Short-Circuit Evaluation in C

Short-Circuit Evaluation in C – Complete Guide with MCQs for CDAC C-CAT

Short-circuit evaluation is one of the most frequently tested concepts in CDAC C-CAT. It determines whether the second operand of a logical expression is evaluated or skipped. Understanding this behavior is essential for solving expression-based MCQs and avoiding runtime errors.

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What is Short-Circuit Evaluation?

Short-circuit evaluation means that the compiler does not evaluate the second operand if the result can already be determined from the first operand.

It applies only to the logical operators:

• && (Logical AND)
• || (Logical OR)

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Short-Circuiting with &&

For the Logical AND (&&) operator:

• If the first operand is false (0), the second operand is not evaluated.
• The overall result is already known to be false.

Example:

printf("%d", 0 && 10 / 0);

Evaluation

0 && ...

Since the first operand is 0, the second operand is skipped.

Output

0

The program does not crash even though 10 / 0 would normally cause undefined behavior.

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Short-Circuiting with ||

For the Logical OR (||) operator:

• If the first operand is true (non-zero), the second operand is not evaluated.
• The result is already known to be true.

Example:

printf("%d", 5 || 10 / 0);

Evaluation

5 || ...

Since 5 is true, the second operand is skipped.

Output

1

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When Does the Second Operand Execute?

Example 1

printf("%d", 1 && 5);

Evaluation

1 && 5

Both operands are evaluated.

Output

1

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Example 2

printf("%d", 0 || 5);

Evaluation

0 || 5

The second operand must be evaluated.

Output

1

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Using Functions

#include <stdio.h>

int fun()
{
    printf("Function Called\n");
    return 1;
}

int main()
{
    printf("%d", 0 && fun());
}

Output

0

fun() is never called.

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Another example

printf("%d", 1 || fun());

Output

1

Again, fun() is skipped.

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Short-Circuiting with Increment Operators

int i = 0;

printf("%d", i++ && 5);

printf("%d", i);

Evaluation

i++ returns 0
i becomes 1
Second operand skipped

Output

0
1

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Another example

int i = 0;

printf("%d", ++i && 5);

Evaluation

++i makes i = 1
1 && 5

Output

1

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Preventing Runtime Errors

A common use of short-circuiting is to avoid division by zero.

Instead of

if (b / a > 5)

write

if (a != 0 && b / a > 5)

If a is zero, the division is never evaluated.

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Pointer Safety

Instead of

if(ptr->data == 10)

write

if(ptr && ptr->data == 10)

If ptr is NULL, the second condition is skipped, preventing a crash.

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Common Mistakes

Mistake 1

Assuming both operands are always evaluated.

0 && fun()

fun() is never executed.

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Mistake 2

Assuming && and & behave the same.

0 && fun();

fun() is skipped.

0 & fun();

fun() is executed because & is a bitwise operator, not a short-circuit operator.

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Mistake 3

Assuming || always evaluates both operands.

10 || fun();

fun() is skipped.

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CDAC C-CAT MCQs

MCQ 1

printf("%d", 0 && 10 / 0);

A. Runtime Error

B. 0

C. 1

D. Undefined

Answer: B

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MCQ 2

printf("%d", 5 || 10 / 0);

A. Runtime Error

B. 0

C. 1

D. Undefined

Answer: C

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MCQ 3

int i = 0;

printf("%d", i++ && 5);

printf("%d", i);

A. 0 0

B. 0 1

C. 1 1

D. 1 0

Answer: B

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MCQ 4

int i = 0;

printf("%d", ++i || 0);

A. 0

B. 1

C. Runtime Error

D. Undefined

Answer: B

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MCQ 5

int i = 0;

printf("%d", i++ || 5);

printf("%d", i);

Evaluation

i++ returns 0
i becomes 1
Second operand executes

Output

A. 1 1

B. 0 1

C. 1 0

D. 0 0

Answer: A

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MCQ 6

int i = 5;

printf("%d", i-- && 0);

printf("%d", i);

Evaluation

i-- returns 5
i becomes 4
5 && 0

Output

A. 0 4

B. 1 5

C. 0 5

D. 1 4

Answer: A

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MCQ 7

printf("%d", 0 || 0 || 10);

A. 0

B. 1

C. 10

D. Error

Answer: B

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MCQ 8

printf("%d", 5 && 6 && 7);

A. 0

B. 1

C. 7

D. 5

Answer: B

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MCQ 9

printf("%d", 0 || 2 && 0);

Operator precedence:

&& before ||

Evaluation

2 && 0 = 0
0 || 0 = 0

A. 0

B. 1

C. 2

D. Error

Answer: A

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MCQ 10

printf("%d", 1 || 0 && 0);

Evaluation

&& before ||
0 && 0 = 0
1 || 0 = 1

A. 0

B. 1

C. Runtime Error

D. Undefined

Answer: B

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Key Takeaways

• Short-circuit evaluation applies only to && and ||.
• && skips the second operand if the first operand is false.
• || skips the second operand if the first operand is true.
• Short-circuiting is commonly used to avoid division by zero and null pointer dereferencing.
• && and || are different from the bitwise operators & and |, which always evaluate both operands.
• For CDAC C-CAT, always determine whether the second operand is evaluated before solving the expression.