NOT for inverting bits in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to invert all bits of the variable num using the NOT operator.

Embedded C

unsigned int num = 0x0F; 
unsigned int inverted = [1] num; 

Drag options to blanks, or click blank then click option'

A|

B!

C&

D~

Attempts:

3 left

💡 Hint

Common Mistakes

Using ! instead of ~ flips the logic but not the bits.

Using & or | does not invert bits.

The bitwise NOT operator in C is ~. It flips every bit of the number.

2fill in blank

medium

Complete the code to invert bits of value and store the result in result.

Embedded C

unsigned char value = 0xAA; 
unsigned char result = [1]value;

Drag options to blanks, or click blank then click option'

A^

B~

C&

D|

Attempts:

3 left

💡 Hint

Common Mistakes

Using & or | operators instead of ~.

Using ^ without a mask does not invert all bits.

The ~ operator inverts all bits of the operand.

3fill in blank

hard

Fix the error in the code to correctly invert bits of data.

Embedded C

unsigned int data = 0x1234;
unsigned int inverted = [1]data;

Drag options to blanks, or click blank then click option'

A~

B!

C^

D&

Attempts:

3 left

💡 Hint

Common Mistakes

Using ! instead of ~ causes incorrect results.

Confusing logical and bitwise operators.

The logical NOT operator ! returns 0 or 1, not bitwise inversion. Use ~ for bitwise NOT.

4fill in blank

hard

Fill both blanks to create a mask and invert only the lower 4 bits of num.

Embedded C

unsigned int num = 0x3C;
unsigned int mask = [1];
unsigned int result = num [2] mask;

Drag options to blanks, or click blank then click option'

A0x0F

B~

C&

D^

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect mask values.

Using ~ which inverts all bits.

Using & or | which do not flip bits.

Create a mask 0x0F with 1s in the lower 4 bits. XOR (^) the number with this mask inverts exactly those bits (XOR with 1 flips a bit, XOR with 0 leaves it unchanged).

5fill in blank

hard

Fill the blanks to invert bits of value only where mask has 1s, leaving other bits unchanged.

Embedded C

unsigned int value = 0x55AA;
unsigned int mask = [1];
unsigned int result = value [2] mask;
// result now has bits inverted only where mask bits are 1

Drag options to blanks, or click blank then click option'

A0x00FF

B^

C~

D&

Attempts:

3 left

💡 Hint

Common Mistakes

Using OR instead of XOR.

Using ~ which inverts all bits unconditionally.

Using & which masks but does not flip.

Use mask 0x00FF to target the lower 8 bits. XOR (^) flips bits where mask is 1.

Practice

1.

What does the ~ operator do in embedded C?

easy

A. It flips every bit of the operand (bitwise NOT).

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

C. It adds one to the operand.

D. It compares two values for equality.

Solution

1. Step 1: Understand the ~ operator

   The ~ operator is the bitwise NOT operator in C, which flips every bit of its operand.

2. Step 2: Compare with other options

   Other options describe different operations: shifting, addition, and comparison, which are not what ~ does.

3. Final Answer:

   It flips every bit of the operand (bitwise NOT). -> Option A

4. Quick Check:

   ~ operator = bitwise NOT [OK]

Hint: Remember ~ flips bits, like turning 0s to 1s and vice versa [OK]

Common Mistakes:

• Confusing ~ with shift operators
• Thinking ~ adds or subtracts
• Mixing ~ with logical NOT (!)

2.

Which of the following is the correct syntax to invert bits of an unsigned integer x in embedded C?

unsigned int x = 5;
unsigned int y = ?;

easy

A. y = x++;

B. y = !x;

C. y = ~x;

D. y = x & ~x;

Solution

1. Step 1: Identify the bit inversion syntax

   To invert bits, use the bitwise NOT operator ~ directly on the variable.

2. Step 2: Check other options for correctness

   y = !x; uses logical NOT, which returns 0 or 1, not bit inversion. y = x++; increments x, not invert bits. y = x & ~x; ANDs x with its inverse, resulting in zero.

3. Final Answer:

   y = ~x; -> Option C

4. Quick Check:

   Bit inversion syntax = y = ~x; [OK]

Hint: Use ~ directly on variable to invert bits [OK]

Common Mistakes:

• Using ! instead of ~
• Using increment operators by mistake
• Combining & with ~ incorrectly

3.

What is the output of the following code snippet?

unsigned char x = 0b00001111;
unsigned char y = ~x;
printf("%u", y);

medium

A. 240

B. 15

C. 0

D. 255

Solution

1. Step 1: Understand the initial value of x

   x is 0b00001111, which is decimal 15.

2. Step 2: Apply bitwise NOT to x

   ~x flips bits: 00001111 becomes 11110000 in 8 bits, which is decimal 240.

3. Final Answer:

   240 -> Option A

4. Quick Check:

   ~15 (8-bit) = 240 [OK]

Hint: Invert bits of 00001111 to get 11110000 = 240 decimal [OK]

Common Mistakes:

• Confusing logical NOT (!) with bitwise NOT (~)
• Forgetting unsigned type size
• Assuming output is 15 or 255

4.

Find the error in the following code that attempts to invert bits of num and print the result:

int num = 5;
int inverted = ~num;
printf("%u", inverted);

medium

A. The variable inverted should be unsigned char.

B. The ~ operator cannot be used on int types.

C. Missing semicolon after variable declaration.

D. Using %u to print a signed int causes incorrect output.

Solution

1. Step 1: Check the type and format specifier

   Variable inverted is signed int, but printed with %u (unsigned). This mismatch can cause wrong output.

2. Step 2: Verify other options

   The ~ operator works on int types. Semicolons are present. Changing to unsigned char is not necessary but can clarify output.

3. Final Answer:

   Using %u to print a signed int causes incorrect output. -> Option D

4. Quick Check:

   Format specifier must match variable type [OK]

Hint: Match printf format (%d for int, %u for unsigned) [OK]

Common Mistakes:

• Using wrong printf format specifier
• Thinking ~ can't be used on int
• Ignoring signed vs unsigned mismatch

5.

You want to invert bits of a 16-bit unsigned integer val but only for the lower 8 bits, leaving the upper 8 bits unchanged. Which code snippet correctly does this?

unsigned short val = 0xABCD;
unsigned short result = ?;

hard

A. result = ~val & 0xFFFF;

B. result = (val & 0xFF00) | (~val & 0x00FF);

C. result = ~val | 0xFF00;

D. result = val ^ 0xFFFF;

Solution

1. Step 1: Understand the requirement

   Invert only the lower 8 bits of val, keep upper 8 bits as is.

2. Step 2: Analyze each option

   result = (val & 0xFF00) | (~val & 0x00FF); keeps upper 8 bits (val & 0xFF00) and inverts lower 8 bits (~val & 0x00FF), then combines them. result = ~val & 0xFFFF; inverts all 16 bits. result = ~val | 0xFF00; incorrectly sets upper bits to 1. result = val ^ 0xFFFF; flips all bits (XOR with 0xFFFF).

3. Final Answer:

   result = (val & 0xFF00) | (~val & 0x00FF); -> Option B

4. Quick Check:

   Mask upper bits, invert lower bits, combine [OK]

Hint: Mask upper bits, invert lower bits, then combine [OK]

Common Mistakes:

• Inverting all bits instead of just lower 8
• Using OR instead of AND for masking
• Forgetting to combine masked parts