Making Levels

Question

How do you make levels spanning a range?

Answer

Say we have data that runs from a minimum value of -3.4 to a maximum value of 5.6. If we want a list of even levels whose endpoints are identical to the enpoints of this range, we use the mkevenlevels command:

>>> import vcs
>>> levels = vcs.mkevenlevels(-3.4, 5.6)
>>> ['%.4g' % levels[i] for i in range(len(levels))]
['-3.4', '-2.5', '-1.6', '-0.7', '0.2', '1.1', '2', '2.9', '3.8', '4.7', '5.6']

This produces exactly 11 levels values (i.e. 10 level intervals), unless you specify a smaller number of intervals:

>>> levels = vcs.mkevenlevels(-3.4, 5.6, 4)
>>> ['%.4g' % levels[i] for i in range(len(levels))]
['-3.4', '-1.15', '1.1', '3.35', '5.6']

To make a list of levels that are "nice" (i.e. at natural breakpoints), but that also span the range, use mkscale:

>>> levels = vcs.mkscale(-3.4, 5.6)
>>> levels
[-4.0, -3.0, -2.0, -1.0, 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0]

The zero keyword also allows you to set whether you want 0 to be a level:

For instance:

>>> print vcs.mkscale(-3.4, 5.6, 5, zero=-1)
[-5.0, -3.0, -1.0, 1.0, 3.0, 5.0, 7.0]
>>> print vcs.mkscale(-3.4, 5.6, 5, zero=1)
[-4.0, -2.0, 0.0, 2.0, 4.0, 6.0]
>>> print vcs.mkscale(-3.4, 5.6, 5, zero=2)
[-4.0, -2.0, 0.0, 2.0, 4.0, 6.0]

Note you can set the maximum number of intervals in mkscale as the third positional argument. It isn't always enforced, however (as seen in the first zero=-1 example above).

The Python help command (e.g. help(vcs.mkscale)) gives some more information. Descriptions of these VCS function are also found in the Quick Start document.

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Notes: This discussion applies to CDAT 3.3.