GrADS Function: const

## 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.