Array assignment in C - c

Question

If I have defined this:
typedef char word[30];
typedef struct {
int num;
word group[30];
} paragraph;
And then in the body:
int
main(int argc, char **argv) {
paragraph p;
word w = "BLA";
return 0;
}
How do I assign w in the array p and then printf w in terms of p
I know this is wrong but e.g.:
p[][1] = w;
printf("%s", p[][1]);
Pleas help a noob out

Answer 1

Arrays are not assignable. Either initialize your array to the proper size and set each element as needed or store a pointer instead.

Answer 2

Assuming you really intend to use the flexible array member introduced in C99, you would need to write code like:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef char word[30];
typedef struct {
int num;
word group[];
} paragraph;
int main(void)
{
paragraph *p = malloc(sizeof(*p) + 1 * sizeof(word));
word w = "BLA";
strcpy(p->group[0], w);
printf("Word: %s\n", p->group[0]);
free(p);
return 0;
}
That's modestly tricky code. Avoid using flexible array members until you understand what all this is doing. Until then, make the group member of the paragraph into a word:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef char word[30];
typedef struct {
int num;
word group;
} paragraph;
int main(void)
{
paragraph p;
word w = "BLA";
strcpy(p.group, w);
printf("Word: %s\n", p.group);
return 0;
}
The key point remains that you cannot directly assign arrays and need to use strcpy() or a relative with strings, and memmove() or perhaps memcpy() with other types. You can assign structures that contain arrays in general (though you can't do that with structures containing a flexible array member), but that's because you can do structure assignment and it is coincidental that the structure contains an array.
BUT for word group, if you put group[30], how does that affect the rest of the code?
Here's code that does as you ask:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef char word[30];
typedef struct
{
int num;
word group[30];
} paragraph;
int main(void)
{
paragraph p;
word w = "BLA";
strcpy(p.group[0], w);
printf("Word: %s\n", p.group[0]);
return 0;
}
Under this structure, assuming sizeof(int) == 4, you would find that sizeof(paragraph) == 904 because you've declared an array of 30 words, and each word is an array of 30 characters. This structure would allow you to add multiple words to the paragraph (and most paragraphs that I write most certainly contain more than one word!). You still have to use strcpy() to copy the word from w to an element of p.group:
int main(void)
{
paragraph p;
word w = "BLA";
strcpy(p.group[0], w);
strcpy(p.group[1], "Porcupine");
strcpy(p.group[2], "Elephant");
p.num = 3;
printf("Word: %s\n", p.group[0]);
printf("Paragraph:");
for (int i = 0; i < p.num; i++)
printf(" %s", p.group[i]);
putchar('\n');
return 0;
}
While it isn't wrong to use a typedef like word, it isn't something I'd normally do. It can all too easily lead to more confusion than enlightenment. It could also lead to wasted memory if most of your words are significantly fewer than 29 characters long. (The onus is on you to ensure that you do not store words longer than 29 characters (plus the terminating null byte) in the p.group array elements.) OTOH, the overhead of memory allocation with 64-bit software could easily render the wasted space for a single word moot.