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# How do I use my scroll bar to increment a std::vector

## 3 posts in this topic

Hello all.

Ok I have my list box its a std::vector.

My scroll bar moves within the ranges of -104 to - 230

my question is how do I use the scroll ranges to move my list elements

my list elements start at 0 to max entrys.

I think its more complicated because the number of entrys you can see at a time is 4 so I can scroll the bar down 4 slots before it needs to move

any elements into view.

What would you all do.This is my first ever scroll bar I normally go for the arrow keys.

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You need to convert your scrollbar position into an index into the vector so you know the first item to draw. Then its easy, you draw the next three items after that one.

So -104 to 230, that's a total range of 334. So your scrollbar range is between 0 and 334 if you normalize it into positive coordinates. Now say you have ten items in your list. You display 4 at a time, so your scrollbar needs to be able to have the first index as 0 - 6 (10 - 4) so that in its bottom position, it is showing the last 4 items.

So 334 / 6 = 55.667. So divide your scrollbar position by 55.667 and truncate it to int to get a number between 0 and 6. That is the index of the vector that is the first one that should be drawn.

Sort of. It depends if you want smooth scrolling or not but that's the idea.

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Thanks All. It works well

init code

//we need to calculate the delta range this range is maped to the total amount in the list box
ScrollDelta = fabs(ScrollRangeMin - ScrollRangeMax);

//the ScrollIncrement is the amount we can move for each slot of the list box
//its the number of list Items devided by the ScrollDelta
//this maps the range of the slider to the number of items in the list box
ScrollIncrement = ScrollDelta / (NumberDlgs - MaxDisplayListItems);

//set the default range and check to see if the number of dlg boxes is less then max display items
VectorEndRange = MaxDisplayListItems;
if(NumberDlgs < MaxDisplayListItems)
VectorEndRange = NumberDlgs;

OnMouse move code

//calculate the display start range
StartListDisplayElement = fabs(fabs(ScrollRangeMin - v.y) / ScrollIncrement);
if(StartListDisplayElement < 0)
StartListDisplayElement = 0;

if(StartListDisplayElement > ListDialogBox.size() - MaxDisplayListItems)
StartListDisplayElement = ListDialogBox.size() - MaxDisplayListItems;

//set the rendering of the list box vector here
VectorEndRange = StartListDisplayElement + MaxDisplayListItems;

if(VectorEndRange > ListDialogBox.size())
VectorEndRange = ListDialogBox.size() - MaxDisplayListItems;

if(VectorEndRange < 0)
VectorEndRange = 0;

Render code
for(UINT ctr = StartListDisplayElement; ctr < VectorEndRange; ctr++)
{
render items
}


Here it is

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