[C Language] Goto, Preprocessor Directives, Structures, and Unions

C language

1. goto

• It is used to immediately jump to a specified label within the program.

    #include <stdio.h>
  
    int main() {
        int num = 0;
  
        printf("Enter a positive number: ");
        scanf("%d", &num);
  
        if (num <= 0) {
            goto error;
        }
  
        printf("You entered: %d\n", num);
        return 0;
  
    error:
        printf("Error: The number is not positive.\n");
        return 1;
    }
  

2. ifdef & endif

• #ifdef and #endif are preprocessor directives and in C.

    #define DEBUG
  
    #ifdef DEBUG
    #include <stdio.h>
    #endif
  
    int main() {
        #ifdef DEBUG
        printf("Debug information\n");
        #endif
  
        // Rest of the program
        return 0;
    }
  

• If DEBUG is not defined (commented out or removed), the DEBUG macro would not be defined and the printf statement would not be compiled.

Struct

• A structure in C is a user-defined data type that allows grouping different data types into a single unit. By using structures, collecting related data together into a group.

    #include <stdio.h>
  
    // Declaration of Student structure
    struct Student {
        char name[50];
        int id;
        float averageScore;
    };
  
    int main() {
        // Declaring and initializing a structure variable
        struct Student student1 = {"John Doe", 12345, 85.6};
  
        // Accessing and printing structure members
        printf("Name: %s\n", student1.name);
        printf("ID: %d\n", student1.id);
        printf("Average Score: %.2f\n", student1.averageScore);
  
        return 0;
    }
  

Union

• It is a user-defined data type that allows multiple members to share the same memory location.

    union myUnion {
        int integer;
        float decimal;
        char character;
    };
    union myUnion u;
  
    u.integer = 5;
    printf("%d\n", u.integer); // Outputs 5
  
    u.decimal = 3.14;
    printf("%f\n", u.decimal); // Outputs 3.14
    // Accessing u.integer at this point may produce an unexpected value
  
    u.character = 'A';
    printf("%c\n", u.character); // Outputs 'A'
    // Accessing u.integer or u.decimal at this point may produce unexpected values