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Jan 7, 2016 10:29 AM
(940 views)

Hello again,

I have to columns of data, let's call them x and y. I want to calculate the center derivative of y with respect to x (dy/dx). If I am using the center derivative, that means I will be "missing" a point at the beginning and end of the dy/dx column.

For example, in row two of dy/dx, I want it to equal dy/dx = y3-y1/x3-x1 and on row three of dy/dx, I want it to equal dy/dx = y4-y2/x4-x2. I am having a hard time not using jmp like I would with excel, so I am not even sure if is possible to do this in the formula editor.

Thanks!

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Solution

This code should give you some ideas. The column formula makes use of the **LAG** function:

dt = open**(**"$sample_data\Big Class.jmp"**)**;

// Create formula column

// Height = X

// Weight = Y

// dy/dx = (y+1) - (y-1)/(x+1) - (x-1)

// y+1: lag(:weight, -1)

// y-1: lag(:weight, 1)

// x+1: lag(:height, -1)

// x-1: lag(:height, 1)

dt << **New Column(** "dy/dx",

Numeric,

Continuous,

Format**(** "Best", **12** **)**,

Formula**(** lag**(**:weight, -**1)** - lag**(**:weight, **1)** / lag**(**:height, -**1)** - lag**(**:height, **1))**

**)**;

3 REPLIES

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Jan 7, 2016 11:06 AM
(729 views)

natalie_,

The Formula Editor has a derivative option. You can access it by clicking the red triangle at the top of the Editor dialog.

Hope this helps.

Dave

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This code should give you some ideas. The column formula makes use of the **LAG** function:

dt = open**(**"$sample_data\Big Class.jmp"**)**;

// Create formula column

// Height = X

// Weight = Y

// dy/dx = (y+1) - (y-1)/(x+1) - (x-1)

// y+1: lag(:weight, -1)

// y-1: lag(:weight, 1)

// x+1: lag(:height, -1)

// x-1: lag(:height, 1)

dt << **New Column(** "dy/dx",

Numeric,

Continuous,

Format**(** "Best", **12** **)**,

Formula**(** lag**(**:weight, -**1)** - lag**(**:weight, **1)** / lag**(**:height, -**1)** - lag**(**:height, **1))**

**)**;

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Jan 8, 2016 10:55 AM
(729 views)

Thank you, it worked well.

I used it in a script, but I had to add As Column.

col <<Set Formula( (lag(As Column("Y"), -1) - lag(As Column("Y"),1))/(lag(As Column("X"),-1)-lag(As Column("X"),1)));