GrADS Function: if

## if()

This function performs an if/then/else expression evaluation. It is available starting with GrADS version 2.1.1.b0.

### Syntax

`if (logical_expr, then_expr, else_expr)`

where:

`logical_expr` - any valid logical expression that has a boolean (yes/no) result
`then_expr` - the result expression if `logical_expr` is true
`else_expr` - the result expression if `logical_expr` is false

All the arguments must be expressions for gridded data -- the logical operators and the if() function have not yet been implemented for station data.

### Usage Notes

1. The `logical_expr` should include one or more of the logical operators:`=, !=, >, >=, <, <=, |, &`
   The result of a logical operation is boolean -- an answer to a yes/no question. If the expression is true the result is 1, and if the expression is false the answer is -1. The `if()` function will evaluate `logical_expr` and wherever the result is >0 it will place the value of `then_expr`, and wherever the result is <0 it will place the value of `else_expr`.
2. The arguments`then_expr` and `else_expr` may be any GrADS expression, including a constant. If you want `then_expr` or `else_expr` to be undefined, then use `maskout()` instead of the `if()` function.

### Examples

Here is a script sample to find the minimum and maximum 2-meter temperature at each grid point over a 12-month period:

  `'define tmin = const(t2m,1e9)'
  'define tmax = const(t2m,-1e9)'
  t = 1
  while (t <= 12)
  'set t 't
  'define tmin = if(t2m<tmin,t2m,tmin)'
  'define tmax = if(t2m>tmax,t2m,tmax)'
  t = t + 1
  endwhile`