Gradually change a number in a variable

Let's say I have an object that I want to be transparent, then gradually becomes opaque. Usually I create a variable within the object named "transparency", and

if "transparency" = 0

then every "tick" obj.transparency= obj.transparency + 5

and if "transparency" = 1, it does the opposite.

Is there any way of handling this better? I'm not that good with programming, and I feel this isn't the right way of doing it.. I also feel like it should go fast like that. I wanted it to go fast at the beggining, then when it gets close to being completely opaque, it slows down the speed it's doing it.. is there a formula for that?

Sorry, it's kinda hard to explain since english isn't my first language.

I guess you could use the system expression "lerp" (explanation topic of lerp)

The lerp function can help. To fade out you could do:

Every tick: set opacity = lerp(self.opacity, 0, 0.5*dT)

to fade in:

Every tick: set opacity = lerp(self.opacity, 100, 0.5*dT)

To alternate between the two, use a sine function:

Every tick: set opacity = 100* sin(time*50)

scirra.com/forum/lerp-clarification_topic48641.html

http://www.scirra.com/forum/fade-in-fade-out-on-passing-event-point_topic48559.html