Column 1.>> [0,1] = <<Ambidextrous Armless Individual.>> [0,2] = <<Data for the third column of the first row.>> [1,0] = <<Row 2, Column 1.>> [1,1] = <<Row 2, Column 2.>> [1,2] = <<Given that the number of rows is 2 and the number of columns is 3, this should be the last input!>> 

Working 4-star code:

 #include <stdio.h> #include <string.h> #include <stdlib.h> static void ReadNames(char ****Names, int *rows, int *cols) { char name[100]; printf("Number of Rows: "); scanf("%d", rows); printf("Number of Columns: "); scanf("%d", cols); int c; while ((c = getchar()) != EOF && c != '\\n') ; *Names = (char ***)malloc(sizeof(char ***)*(*rows)); for (int i = 0; i < (*rows); i++) (*Names)[i] = (char **)malloc(sizeof(char **)*(*cols)); for (int i = 0; i < (*rows); i++) { for (int j = 0; j < (*cols); j++) { printf("R%dC%d: ", i, j); if (fgets(name, sizeof(name), stdin) == 0) { fprintf(stderr, "Unexpected EOF\\n"); exit(1); } name[strlen(name)-1] = '\\0'; // Zap newline (*Names)[i][j] = strdup(name); } } } int main(void) { int rows; int cols; char ***data = 0; ReadNames(&data, &rows, &cols); printf("Rows = %d, cols = %d.\\n", rows, cols); for (int i = 0; i < rows; i++) { for (int j = 0; j < cols; j++) printf("[%d,%d] = <<%s>>\\n", i, j, data[i][j]); } for (int i = 0; i < rows; i++) { for (int j = 0; j < cols; j++) free(data[i][j]); free(data[i]); } free(data); return 0; } 

Alternative 3-star code

Using three levels of pointer is bad enough; four is horrid. This code restricts itself to three levels of pointer. I assume C99 compatibility, so variables can be declared when convenient in a function. The changes to work with C89/C90 compilers (which are 14 years retrograde now) are simple enough.

 #include <stdio.h> #include <stdlib.h> #include <string.h> static char ***ReadNames(int *r, int *c) { int i, j; char name[100]; printf("Number of Rows: "); scanf("%d", r); printf("Number of Columns: "); scanf("%d", c); int x; while ((x = getchar()) != EOF && x != '\\n') ; char ***Names = (char ***)malloc(sizeof(char ***)*(*r)); for (i = 0; i < (*r); i++) Names[i] = (char **)malloc(sizeof(char **)*(*c)); for (i = 0; i < (*r); i++) { for (j = 0; j < (*c); j++) { if (fgets(name, sizeof(name), stdin) == 0) { fprintf(stderr, "Unexpected EOF\\n"); exit(1); } name[strlen(name)-1] = '\\0'; Names[i][j] = strdup(name); } } return Names; } static void PrintNames(char ***Names, int r, int c) { int i, j; for (i = 0; i < r; i++) { for (j = 0; j < c; j++) printf("%s ", Names[i][j]); printf("\\n"); } } int main(void) { int rows; int cols; char ***data = ReadNames(&rows, &cols); PrintNames(data, rows, cols); printf("Rows = %d, cols = %d.\\n", rows, cols); for (int i = 0; i < rows; i++) { for (int j = 0; j < cols; j++) printf("[%d,%d] = <<%s>>\\n", i, j, data[i][j]); } for (int i = 0; i < rows; i++) { for (int j = 0; j < cols; j++) free(data[i][j]); free(data[i]); } free(data); return 0; } 

Example output

 Number of Rows: 3 Number of Columns: 4 R1C1 R1C2 R1C3 R1C4-EOR R2C1 R2C2 R2C3 R2C4-EOR R3C1 R3C2 R3C3 R3C4-EOR R1C1 R1C2 R1C3 R1C4-EOR R2C1 R2C2 R2C3 R2C4-EOR R3C1 R3C2 R3C3 R3C4-EOR Rows = 3, cols = 4. [0,0] = <<R1C1>> [0,1] = <<R1C2>> [0,2] = <<R1C3>> [0,3] = <<R1C4-EOR>> [1,0] = <<R2C1>> [1,1] = <<R2C2>> [1,2] = <<R2C3>> [1,3] = <<R2C4-EOR>> [2,0] = <<R3C1>> [2,1] = <<R3C2>> [2,2] = <<R3C3>> [2,3] = <<R3C4-EOR>> 

Both programs run clean under valgrind .

Answer 2

So, this is the code working...

    *Names=(char **)malloc(sizeof(char **)*(*r));    
for(i=0;i<(*r);i++)    
    *(*Names+i)=(char*)malloc(sizeof(char *)*(*c));    
for(i=0;i<(*r);i++)
for(j=0;j<(*c);j++)
{
fflush(stdin);
gets(name);
strcpy((*(*Names+i)+j),name);
}

but while printing those names stored...i have a function provided..

int PrintNames(char **Names, int r, int c)
{

int i,j;
for(i=0;i<r;i++)
{   printf("\n");
    for(j=0;j<c;j++)
        printf("%s ",*(*(Names+i)+j));
}
return 1;

}

Now this PrintNames is also called through main...as "PrintNames(p, r, c);"....but program stops while printing...What could be wrong?