Logical operators - Step-by-Step Execution

Concept Flow - Logical operators

Start

↓

Evaluate operand1

↓

Evaluate operand2

↓

Apply logical operator (&&, ||, !)

↓

Result is 0 (false) or 1 (true)

↓

Use result in condition or expression

↓

End

Logical operators combine or invert true/false values to produce a true or false result used in conditions.

Execution Sample

C

int a = 1;
int b = 0;
int c = a && b;
int d = a || b;
int e = !b;

This code uses logical AND, OR, and NOT operators on variables a and b.

Execution Table

Step	Expression	Operand1	Operand2	Operator	Result	Explanation
1	a && b	1 (true)	0 (false)	&& (AND)	0 (false)	AND is true only if both operands are true
2	a || b	1 (true)	0 (false)	|| (OR)	1 (true)	OR is true if at least one operand is true
3	!b	0 (false)	N/A	! (NOT)	1 (true)	NOT inverts the operand's truth value
4	End	N/A	N/A	N/A	N/A	All logical operations evaluated

💡 All logical operations completed, results stored in c, d, and e.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
a	1	1	1	1	1
b	0	0	0	0	0
c	undefined	0	0	0	0
d	undefined	undefined	1	1	1
e	undefined	undefined	undefined	1	1

Key Moments - 3 Insights

Why does 'a && b' result in 0 even though 'a' is 1?

Because AND (&&) requires both operands to be true (non-zero). Here, 'b' is 0 (false), so the result is 0 (false), as shown in execution_table step 1.

What does the '!' operator do to the value of 'b'?

The NOT (!) operator flips the truth value. Since 'b' is 0 (false), '!b' becomes 1 (true), as shown in execution_table step 3.

Why is the result of 'a || b' equal to 1?

OR (||) returns true if at least one operand is true. 'a' is 1 (true), so the whole expression is true, as shown in execution_table step 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the result of 'a && b' at step 1?

A Undefined

B 1 (true)

C 0 (false)

D 2

💡 Hint

Check the 'Result' column in execution_table row 1.

At which step does the NOT operator (!) get applied?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look for the row where Operator is '! (NOT)' in execution_table.

If 'b' was changed to 1, what would be the result of 'a && b'?

A 0 (false)

B 1 (true)

C Undefined

D 2

💡 Hint

AND is true only if both operands are true; see execution_table step 1 explanation.

Concept Snapshot

Logical operators in C:
- && (AND): true if both true
- || (OR): true if any true
- ! (NOT): flips true/false
Operands are treated as true (non-zero) or false (0)
Used in conditions and expressions

Full Transcript

This lesson shows how logical operators work in C. We start by evaluating each operand's value. Then we apply the logical operator: AND (&&) returns true only if both operands are true; OR (||) returns true if at least one operand is true; NOT (!) flips the truth value. The results are 0 or 1, representing false or true. Variables a and b are used to demonstrate these operators. The execution table traces each step, showing how results are computed. Key moments clarify common confusions, like why AND with one false operand is false, and how NOT inverts values. The quiz tests understanding by asking about specific steps and hypothetical changes. The snapshot summarizes the main points for quick review.

Practice

1. Which logical operator in C means AND and returns true only if both conditions are true?

easy

A. !

B. ||

C. &&

D. &

Solution

1. Step 1: Understand logical AND operator

   The logical AND operator (&&) returns true only if both conditions it connects are true.

2. Step 2: Compare with other operators

   The || operator means OR, ! means NOT, and & is bitwise AND, not logical AND.

3. Final Answer:

   && -> Option C

4. Quick Check:

   AND = && [OK]

Hint: AND operator is double ampersand && [OK]

Common Mistakes:

• Confusing & (bitwise) with && (logical)
• Mixing OR and AND operators
• Using ! for AND

2. Which of the following is the correct syntax to negate a condition in C?

easy

A. condition !

B. !condition

C. not condition

D. condition != true

Solution

1. Step 1: Identify negation syntax in C

   The NOT operator in C is a single exclamation mark placed before the condition: !condition.

2. Step 2: Check other options

   condition ! places ! after condition which is invalid, C uses 'not' which is not C syntax, D is a comparison, not negation.

3. Final Answer:

   !condition -> Option B

4. Quick Check:

   NOT = ! before condition [OK]

Hint: Negate condition with ! before it [OK]

Common Mistakes:

• Putting ! after condition
• Using 'not' keyword
• Confusing != with negation

3. What is the output of this C code?

int a = 5, b = 10;
if (a > 3 && b < 15) {
    printf("True\n");
} else {
    printf("False\n");
}

medium

A. No output

B. False

C. Compilation error

D. True

Solution

1. Step 1: Evaluate first condition (a > 3)

   a is 5, which is greater than 3, so this condition is true.

2. Step 2: Evaluate second condition (b < 15)

   b is 10, which is less than 15, so this condition is true.

3. Step 3: Apply logical AND (&&)

   Both conditions are true, so the combined condition is true, entering the if block.

4. Final Answer:

   True -> Option D

5. Quick Check:

   True && True = True [OK]

Hint: Both conditions true means && condition is true [OK]

Common Mistakes:

• Assuming OR instead of AND
• Misreading variable values
• Ignoring logical operator precedence

4. Find the error in this C code snippet:

int x = 0;
if (x = 5 || x == 0) {
    printf("Yes\n");
}

medium

A. Using assignment '=' instead of comparison '==' in if condition

B. Missing semicolon after printf statement

C. Incorrect use of logical AND instead of OR

D. Variable x is not declared

Solution

1. Step 1: Check the if condition syntax

   The condition uses 'x = 5 || x == 0'. The '=' is assignment, not comparison.

2. Step 2: Understand effect of assignment in condition

   Assignment returns the assigned value (5), which is true, so condition always true, likely a bug.

3. Final Answer:

   Using assignment '=' instead of comparison '==' in if condition -> Option A

4. Quick Check:

   Use '==' for comparison, not '=' [OK]

Hint: Use '==' to compare, '=' assigns value [OK]

Common Mistakes:

• Confusing '=' and '=='
• Ignoring logical operator precedence
• Assuming code won't compile

5. Given the code below, what will be printed?

int a = 0, b = 1, c = 2;
if (!(a || b) && c > 1) {
    printf("Inside if\n");
} else {
    printf("Inside else\n");
}

hard

A. Inside else

B. Inside if

C. Compilation error

D. No output

Solution

1. Step 1: Evaluate inner OR condition (a || b)

   a is 0 (false), b is 1 (true), so a || b is true.

2. Step 2: Apply NOT operator !(a || b)

   NOT true is false, so this part is false.

3. Step 3: Evaluate c > 1

   c is 2, which is greater than 1, so true.

4. Step 4: Combine with AND: false && true

   False AND anything is false, so condition is false, else block runs.

5. Final Answer:

   Inside else -> Option A

6. Quick Check:

   !(true) && true = false [OK]

Hint: NOT true is false; AND with false is false [OK]

Common Mistakes:

• Ignoring NOT operator effect
• Misreading OR condition
• Assuming if block runs when condition false