Union basics - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of union member assignment

What is the output of this C program?

C

#include <stdio.h>

union Data {
    int i;
    float f;
    char str[20];
};

int main() {
    union Data data;
    data.i = 10;
    data.f = 220.5f;
    printf("%d\n", data.i);
    return 0;
}

A Some unpredictable integer value

B 220

C 10

D Compilation error

💡 Hint

Remember that all members of a union share the same memory space.

In a union, all members share the same memory. Assigning to data.f overwrites data.i. Printing data.i after assigning data.f will show an unpredictable integer value representing the float bits.

❓ Predict Output

intermediate

Size of union vs struct

What will be the output of this program?

C

#include <stdio.h>

union U {
    int i;
    double d;
    char c;
};

struct S {
    int i;
    double d;
    char c;
};

int main() {
    printf("%zu %zu\n", sizeof(union U), sizeof(struct S));
    return 0;
}

A 16 24

B 16 16

C 8 16

D 24 24

💡 Hint

Union size is the size of its largest member; struct size is sum plus padding.

The union size equals the largest member size (double = 8 or 16 bytes depending on platform, here 16). The struct size is sum of members plus padding, here 24 bytes.

🔧 Debug

advanced

Why does this union code cause unexpected output?

Consider this code snippet. Why does printing data.str after assigning data.i show unexpected characters?

C

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

union Data {
    int i;
    char str[4];
};

int main() {
    union Data data;
    data.i = 0x41424344; // Hex for 'ABCD'
    printf("%s\n", data.str);
    return 0;
}

A Because the integer value is too large to fit in the union

B Because the string is not null-terminated, printf reads beyond the array causing garbage output

C Because union members cannot be accessed after assignment to another member

D Because printf cannot print char arrays inside unions

💡 Hint

Think about how strings are represented in C and what printf expects.

The char array in the union is not null-terminated after assigning the integer. printf expects a null-terminated string, so it reads past the array causing garbage output.

🧠 Conceptual

advanced

Union member alignment and padding

Which statement about union member alignment and padding is true?

A Union size is always the sum of all member sizes including padding

B Union members are stored in separate memory locations to avoid padding

C Union size is the size of its largest member, aligned to the strictest member's alignment requirement

D Union size is always equal to the smallest member size

💡 Hint

Think about how unions share memory and how alignment affects size.

Unions share the same memory for all members. The size is the largest member size, rounded up to meet the strictest alignment requirement among members.

❓ Predict Output

expert

Output of union with bit fields and overlapping members

What is the output of this program?

C

#include <stdio.h>

union U {
    struct {
        unsigned int a:4;
        unsigned int b:4;
    } bits;
    unsigned char byte;
};

int main() {
    union U u;
    u.byte = 0;
    u.bits.a = 9;  // 1001 in binary
    u.bits.b = 6;  // 0110 in binary
    printf("%u\n", u.byte);
    return 0;
}

A Compilation error

B 150

C 90

D 105

💡 Hint

Calculate the bits: a in lower 4 bits, b in upper 4 bits.

Bits a=9 (1001) in lower 4 bits, b=6 (0110) in upper 4 bits. Combined byte = 01101001 binary = 0x69 = 105 decimal.