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Autores Arthur van Stratum
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Category tags s2, level2, category_bit

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[S2] Bitwise operation: setting a bit

In this exercise, we will work with operation on bits. When we speak about the position of a bit, index 0 corresponds to lowest order bit, 1 to the second-lowest order bit, ...

In C source code, you can write a number in binary (base 2) by prefixing it via 0b., e.g. 0b11010 = 26.

This exercise will introduce some non-standard data types which guarantee that the variable has a fixed number of bits. Indeed, on some machines, a int could use 2, 4 or 8 bytes. Hence, if we want to perform bitwise operations, we have to know first on how many bits we are working.

For this, C introduces a new class of variable types :

  • int8_t (signed integer of 8 bits)
  • uint8_t (unsigned integer of 8 bits)
  • uint16_t (unsigned integer of 16 bits)

You can mix uint or int with bit-lengths 8, 16, 32 and 64). These types are defined in <stdint.h>

Write the body of a function that returns the value of variable x, but modified with the bit at position pos set to value.

Remember that in C (this is defined in stdbool.h), true corresponds to integer 1 while false corresponds to integer 0.

Remember than in C, you can create a mask as a binary value and use it with the bitwise and (&) and or (|) operations. For example:

uint8_t a=0b00000000;
uint8_t b=0b00001000;
uint8_t c=0b11111101;
uint8_t d=0b11011011;
// ~(a) returns 0b11111111
// (c&a) returns 0b00000000
// (c&b) returns 0b00001000
// (a|b) returns 0b00001000
// (d&c) returns 0b11011001

#include <stdint.h>
#include <stdbool.h>
/*
 * @pre 0<= pos < 64
 */
uint64_t set_bit(uint64_t x, int pos, bool value) {