Bitwise NOT in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this bitwise NOT operation?

Consider the following C code snippet. What will it print?

C

#include <stdio.h>

int main() {
    unsigned char x = 0b00001111; // 15 in decimal
    unsigned char y = ~x;
    printf("%u\n", y);
    return 0;
}

A 240

B 15

C 0

D 255

💡 Hint

Remember that bitwise NOT flips all bits. For an 8-bit unsigned char, 0b00001111 becomes 0b11110000.

The bitwise NOT operator (~) flips every bit. For 0b00001111 (decimal 15), flipping bits gives 0b11110000, which is 240 in decimal.

❓ Predict Output

intermediate

What is the output of bitwise NOT on a signed integer?

What will this C program print?

C

#include <stdio.h>

int main() {
    int x = 0;
    int y = ~x;
    printf("%d\n", y);
    return 0;
}

A 0

B -1

C 1

D 2147483647

💡 Hint

Bitwise NOT flips all bits. For zero, all bits flipped become all ones, which is -1 in two's complement.

In two's complement, ~0 flips all bits to 1, which represents -1 as a signed integer.

🔧 Debug

advanced

Why does this bitwise NOT code produce unexpected output?

This code is intended to print the maximum unsigned integer value after applying bitwise NOT to 0, but it prints -1 instead. Why?

C

#include <stdio.h>

int main() {
    unsigned int x = 0;
    unsigned int y = ~x;
    printf("%d\n", y);
    return 0;
}

A The bitwise NOT operator (~) doesn't work correctly on unsigned integers.

B There is integer overflow when applying ~ to 0.

C The printf format specifier "%d" expects a signed int, and the bit pattern of UINT_MAX (all 1s) is interpreted as -1 in two's complement.

D unsigned int y = ~x promotes x to signed before applying ~.

💡 Hint

The issue is with how printf interprets the unsigned value when using the wrong format specifier.

The ~ operator on unsigned int 0 produces UINT_MAX (all bits 1). However, printf("%d", y) passes an unsigned int to a format expecting signed int, leading to undefined behavior. In practice, on two's complement systems, it prints -1.

❓ Predict Output

advanced

What is the output of this bitwise NOT on a 16-bit unsigned short?

Given this C code, what will be printed?

C

#include <stdio.h>

int main() {
    unsigned short x = 0x00FF; // 255 decimal
    unsigned short y = ~x;
    printf("%u\n", y);
    return 0;
}

A 65280

B 255

C 65535

D 0

💡 Hint

Bitwise NOT flips all bits in 16 bits. 0x00FF flipped becomes 0xFF00.

0x00FF in binary is 0000000011111111. Flipping all bits gives 1111111100000000, which is 65280 in decimal.

❓ Predict Output

expert

What is the output of this bitwise NOT on a signed char with negative value?

Analyze this C code and determine what it prints.

C

#include <stdio.h>

int main() {
    signed char x = -1;
    signed char y = ~x;
    printf("%d\n", y);
    return 0;
}

A 127

B -1

C -128

D 0

💡 Hint

Bitwise NOT flips all bits. For -1 in two's complement, all bits are 1, flipping gives all 0 bits.

In two's complement, -1 is represented by all bits set to 1. Bitwise NOT flips all bits to 0, which is 0 as a signed char.

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