How to control only the WIDTH of a rectangle with slider control using expression

A Size experssion like this maybe?

w = effect("Width Control")("Slider");
h = effect("Height Control")("Slider");
[w,h]

Use Dan's expression for the anchor point and change it to:

w = content("Rectangle 1").content("Rectangle Path 1").size[1];

[value[0],-w/2]

w = content("Rectangle 1").content("Rectangle Path 1").size[...]; -> taking the size of the rectangle and store it in the variable w.

...size[] is an array with two values, x and y. To access on of those values, you write size[0] for x and size[1] for y.

[value[0],-w/2] is an array for x and y values, too. value[0] is the x value of the anchorpoint. This is a way to use an expression along with the possibility to change a value by entering it or drag it.

-w/2 takes half of the y-size of the rectangle and moves the y-value of the anchorpoint by this amount, aka to the top of the rectangle. If you change -w/2 to w/2, the anchor point is pinned at the bottom of the rectangle.

It's easier to understand, if you use actual values:

Rectangle is x = 100, y = 200.

Anchorpoint is at x = 0, y = 0 (in the middle of the rectangle)

Coordinates of the top edge are 0,-100

Coordinates of the bottom edge are 0,100.

(Coordinates of the left edge are -50,0.

Coordinates of the right edge are 50,0.)

w = content("Rectangle 1").content("Rectangle Path 1").size[1];

= 200 (y-value of the rectangle)

-w/2

= -200/2

= -100

Coordinates for the anchorpoint are:

[value[0],-w/2]

= [0, -100]

= top edge.

Cheers,

Martin

Thanks, Dan for your quick reply.

I tried your expression and it works like a charm.

However, I should have mentioned that I wanted this shape layer to grow and shrink from the left side (with an anchor point at the far left).

I have the anchor point at the far left, but nonetheless with this expression, it grows and shrinks from the middle.

How can I fix that?

Thanks again so much for your help.