const()

const (expr, value, <-u|-a>)

This is a powerful function that allows the user to change the missing values of a variable, set all the non-missing values of a variable to a constant, or set all possible values of a variable (both valid and missing) to a constant.

expr   a valid GrADS expression
value  a constant, either an integer or floating point value
-u     all missing data are set to value; non-missing data are unchanged
-a     all data are set to value, both missing and non-missing

Default behaviour is to set all non-missing data equal to value; missing data are unchanged

Usage Notes

1. The const function operates on both gridded and station data.
2. If value is given as an integer, it will still be treated as as floating point.

Examples

1. The const function assigns a new value to missing data, so that missing data may participate in operations:
   const(z, 0, -u)

2. The const function is used with the set gxout linefill graphics output option to define a straight horizontal line:
   set lon -90
set lat -90 90
set gxout linefill
set lev 500
d const(t, -20);t-273


3. In this example, const is used to calculate a daily timeseries of the fraction of the globe convered by precipitation greater than 10mm/day:
   set lon 0 360
set lat -90 90
set t 1 last
define ones = const(const(maskout(p,p-10),1),0,-u)
set x 1
set y 1
display tloop(aave(ones,lon=0,lon=360,lat=0,lat=360))


   Notes: The defined variable "ones" contains 1 wherever the precip value is greater than 10, and 0 whever the precip value is less than 10. This is done via nested functions; first maskout sets all values less than 10 to missing, then const sets all non-missing values to 1, then const is used with the -u flag to set all the missing data values to 0. The aave function calculates an area weighted average. Since we are averaging zeros and ones, the result is the fraction of the area where there are ones. See the tloop function for a description of how to perform time series of areal averages.