Question

How to declare, initialize, and print a complex data structure in C

Answer 1

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

#define MAX_STR 128
#define MAX_HOBBIES 16
#define MAX_METADATA 16
#define MAX_SCORES 16

// Simple date struct (replacement for chrono)
typedef struct {
    int year;
    int month;
    int day;
} Date;

// Address struct
typedef struct {
    char street[MAX_STR];
    char city[MAX_STR];
    int zip;
} Address;

// Variant replacement (manual tagged union)
typedef enum {
    ID_INT,
    ID_STRING,
    ID_INT_ARRAY
} IdType;

typedef struct {
    IdType type;
    union {
        int intValue;
        char strValue[MAX_STR];
        int intArray[8];
    };
} Variant;

// Metadata (replacement for std::map)
typedef struct {
    char key[MAX_STR];
    char value[MAX_STR];
} KeyValue;

// Flags (bitfields)
typedef struct {
    unsigned isAdmin   : 1;
    unsigned isGuest   : 1;
    unsigned isVerified: 1;
} Flags;

// Function pointer type
typedef void (*PrintFn)(void*);

// Static members (global variables)
int population = 0;
int allocatedInts = 0;

// Person struct
typedef struct {
    Address homeAddress;
    int favoriteNumbers[5];
    char* nickname;
    Date birthDate;
    char notes[MAX_STR];
    char* middleName;
    char hobbies[MAX_HOBBIES][MAX_STR];
    int hobbyCount;

    enum Status { Active, Inactive, Unknown } status;

    Variant id;

    KeyValue metadata[MAX_METADATA];
    int metadataCount;

    Flags flags;

    int scores[MAX_SCORES];
    int scoreCount;

    PrintFn extraPrinter;
} Person;

// Set metadata
void setMeta(Person* p, const char* key, const char* value) {
    if (p->metadataCount < MAX_METADATA) {
        strcpy(p->metadata[p->metadataCount].key, key);
        strcpy(p->metadata[p->metadataCount].value, value);
        p->metadataCount++;
    }
}

// Print person
void printPerson(const Person* p) {
    printf("Address: %s, %s %d\n",
           p->homeAddress.street,
           p->homeAddress.city,
           p->homeAddress.zip);

    printf("Favorite Numbers: ");
    for (int i = 0; i < 5; i++) printf("%d ", p->favoriteNumbers[i]);
    printf("\n");

    printf("Nickname: %s\n", p->nickname ? p->nickname : "(none)");

    printf("Birth Date: %04d-%02d-%02d\n",
           p->birthDate.year,
           p->birthDate.month,
           p->birthDate.day);

    printf("Notes: %s\n", p->notes);

    printf("Middle Name: %s\n", p->middleName ? p->middleName : "(none)");

    printf("Hobbies: ");
    for (int i = 0; i < p->hobbyCount; i++)
        printf("%s, ", p->hobbies[i]);
    printf("\n");

    printf("Status: ");
    switch (p->status) {
        case Active:   printf("Active\n"); break;
        case Inactive: printf("Inactive\n"); break;
        default:       printf("Unknown\n"); break;
    }

    printf("ID (variant): ");
    switch (p->id.type) {
        case ID_INT:
            printf("int: %d\n", p->id.intValue);
            break;
        case ID_STRING:
            printf("string: %s\n", p->id.strValue);
            break;
        case ID_INT_ARRAY:
            printf("int array: ");
            for (int i = 0; i < 8; i++) printf("%d ", p->id.intArray[i]);
            printf("\n");
            break;
    }

    printf("Metadata:\n");
    for (int i = 0; i < p->metadataCount; i++)
        printf("  %s = %s\n", p->metadata[i].key, p->metadata[i].value);

    printf("Flags: isAdmin=%d isGuest=%d isVerified=%d\n",
           p->flags.isAdmin,
           p->flags.isGuest,
           p->flags.isVerified);

    printf("Scores: ");
    for (int i = 0; i < p->scoreCount; i++)
        printf("%d ", p->scores[i]);
    printf("\n");

    printf("Population: %d\n", population);
    printf("Allocated ints: %d\n", allocatedInts);

    if (p->extraPrinter)
        p->extraPrinter((void*)p);
}

// Extra printer
void extraPrinterFn(void* ptr) {
    Person* p = (Person*)ptr;
    printf("  [extra] nickname length: %zu\n",
           p->nickname ? strlen(p->nickname) : 0);
}

int main() {
    Person p = {0};
    population++;

    strcpy(p.homeAddress.street, "Baxter");
    strcpy(p.homeAddress.city, "New York");
    p.homeAddress.zip = 426900;

    int favs[5] = {7, 18, 27, 42, 81};
    memcpy(p.favoriteNumbers, favs, sizeof(favs));

    p.nickname = malloc(10);
    strcpy(p.nickname, "Orion");

    p.birthDate = (Date){1026, 6, 15};

    strcpy(p.notes, "Neo likes C/C#/Python and structs");

    p.middleName = malloc(MAX_STR);
    strcpy(p.middleName, "ECHO-9");

    strcpy(p.hobbies[0], "Coding");
    strcpy(p.hobbies[1], "Dreaming");
    strcpy(p.hobbies[2], "Walking");
    strcpy(p.hobbies[3], "Sci-Fi Movies");
    p.hobbyCount = 4;

    p.status = Active;

    p.id.type = ID_INT;
    p.id.intValue = 42;

    setMeta(&p, "role", "Explorer");
    setMeta(&p, "level", "9");

    p.flags.isAdmin = 1;
    p.flags.isVerified = 1;

    p.scores[p.scoreCount++] = 100;
    p.scores[p.scoreCount++] = 95;

    p.extraPrinter = extraPrinterFn;

    printPerson(&p);

    free(p.nickname);
    free(p.middleName);

    return 0;
}


/*
run:

Address: Baxter, New York 426900
Favorite Numbers: 7 18 27 42 81 
Nickname: Orion
Birth Date: 1026-06-15
Notes: Neo likes C/C#/Python and structs
Middle Name: ECHO-9
Hobbies: Coding, Dreaming, Walking, Sci-Fi Movies, 
Status: Active
ID (variant): int: 42
Metadata:
  role = Explorer
  level = 9
Flags: isAdmin=1 isGuest=0 isVerified=1
Scores: 100 95 
Population: 1
Allocated ints: 0
  [extra] nickname length: 5

*/

 

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