Programming Languages – Why Boolean Value Stored as Byte Instead of Bit?

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I recently started learning to write code, and in my book I came across this question. "Why is a Boolean value stored as a byte inside of a computer when it only requires one bit?" can someone shed more light on this question?

Best Answer

It has to do with what the CPU can easily address. For example on an x86 processor there is an eax (32 bits), ax (16 bits) and a ah (8 bits) but no single bit register. So in order for it to use a single bit the CPU will have to do a read/modify/write to change the value. If it is stored as a byte a single read or write can be used to inspect/change the value.

Additionally one might wonder if it would be better to use a single bit vs a full byte, after all a byte will be wasting 7 bits. Unless space is a constraint the one should go for the byte because, at least the x86 and I think others, there is usually an instructions to quickly set/clear a bool which is much quicker than the read/modify/write of a single bit. From personal measurements I have seen the read/mod/write method be 5x slower than the single instruction method.

Related Solutions

C++ – Understanding Backtracking

... I can't seem to be able to get my mind around the concept of going back in the recursion stack and starting again in order to solve the problem.

In backtracking, you are not starting again. Instead, you iterate through all options at the current situation.

Think about finding solution for a maze. At one point where you have two different paths, you try the left one first. If the left one does not lead you to the exit, you return to the point and try the other path. That's how backtracking works. In 8 Q and other problems where backtracking can be used, the confusing part is in the problem domain - how to iterate through your options in a given situation in a deterministic way.

EDIT: the following is a pseudo code helping understanding backtracking.

# depending on the problem, backtracking is not necessarily calling the
# method itself directly. for now, let's just stick with the simple case.

def backtracking(state)
  option_list = state.get_all_options
  option_list.each {|option|
    state.apply option
    return resolved if state.is_resolved
    return resolved if backtracking(state) == resolved
    state.undo option
  }
  return not_resolved
end

For 8Q question:

state.get_all_options would return a list of the possible positions for the next queen
state.is_resolved would test if all queens are on the board and if they are good with each other.
state.apply and state.undo will modify the board to apply or undo a positioning.

C++ Pointers – Benefits of Incrementing Pointers

When you have an array, you can set up a pointer to point to an element of the array:

int a[10];
int *p = &a[0];

Here p points to the first element of a, which is a[0]. Now you can increment the pointer to point to the next element:

p++;

Now p points to the second element, a[1]. You can access the element here using *p. This is different from Java where you would have to use an integer index variable to access elements of an array.

Incrementing a pointer in C++ where that pointer does not point to an element of an array is undefined behaviour.

Best Answer

Related Solutions

C++ – Understanding Backtracking

C++ Pointers – Benefits of Incrementing Pointers

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