How to access a structure element dynamically in C?

Question

I have two structures and an array of const characters:

typedef struct {
    int skip_lines;
    int num; // count of files
    int i; // number of the file to define order; extremly important to set correctly and then not to change!
    char filename[70];
    char main_directory[16];
    char submain_directory[100];
} FILE_;

typedef struct {
    FILE_ radiation_insolation[7];
    FILE_ radiation_radiation[5];
    FILE_ winds[9];
    FILE_ pressure[1];
    FILE_ humidity[1];
    FILE_ temperature[4];
} FILES;

char *tables[] = {"radiation_insolation", "radiation_radiation", "winds", "pressure", "humidity", "temperature" };

I also have FILES files;in the main function and initiates a function that loads data from a file. Therefore, each member of the files contains data.

Then I need to access the following data:

files->radiation_insolation[0].skip_lines
files->radiation_radiation[0].skip_lines
files->radiation_winds[0].skip_lines
files->pressure[0].skip_lines
files->humidity[0].skip_lines
files->temperature[0].skip_lines

My plan is to create a cycle for the dynamic process of each member.

for(i = 0; i<6; i++) {
        // do some job
    }

My question is how to do this when I need to access, for example. files-> radiation_insolation using tables [i] in a loop? How to create a member name so that the compiler knows who to contact?

In PHP, you can use something like $ files → $ tables [i]. But how to do it in C?

Answer 1

: . , C, FILE_, , undefined. , , , , , .

, , :

enum {
   radiation_isolation = 0,
   radiation_radiation = 7,
   winds = 12,
   // etc
}

FILE_ files[total_files];

FILE_ *isolation_3 = &files[radiation_isolation + 3];

, , .

Answer 2

C. - , , .

:

// bad idea
#define FILES_ITEM(var, field, index, member) var.field[index].member

- , :

int main (void)
{
  FILES files;

  for(size_t i=0; i<7; i++)
  {
    files.radiation_insolation[i].skip_lines = i;
    printf("%d ", files.radiation_insolation[i].skip_lines);
  }
}

, , .

---

C11 , , :

#define FILE_ITEMS_N (7 + 5 + 9 + 1 + 1 + 4)

typedef union {

  struct
  {
    FILE_ radiation_insolation[7];
    FILE_ radiation_radiation[5];
    FILE_ winds[9];
    FILE_ pressure[1];
    FILE_ humidity[1];
    FILE_ temperature[4];
  };

  FILE_ items [FILE_ITEMS_N];

} FILES;

:

files.radiation_insolation[0].skip_lines = 123;

:

files.items[item].skip_lines = 123;

---

C11 §6.7.2.1:

14 , .

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17 .

, , .

, , C11 6.5/7:

, lvalue, :

/-/

- , ( , , )

Answer 3

union of FILES FILE_ 27 :

typedef union
{
    FILES f;
    FILE_ all[27];
}
FILES_U;

files->f.radiation_radiation[0] , files->all[7]. tables all (0,7,12...) .

Answer 4

(ab) x-. . , , .

.. , FILE_ENTRY undefined:

#define X_FILE_LIST(X_FILE_ENTRY) \
    X_FILE_ENTRY(radiation_insolation, 7) \
    X_FILE_ENTRY(radiation_radiation, 5) \
    X_FILE_ENTRY(winds, 9) \
    X_FILE_ENTRY(pressure, 1) \
    X_FILE_ENTRY(humidity, 1) \
    X_FILE_ENTRY(temperature, 4)

FILE_ENTRY(name, len), :

// number of entries
#define X_EXPAND_AS_COUNT(name, len) 1 + 
const int FILES_count = X_FILE_LIST(X_EXPAND_AS_COUNT) 0;

// struct definition
#define X_EXPAND_AS_FIELD(name, len) FILE_ name[len];
typedef struct {
    X_FILE_LIST(X_EXPAND_AS_FIELD)
}
FILES;

// byte offsets of each field
#define X_EXPAND_AS_BYTEOFFSET(name, len) offsetof(FILES, name),
int FILES_byte_offsets[] = {
    X_FILE_LIST(X_EXPAND_AS_BYTEOFFSET)
};

// FILE_ offsets of each field
#define X_EXPAND_AS_FILEOFFSET(name, len) offsetof(FILES, name)/sizeof(FILE_),
int FILES_offsets[] = {
    X_FILE_LIST(X_EXPAND_AS_FILEOFFSET)
};

