C - Why this function prints the array as NULL Strings?

Question

I'm having a problem dealing with an array in C. You see, this is a portion of my code which basically reads a file and organizes coordinates of the vertices of parcels of the 3rd-levels administrive divisions in Portugal - which we call Freguesias. In this part of the exercise, I need to write the name of all 2rd-levels administrive divisions - Concelhos (which is already well defined in my code in the array Cartography cartography, that isn't the problem) that appear in the file.

I want to do a function that shows what Concelhos appear in the file and I want to write with this exact subfunctions and functions so I can change some things later, but for some reason it doesn't printf the strings in "command_list_concelhos", it just prints NULL strings. I don't know why this happens, specially since it does rightly so if I do a printf inside and outside the for in "read_string_concelhos".

Sorry if this question is wrongly explained, too big or just a small detail that I am missing, but I don't have a better way to explain it...

#define MAX_STRING 256
#define MAX_NAMES  50

typedef char String[MAX_STRING];

typedef struct {
    String list[MAX_NAMES];
    int n_strings;
}   StringList;

int read_string_concelhos(StringList s ,Cartography cartography, int n)
{
    int i, j=1;
    strcpy (s.list[j-1], cartography[0].identification.concelho);
    for ( i = 0 ; i < n ; i++){
        if ( strcmp(cartography[i].identification.concelho, s.list[j-1]) != 0){
            strcpy(s.list[j] , cartography[i].identification.concelho);
            j++;
        }
    }
    return j; // n_strings
}

void command_list_concelhos(Cartography cartography, int n)
{
    StringList s;
    s.n_strings = read_string_concelhos(s, cartography, n);
    int i;
    for(i = 0; i < s.n_strings; i++ )
    {
        printf("\n", s.list[i]);
    }
}

Fail

How it should look like

When the code passes s to read_string_concelhos, s is a copy of the structure. So any changes made to s only affect the copy. The s in command_list_concelhos is not changed when read_string_concelhos changes its copy of s. — user3386109
– user3386109, Commented Dec 9, 2018 at 2:17
@user3386109 I do understand what's the problem, but can you tell me how can I fix this please? I suppose I'm missing some detail. — Diogo MDD
– Diogo MDD, Commented Dec 9, 2018 at 22:10

Ze Chen · Accepted Answer · 2018-12-10 15:04:54Z

1

int read_string_concelhos(StringList s ,Cartography cartography, int n)
should be changed to
int read_string_concelhos(StringList* s ,Cartography cartography, int n)

And inside the function int read_string_concelhos(StringList* s ,Cartography cartography, int n) { ... }, all s.list[...] should be changed to s->list[...]. In this way, parameter s is a pointer therefore strcmp would paste to s declared in command_list_concelhos which is the desired behaviour.

answered Dec 10, 2018 at 15:04

Ze Chen

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