Bitwise NOT in C - Mini Project: Build & Apply

Bitwise NOT Operation in C

📖 Scenario: You are working with a small embedded system that uses 8-bit registers. You need to invert the bits of a given number to control some hardware flags.

🎯 Goal: Learn how to use the bitwise NOT operator ~ in C to invert bits of an 8-bit number.

📋 What You'll Learn

Create an 8-bit unsigned integer variable with a specific value

Create a variable to store the inverted bits

Use the bitwise NOT operator ~ to invert the bits

Print the original and inverted values

💡 Why This Matters

🌍 Real World

Bitwise NOT is used in embedded systems to toggle hardware flags and invert control bits quickly.

💼 Career

Understanding bitwise operations is essential for firmware developers, systems programmers, and anyone working close to hardware.

1

Create an 8-bit unsigned integer variable

Create a variable called num of type unsigned char and set it to 170.

C

# Your code here

Hint

Use unsigned char to represent an 8-bit number and assign the value 170.

2

Create a variable to store the inverted bits

Create a variable called inverted of type unsigned char to hold the result of the bitwise NOT operation.

C

unsigned char num = 170;
# Your code here

Hint

Declare inverted as unsigned char without initializing it yet.

3

Invert the bits using the bitwise NOT operator

Assign to inverted the bitwise NOT of num using the ~ operator.

C

unsigned char num = 170;
unsigned char inverted;
# Your code here

Hint

Use ~num to invert all bits of num and assign it to inverted.

4

Print the original and inverted values

Use printf to display the values of num and inverted as unsigned integers on separate lines.

C

unsigned char num = 170;
unsigned char inverted;
inverted = ~num;
# Your code here

Hint

Use printf("Original: %u\n", num); and printf("Inverted: %u\n", inverted); to print the values.

Practice

1. What does the bitwise NOT operator ~ do to an integer in C?

easy

A. It adds 1 to the integer.

B. It shifts all bits to the left by one position.

C. It sets all bits to zero.

D. It flips every bit of the integer, changing 0s to 1s and 1s to 0s.

Solution

1. Step 1: Understand the bitwise NOT operator

   The bitwise NOT operator ~ in C flips every bit of the integer. This means all 0 bits become 1, and all 1 bits become 0.

2. Step 2: Compare with other bitwise operations

   Unlike shifting or adding, ~ only toggles bits without changing their positions or values numerically.

3. Final Answer:

   It flips every bit of the integer, changing 0s to 1s and 1s to 0s. -> Option D

4. Quick Check:

   Bitwise NOT = flip bits [OK]

Hint: Remember: ~ flips bits, not shifts or adds [OK]

Common Mistakes:

• Confusing ~ with bit shift operators << or >>
• Thinking ~ adds or subtracts from the number
• Assuming ~ sets bits to zero instead of flipping

2. Which of the following is the correct syntax to apply bitwise NOT to an integer variable x in C?

easy

A. int y = ~x;

B. int y = not x;

C. int y = !x;

D. int y = x~;

Solution

1. Step 1: Recall the bitwise NOT operator syntax

   In C, the bitwise NOT operator is the tilde symbol ~ placed before the variable.

2. Step 2: Check each option

   int y = not x; uses not which is not valid in C. int y = !x; uses logical NOT !. int y = x~; has incorrect placement of ~. Only int y = ~x; correctly uses ~x.

3. Final Answer:

   int y = ~x; -> Option A

4. Quick Check:

   Bitwise NOT syntax = ~x [OK]

Hint: Use ~ before variable for bitwise NOT [OK]

Common Mistakes:

• Using 'not' instead of ~
• Confusing logical NOT (!) with bitwise NOT (~)
• Placing ~ after the variable

3. What is the output of the following C code?

int x = 5;
int y = ~x;
printf("%d", y);

medium

A. 5

B. -6

C. -5

D. 6

Solution

1. Step 1: Convert 5 to binary and apply bitwise NOT

   5 in binary (32-bit) is 00000000 00000000 00000000 00000101. Applying ~ flips bits to 11111111 11111111 11111111 11111010.

2. Step 2: Interpret the result as a signed integer

   The result is the two's complement representation of -6.

3. Final Answer:

   -6 -> Option B

4. Quick Check:

   ~5 = -6 in two's complement [OK]

Hint: ~x equals -(x+1) for signed integers [OK]

Common Mistakes:

• Thinking ~5 equals 6 instead of -6
• Ignoring two's complement representation
• Confusing bitwise NOT with logical NOT

4. Identify the error in the following C code snippet:

int x = 10;
int y = ~ x;
printf("%d", y);

medium

A. There is a syntax error due to space between ~ and x.

B. The code will print 9 instead of expected -11.

C. No error; the code works and prints -11.

D. The printf format specifier is incorrect.

Solution

1. Step 1: Check syntax of bitwise NOT usage

   In C, spaces between operators and operands are allowed. So ~ x is valid and equivalent to ~x.

2. Step 2: Verify output and printf usage

   Applying ~10 results in -11. The printf format specifier %d is correct for int.

3. Final Answer:

   No error; the code works and prints -11. -> Option C

4. Quick Check:

   Space allowed after ~, output = -11 [OK]

Hint: Spaces don't break ~ operator usage [OK]

Common Mistakes:

• Thinking space causes syntax error
• Expecting positive output instead of negative
• Confusing format specifier for int

5. Given a signed 8-bit integer n, which expression correctly computes the bitwise NOT of n and then adds 1, effectively calculating the two's complement negation of n in C?

hard

A. ~n + 1

B. ~(n + 1)

C. -~n

D. -n + 1

Solution

1. Step 1: Understand two's complement negation

   Two's complement negation of a number n is computed as ~n + 1.

2. Step 2: Analyze each option

   ~n + 1 matches the formula. ~(n + 1) negates after adding 1, which is incorrect. -~n negates the bitwise NOT, not the number. -n + 1 is arithmetic negation plus 1, which is not two's complement negation.

3. Final Answer:

   ~n + 1 -> Option A

4. Quick Check:

   Two's complement negation = ~n + 1 [OK]

Hint: Two's complement negation = ~n + 1 [OK]

Common Mistakes:

• Using -(~n) instead of ~n + 1
• Applying ~ after adding 1
• Confusing arithmetic negation with bitwise negation