Stacks

A stack is a last in, first out data structure. A stack can only have data added or removed from the top. Imagine a stack of plates in a restaurant, you can only take the top plate and you can't grab one from the middle.

The stack has limited size and this must be defined. The Stack Pointer will always point to the value at the top of the stack. It will start a zero for an empty stack and the maximum value should be equal to the size of the stack.

Check for Full

A stack must have a maximum size of the stack defined, this is often called StackMax. This will be used to check if the stack is currently full, it will do this by comparing the stack pointer and the StackMax.

procedure TestFull (ref Stack, ref SP)
begin
	if SP = StackMax then
		output ‘no room on stack’
	else
		output ‘room available on stack’
	endif
end { TestFull}

Check for Empty

Testing for empty only requires the stack pointer, so if the stack pointer is zero the stack is empty.

procedure TestEmpty (ref Stack, ref SP)
begin
	if SP = 0 then
		output ‘stack is empty’
	else
                output ‘stack is not empty’
	endif
end {TestEmpty}

Push

When an item is pushed it is added to the top of the stack. The stack pointer points to the item at the top of the stack, so the stack pointer is incremented and then the data item is stored in that location of the stack. remember this can only be carried out if the stack is not currently full.

Algorithm

procedure TestAndPush (ref Stack, ref SP)
begin
	if SP = StackMax then
		output ‘no room on stack’
	else
		input DataItem
		SP := SP + 1
		Stack[SP] : = DataItem
	endif
end { TestAndPush}

Pop

Pop will remove the top item from the stack. You will need to store the item at the top of the stack (location of stack pointer), and then decrement the stack pointer by 1. The item may not be deleted it will just be overwritten the next time that location is used.

Remember a pop can only be performed if the stack is not currently empty.

procedure TestAndPop (ref Stack, ref SP)
begin
	if SP = 0 then
		output ‘stack is empty’
	else
		DataItem := Stack[SP]
		SP := SP – 1
		output DataItem
	endif
end {TestAndPop}

Peek

Allows you to get the value from the top of the stack, without actually popping it. This is achieved by following the same algorithm for a pop, however the stack pointer is never changed.

Again remember this can only be carried out on a stack that current contains values, if it is empty it will crash.

procedure TestAndPeek (ref Stack, ref SP)
begin
	if SP = 0 then
		output ‘stack is empty’
	else
		DataItem := Stack[SP]
		output DataItem
	endif
end {TestAndPeek}