在C中返回2d数组...

Question

I'm a total noob in C. I can't make the connect between this function and main. I'm trying to print out a 2d array and I keep getting segmentation fault. Any help would be greatly appreciated.

EDIT: When I changed the last line 'printf("%d:[%s]\\n",i,*(p+i))' from %s to %c, I get the first word in the file i'm reading from. So turns out that something is in fact being returned from my function. Now just need to figure out how to get it to return words from other lines in the file.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define num_strings 20
#define size_strings 20

int *read_file(){
    int j = 0;
    static char text[num_strings][size_strings];

    FILE *fp;
    int x;

    fp = fopen("dictionary2.txt", "r");

    char s[100];
    while(!feof(fp)) {
        x = fscanf(fp,"%[^\n]",s);
        fgetc(fp);

        if (x==1) {
            strcpy(text[j],s);
            j++;
        }
    }
    return text;
}

int main() {
    int *p;
    p = read_file();
    int i;
    for(i = 0; i < 10; i++) {
        printf("%d:[%s]\n",i,*(p+i));
    }
    return(0);
}

Answer 1

In general, you should be creating your array in main() and passing it in, this kind of behavior is very unorthodox. However, if you do insist on doing it this way, you have to return a pointer to your array, since you cannot return arrays in C.

This is the kind of thing you'll need:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define num_strings 20
#define size_strings 20

typedef char (*PARR)[num_strings][size_strings];

PARR read_file(int * wordsread)
{
    static char text[num_strings][size_strings];
    FILE *fp;

    if ( (fp = fopen("dictionary2.txt", "r")) == NULL ) {
        fprintf(stderr, "Couldn't open file for reading\n");
        exit(EXIT_FAILURE);
    }

    char s[100];
    int j = 0;

    while ( j < num_strings && fgets(s, sizeof s, fp) ) {
        const size_t sl = strlen(s);
        if ( s[sl - 1] == '\n' ) {
            s[sl - 1] = 0;
        }

        if ( (strlen(s) + 1) > size_strings ) {
            fprintf(stderr, "String [%s] too long!\n", s);
            exit(EXIT_FAILURE);
        }

        strcpy(text[j++], s);
    }

    fclose(fp);
    *wordsread = j;
    return &text;
}

int main(void)
{
    int wordsread = 0;
    PARR p = read_file(&wordsread);

    for ( int i = 0; i < wordsread; ++i ) {
        printf("%d:[%s]\n", i, (*p)[i]);
    }

    return 0;
}

which, with a suitable input file, outputs:

paul@horus:~/src/sandbox$ ./twoarr
0:[these]
1:[are]
2:[some]
3:[words]
4:[and]
5:[here]
6:[are]
7:[some]
8:[more]
9:[the]
10:[total]
11:[number]
12:[of]
13:[words]
14:[in]
15:[this]
16:[file]
17:[is]
18:[twenty]
19:[s'right]
paul@horus:~/src/sandbox$ 

Note this only works because you declared your array in read_file() as static - don't return pointers to local variables with automatic storage duration in this way.

Answer 2

Try moving your #define s back and changing your function header to return a pointer to arrays of size_strings characters, as follows:

    #define num_strings 20
    #define size_strings 20

    char (*read_file())[size_strings] {

Or alternately, with a typedef:

    #define num_strings 20
    #define size_strings 20

    typedef char (*PCharArr)[size_strings];

    PCharArr read_file() {

...and change the type of p in main accordingly:

    char (*p)[size_strings];

That will return (a pointer to the first element of) an array of character arrays, which is more or less equivalent to a 2D array of char .

Answer 3

Update, oh I see, you pasted the code from main to the function, I know what happened here, you assumed p[20][20] is the same as ap* or maybe ap**, that's not correct, since now if you do *(p+1), the compiler doesn't know each element in p is 20 wide instead of 1 wide. You approach here should be to declare a pointer to an array of strings in read_file and return that instead:

static char text[num_strings][size_strings];
static char *texts[num_strings]

...
while....
    ....
       if (x==1)
        {strcpy(text[j],s);texts[j]=text[j];j++;}


return texts;

your p should be char* not int*. You also need to terminate the loop if 20 items have been read in.