C Programming — Structures & Unions Questions & Answers

Q.1 Hard Structures & Unions

In competitive programming for GATE/ISRO 2025, which scenario would union be preferable over structure?

A When you need to store multiple data types independently

B When memory optimization is critical and you need mutually exclusive data

C When creating complex data structures like trees

D When implementing object-oriented features

Correct Answer: B. When memory optimization is critical and you need mutually exclusive data

EXPLANATION

Unions are preferred when you have mutually exclusive data (only one member is used at a time) and memory is critical. A classic example is storing different data types in a tagged union pattern used in compilers and interpreters.

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Q.2 Hard Structures & Unions

What will be the output of the following code?
union Test {
int a;
char b;
};
union Test t;
t.a = 65;
printf("%d %c", t.a, t.b);

A 65 A

B 65 65

C 0 A

D Depends on endianness

Correct Answer: D. Depends on endianness

EXPLANATION

Since union members share memory, the output depends on the system's endianness. On little-endian systems, t.b would be 65 (or 'A'), but on big-endian systems, the result differs.

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Q.3 Hard Structures & Unions

In competitive programming, when you need a structure that can hold either an integer or a floating-point number (but not both simultaneously) in the most memory-efficient way, which should you use?

A struct with both members

B union with both members

C An array of size 2

D A single variable with type casting

Correct Answer: B. union with both members

EXPLANATION

Union is memory-efficient as members share space. Only one can be accessed at a time, which suits the requirement.

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Q.4 Hard Structures & Unions

What is the output of this complex code?
struct S { char c; int i; } s;
printf("%lu", sizeof(s));

A 5

B 8

C 6

D 4

Correct Answer: B. 8

EXPLANATION

char(1 byte) + padding(3 bytes) + int(4 bytes) = 8 bytes due to alignment requirements.

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Q.5 Hard Structures & Unions

If you have a structure with bit fields, which statement about their behavior is true?

A Bit fields can improve memory efficiency for small values

B Bit fields guarantee portability across all platforms

C Bit fields allow taking addresses (&) of members

D Bit fields support all data types

Correct Answer: A. Bit fields can improve memory efficiency for small values

EXPLANATION

Bit fields save memory by storing multiple values in single bytes, but behavior varies by compiler/platform, and you cannot take addresses of bit field members.

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Q.6 Hard Structures & Unions

In competitive programming, when implementing a graph using adjacency lists, which data structure is most suitable?

A Array of unions

B Array of structures containing linked lists

C Single structure

D Union of arrays

Correct Answer: B. Array of structures containing linked lists

EXPLANATION

An array of structures, where each structure contains a linked list (using self-referential pointers), is ideal for adjacency list representation.

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Q.7 Hard Structures & Unions

If a structure contains a flexible array member, where must it be declared?

A At the beginning of the structure

B At the end of the structure

C Anywhere in the structure

D In a separate union

Correct Answer: B. At the end of the structure

EXPLANATION

A flexible array member must be the last member of a structure. This is a C99 feature used for dynamic-sized arrays.

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Q.8 Hard Structures & Unions

In a structure, what does the #pragma pack(1) directive do?

A Removes padding between members

B Adds padding between members

C Compresses structure data

D Aligns structure to word boundary

Correct Answer: A. Removes padding between members

EXPLANATION

#pragma pack(1) removes padding, making members occupy consecutive memory locations for compact storage.

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Q.9 Hard Structures & Unions

What will be the output of this code?
struct X { int a; float b; };
struct Y { struct X x; int c; };
printf("%zu", sizeof(struct Y));

A 12

B 13

C 16

D Cannot determine

Correct Answer: C. 16

EXPLANATION

Nested struct X: int(4) + float(4) = 8 bytes. Y: struct X(8) + padding(0) + int(4) = 12, but with overall alignment it becomes 16 bytes.

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Q.10 Hard Structures & Unions

In competitive programming, when should you use union instead of struct?

A When you need to store multiple values simultaneously

B When memory optimization and type reinterpretation are critical

C When you have more than 5 members

D Never - struct is always better

Correct Answer: B. When memory optimization and type reinterpretation are critical

EXPLANATION

Unions are optimal for memory-constrained scenarios and hardware programming where type casting is needed.

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