// sizes of each array
#define X_EXPAND_AS_LEN(name, len) len,
int FILES_sizes[] = {
    X_FILE_LIST(X_EXPAND_AS_LEN)
};

// names of each field
#define X_EXPAND_AS_NAME(name, len) #name,
const char * FILES_names[] = {
    X_FILE_LIST(X_EXPAND_AS_NAME)
};

:

const int FILES_count = 1 + 1 + 1 + 1 + 1 + 1 + 0;

typedef struct {
    FILE_ radiation_insolation[7];
    FILE_ radiation_radiation[5]; 
    FILE_ winds[9]; 
    FILE_ pressure[1]; 
    FILE_ humidity[1]; 
    FILE_ temperature[4];
}
FILES;

int FILES_byte_offsets[] = {
    ((size_t)&(((FILES*)0)->radiation_insolation)),
    ((size_t)&(((FILES*)0)->radiation_radiation)),
    ((size_t)&(((FILES*)0)->winds)),
    ((size_t)&(((FILES*)0)->pressure)),
    ((size_t)&(((FILES*)0)->humidity)),
    ((size_t)&(((FILES*)0)->temperature)),
};

int FILES_offsets[] = {
    ((size_t)&(((FILES*)0)->radiation_insolation))/sizeof(FILE_), 
    ((size_t)&(((FILES*)0)->radiation_radiation))/sizeof(FILE_),
    ((size_t)&(((FILES*)0)->winds))/sizeof(FILE_), 
    ((size_t)&(((FILES*)0)->pressure))/sizeof(FILE_),
    ((size_t)&(((FILES*)0)->humidity))/sizeof(FILE_),
    ((size_t)&(((FILES*)0)->temperature))/sizeof(FILE_),
};

int FILES_sizes[] = { 7, 5, 9, 1, 1, 4, };

const char * FILES_names[] = {
    "radiation_insolation", "radiation_radiation",
    "winds", "pressure", "humidity", "temperature",
};

, - :

for (int i = 0; i < FILES_count; i++)
{
    FILE_ * first_entry = (FILE_ *)&files + FILES_offsets[i];
    for (int j = 0; j < FILES_sizes[i]; j++)
    {
        FILE_ * file = first_entry + j;
        printf("%s[%d].skip_lines = %d \n",
            FILES_names[i],
            j,
            file->skip_lines);
    }
}

FILES :

// output of the program above
radiation_insolation[0].skip_lines = 0
radiation_insolation[1].skip_lines = 0
radiation_insolation[2].skip_lines = 0
radiation_insolation[3].skip_lines = 0
radiation_insolation[4].skip_lines = 0
radiation_insolation[5].skip_lines = 0
radiation_insolation[6].skip_lines = 0
radiation_radiation[0].skip_lines = 0
radiation_radiation[1].skip_lines = 0
radiation_radiation[2].skip_lines = 0
radiation_radiation[3].skip_lines = 0
radiation_radiation[4].skip_lines = 0
winds[0].skip_lines = 0
winds[1].skip_lines = 0
winds[2].skip_lines = 0
winds[3].skip_lines = 0
winds[4].skip_lines = 0
winds[5].skip_lines = 0
winds[6].skip_lines = 0
winds[7].skip_lines = 0
winds[8].skip_lines = 0
pressure[0].skip_lines = 0
humidity[0].skip_lines = 0
temperature[0].skip_lines = 0
temperature[1].skip_lines = 0
temperature[2].skip_lines = 0
temperature[3].skip_lines = 0

"", :

FILE_ * get_entry_by_name_and_index(FILES * files, const char * name, int idx)
{
    // NOTE: no bounds checking/safe string function, etc

    for (int i = 0; i < FILES_count; i++)
    {
        if (strcmp(FILES_names[i], name) == 0)
        {
            int base_offset = FILES_offsets[i];
            return (FILE_ *)files + base_offset + idx;
        }
    }

    return NULL;
}

, files.winds[4]:

FILE_ * item = get_entry_by_name_and_index(&files, "winds", 4);
assert((void*)item == (void*)&files.winds[4]);

Answer 5

, , , . -, :

struct foo {
    int a;
    int b;
    int c;
};

, , , . , , :

struct foo myfoo = { 100, 200, 300 };
int *p = &myfoo.a;

int sum = 0;
sum += p[0];   /* = *(p+0) = address of a + 0 int = a */
sum += p[1];   /* = *(p+1) = address of a + 1 int = b */ 
sum += p[2];   /* = *(p+2) = address of a + 2 int = c */

, .

, , .