How to make a Sentence out of 2D char array using pointers - c

Question

I am newbie at C so please try to be patient if I am not clear enough.
I have an assignment which I need to make a function that gets a squared char matrix and make a string out of it.
The function should return a pointer to a string eventually,
so that in main I can initialize a string with it and print it in the end.
(also i am restricted to use only pointers inside this function and not regular arrays syntax)
for example the matrix is:
R O M E
G O A L
H E A D
D E A D
and i want to make a sentence like that: "ROME GOAL HEAD DEAD".
I tried to make a for loop which runs only on the matrix rows so I could take each row and copy to a string which I already prepared before (with enough space in it) in the first row I used strcpy() and the other strcat().
Nothing happened.
Sorry for my english, and thanks.
char * makeString(char *smallMat, int rows, char *pStr ) {
int i;
char sentence[(rows * rows) + rows + rows];
pStr = &sentence;
for (i = 0; i < rows; ++i) {
if (i == 0) {
strcpy(sentence, *(smallMat + i));
}
else{
strcat(sentence, ' ' + *(smallMat + i));
}
}
return pStr;
}

Answer 1

As #anonmess pointed out, you cannot use strcpy() etc. for non-0-terminated character sequences. As you said yourself, the task is to use pointers. If you used strcpy() (and if it worked), you would work around the assignment ;)
Here is a full solution that does not use pStr.
#include <stdio.h> // printf()
#include <stdlib.h> // malloc()
static char smallMat[4][4] = {
{ 'R', 'O', 'M', 'E' },
{ 'G', 'O', 'A', 'L' },
{ 'H', 'E', 'A', 'D' },
{ 'D', 'E', 'A', 'D' }
};
char* makeString(char* smallMat, int rows) {
int currentRow, currentColumn;
/* space for all characters (rows * rows)
* + one character for each space + terminating '\0'. */
char* sentence = malloc((rows * rows) + rows);
char* p = sentence;
for (currentRow = 0; currentRow < rows; ++currentRow) {
for (currentColumn = 0; currentColumn < rows; ++currentColumn) {
*p++ = *(smallMat + (currentRow * rows) + currentColumn);
}
*p++ = ' ';
}
*--p = '\0'; /* replace the last space with terminating 0,
* so it can be printed. */
return sentence;
}
int main() {
char* output = makeString(smallMat, 4);
printf(output);
free(output);
return 0;
}

Answer 2

The easiest way to go about creating a sentence from your 2D array of SIZE rows and columns of characters is simply to create an array of SIZE * SIZE + SIZE characters to hold the sentence (SIZE * SIZE for each character, plus SIZE - 1 spaces, plus the nul-terminating character)
After declaring an array to hold your sentence, pass both the new array and your 2D array to your makestring() function and loop over each character in the 2D array, adding a space before each word (except for the 1st word) and finally nul-terminate your new array creating a C-string.
You cannot use strcpy (or any other function expecting a C-string) on the row of characters in your 2D array as the rows of characters in your 2D array are not nul-terminated strings. (your 2D array is just that, a 2D array of characters). You must loop over and assign each character to a position within your 1D array.
Putting it altogether, you could write makestring() similar to:
#define SIZE 4
char *makestring (char *str, char (*a)[SIZE])
{
char *p = str; /* a pointer to 1st character in str */
for (int i = 0; i < SIZE; i++) { /* for each row */
if (i) /* if row not 1st */
*p++ = ' '; /* add space */
for (int j = 0; j < SIZE; j++) /* for each char */
*p++ = a[i][j]; /* copy to str */
}
*p = 0; /* nul-terminate string */
return str; /* return pointer to string (for convenient use in-line) */
}
(note: returning a pointer to the beginning of str would allow the function to be used in-line such as: printf ("str: '%s'\n", makestring (str, matrix)); As mentioned, this is just for convenience, you can make the function of type void as you are not allocating and have provided a pointer to an array that will be updated within the function and can be tested back in the caller before use.)
Adding a short test program, you could test your function as follows:
#include <stdio.h>
#define SIZE 4
char *makestring (char *str, char (*a)[SIZE])
{
char *p = str; /* a pointer to 1st character in str */
for (int i = 0; i < SIZE; i++) { /* for each row */
if (i) /* if row not 1st */
*p++ = ' '; /* add space */
for (int j = 0; j < SIZE; j++) /* for each char */
*p++ = a[i][j]; /* copy to str */
}
*p = 0; /* nul-terminate string */
return str; /* return pointer to string (for convenient use in-line) */
}
int main (void) {
char matrix[SIZE][SIZE] = { {'R','O','M','E'}, /* square 2D array */
{'G','O','A','L'},
{'H','E','A','D'},
{'D','E','A','D'} },
str[SIZE * SIZE + SIZE] = ""; /* array to hold sentence */
makestring (str, matrix); /* call makestring */
if (*str != 0) /* validate str not empty-string */
printf ("str: '%s'\n", str); /* output string */
}
Example Use/Output
Running the program would produce:
$ ./bin/makestring
str: 'ROME GOAL HEAD DEAD'
(note: single-quotes added around the string to affirmatively show the beginning and end of the string)
Look things over and let me know if you have further questions.