C：多个文件的结构列表和fscanf

Question

As a project, I have to read multiple tabular seperated files (containing x and y coordinates starting from 0,0 at left top) and save the content within a dynamically allocated linked list. With a single read, everything is fine, but when I read and try to append the elements of the second file into the list, the items are not appended somehow.

Could someone help me find where the error is and how to fix it? Am I kind of doing things wrong with the pointers on the list maybe?

The structs:

typedef enum{
    BLUME=-1, FREI
}belegung;

typedef struct position{
    int zeile;
    int spalte;
    belegung element;
    struct position* next;
}position;

typedef struct feld{
    int zeilen;
    int spalten;
    int anzBlumen;
    position* positionen;
}feld;

The function to read and add the elements:

int read(feld* f, char* file){
    FILE* in;
    if((in = fopen(file, "r")) == NULL)
        return -1;


    int count = 0;
    if (f->zeilen == 0 && f->spalten == 0 && f->positionen == NULL)
        count = firstread(in, f);

    else {
        printf("already have flowers\n");
        int a, b;
        int oldS = f->spalten;

        position* ptr = f->positionen; // CAUTION: p = NULL
        while (ptr != NULL) {
            ptr = ptr->next;
            if (ptr != NULL)
                printf("readA: ptr @ %p -> (%d, %d) = %d\n", ptr, ptr->zeile,
                    ptr->spalte, ptr->element);
        }

        // read first line
        if (fscanf(in, "%d\t%d", &a, &b) != EOF) {
            f->zeilen = max(f->zeilen, a);
            f->spalten = oldS + b;
            count++;
        }
        // read first flower


        if (fscanf(in, "%d\t%d", &a, &b) != EOF) {
            ptr = calloc(1, sizeof(position));
            ptr->zeile = a;
            ptr->spalte = b + oldS;
            ptr->element = -1;
            ptr->next = NULL;
            count++;
            printf("Flower: (%d, %d)->(%d, %d) at %p\n", a, b, ptr->zeile,
                    ptr->spalte, ptr);
        }
        // read flower lines
        while (fscanf(in, "%d\t%d", &a, &b) != EOF) {
            ptr->next = calloc(1, sizeof(position));
            ptr = ptr->next;
            ptr->zeile = a;
            ptr->spalte = b + oldS;
            ptr->element = -1;
            ptr->next = NULL;
            printf("Flower: (%d, %d)->(%d, %d) at %p\n", a, b, ptr->zeile,
                    ptr->spalte, ptr);
            count++;
        }

        f->anzBlumen += count - 1;
    }


    printf("Flowerscan done\n");
    // get 'next' to write element in ptr
    preinitFreefeld(f);

    //setFreefeld(f);
    printf("setting up free field done\n");
    fclose(in);
    return count;
}

int firstread(FILE* in, feld* f) {
    int a, b;
    position* p = NULL;

    int count = 0;
    // read first line
    if (fscanf(in, "%d\t%d", &a, &b) != EOF) {
        f->zeilen = a;
        f->spalten = b;
        count++;
    }
    // read first flower
    p = calloc(1, sizeof(position));
    position* ptr = p;
     if (fscanf(in, "%d\t%d", &a, &b) != EOF) {
        ptr->zeile = a;
        ptr->spalte = b;
        ptr->element = BLUME;
        ptr->next = NULL;
        count++;
    }
    f->positionen = p;
    // read flower lines
    while (fscanf(in, "%d\t%d", &a, &b) != EOF) {
        if (ptr->next == NULL)
            ptr->next = calloc(1, sizeof(position));
        ptr = ptr->next;
        ptr->zeile = a;
        ptr->spalte = b;
        ptr->element = BLUME;
        ptr->next = NULL;
        count++;
    }

    f->positionen = p;
    f->anzBlumen = count - 1;
    return count;
}

Example input file:

3   5
2   2
2   3
0   0
0   1
2   4
1   4
0   2

In that input, the first line specifies the amount of lines and rows to use on screen, the next lines where the elements are positioned with the first number in line being the line or x in (x, y).

Example Output:

-1  -1  -1  0   0
0   0   0   0   -1
0   0   -1  -1  -1

Answer 1

when I read and try to append the elements of the second file into the list, the items are not appended somehow.

Look at this code:

    position* ptr = f->positionen; // CAUTION: p = NULL
    while (ptr != NULL) {
        ptr = ptr->next;
        if (ptr != NULL)
            printf("readA: ptr @ %p -> (%d, %d) = %d\n", ptr, ptr->zeile,
                ptr->spalte, ptr->element);
    }

Here you continue until ptr is NULL. In other words ptr no longer points to an element in the list. You probably wanted:

    while (ptr->next != NULL) {

and then you want to append new elements using

        ptr->next = calloc(1, sizeof(position)

But you have:

    if (fscanf(in, "%d\t%d", &a, &b) != EOF) {
        ptr = calloc(1, sizeof(position));
        ^^^^

which means that the new elements won't be appended to the list from readfirst but that you start a new linked list instead.

I haven't looked into all details of your code but I think you to change the code to something like:

if (f->positionen == NULL)    // Only check for pointer being NULL
    count = firstread(in, f);

else {
    printf("already have flowers\n");
    int a, b;
    int oldS = f->spalten;

    position* ptr = f->positionen;
    while (ptr->next != NULL) {    // Check for ptr->next
        ptr = ptr->next;
        // if (ptr != NULL)  Delete this line - not needed anymore
            printf("readA: ptr @ %p -> (%d, %d) = %d\n", ptr, ptr->zeile,
                ptr->spalte, ptr->element);
    }

    // read first line
    if (fscanf(in, "%d\t%d", &a, &b) != EOF) {
        f->zeilen = max(f->zeilen, a);
        f->spalten = oldS + b;
        count++;
    }

    // Delete this block - you already have a "first flower"
    // read first flower
    //if (fscanf(in, "%d\t%d", &a, &b) != EOF) {
    //    ptr = calloc(1, sizeof(position));
    //    ptr->zeile = a;
    //    ptr->spalte = b + oldS;
    //    ptr->element = -1;
    //    ptr->next = NULL;
    //    count++;
    //    printf("Flower: (%d, %d)->(%d, %d) at %p\n", a, b, ptr->zeile,
    //            ptr->spalte, ptr);
    //}

    // read flower lines
    while (fscanf(in, "%d\t%d", &a, &b) != EOF) {
        ptr->next = calloc(1, sizeof(position));
        ptr = ptr->next;
        ptr->zeile = a;
        ptr->spalte = b + oldS;
        ptr->element = -1;
        ptr->next = NULL;
        printf("Flower: (%d, %d)->(%d, %d) at %p\n", a, b, ptr->zeile,
                ptr->spalte, ptr);
        count++;
    }

    f->anzBlumen += count - 1;
}