`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

; missing data are unchanged *value*

- The
`const`

function operates on both gridded and station data. - If

is given as an integer, it will still be treated as as floating point.*value*

- The
`const`

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

- 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

- 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.