Chapters:
1: Introduction
2: Simple example
3: Invocation
4: Finer Control
5: X-Y Plots
6: Contour Plots
7: Image Plots
8: Examples
9: Gri Commands
10: Programming
11: Environment
12: Emacs Mode
13: History
14: Installation
15: Gri Bugs
16: Test Suite
17: Gri in Press
18: Acknowledgments
19: License
Indices:
Concepts
Commands
Variables

### 10.9.7: Manipulation of Columns etc

#### 10.9.7.1: Columns

Individual data in the ``x`', ``y`', ``z`', ``u`', ``v`' and ``weight`' columns can be accessed with the ``@`' operator. The first point has index 0. Examples:

    show "first x is " {rpn x 0 @ }
    show "last  x is " {rpn x ..num_col_data.. 1 - @ }
    show "and here are all the data:"
    .i. = 0
    while {rpn .i. ..num_col_data.. >}
        show {rpn x .i. @ }
        .i. += 1
    end while

The mean value is available from the ``mean`' operator (e.g., ``.xmean. = {rpn x mean }`', while the standard deviation is given by ``stddev`', the skewness is given by ``skewness`', and the kurtosis is given by ``kurtosis`' (using the definition that yields 3 for a gaussian distribution).

The minimal and maximal values are given by ``min`' and ``max`'.

The area under the curve y=y(x) is found by ``{rpn y x area }`', defined by ``0.5 * sum ( (y[i] + y[i-1]) * (x[i] - x[i-1]) )`' for ``i`' ranging from 1 to ``..num_col_data..`'-1.

#### 10.9.7.2: Grid

Grid data can be accessed with e.g. ``{rpn grid min }` ', ``{rpn grid max }` ', and ``{rpn grid mean }` '.

The value of the grid at a given ``(.x.,.y.)`' coordinate may be found by by e.g. ``{rpn grid .x. .y. interpolate}`'. The interpolation scheme is the same as that used in converting grids to images.