Summary
if(<conditionExpression>, <thenExpression>, [<elseExpression>])
Returns the points from thenExpression
while conditionExpression
returns nonzero values.
Returns points from elseExpression
otherwise.
Parameters
Parameter  Description 

conditionExpression  Expression describing the time series to be used as the condition. For example, if the condition is cpu.loadavg.1m>100, then the condition evaluates to true if the load is greater than 100, and to false otherwise. This expression typically uses a comparison operator. 
thenExpression  Required expression that describes the time series to return when the condition is met.

elseExpression  Optional expression that describes the time series to return when the condition is not met. If you include elseExpression:

Description
The if()
conditional function returns data values based on the specified condition. You can determine what values are returned if the condition is true, and what values are returned if the condition is false. The Wavefront system treats a 0 value as false, and all other values as true.
Each of the expressions can be constant or a ts() expression (query).
Interpolation
We use interpolation to connect points in each of the expressions. We use interpolated values in conditionalExpression
to determine whether to return values from thenExpression
or elseExpression
. For example:

Assume you have an
if()
query like the following:if(ts(my.metricA) > 0, ts(my.metricB), ts(my.metricC))
. my.metricA
reports a value of 0 at 12:00pm andmy.metricC
reports values at 11:59am and 12:01pm. Because
my.metricA
is false, we showmy.metricC
and we show an interpolated value formy.metricC
at 12:00pm.
Series Matching and if()
We perform series matching when ts() expressions are used for 2 or more parameters.
For example, assume we have a query that’s structured like this:
if(ts(my.metricA) > 100, ts(my.metricB))
my.metricA
describes 100 time seriesmy.metricB
describes 50 time series, If some elements in the
my.metricA
time series, for example, the hosts, map to elements in the time series inmyMetricB
, then we can returnmyMetricB
time series for those series wheremy.metricA
is greater than 100.  However, if the time series don’t include elements that map, then it’s not possible to determine what to return if the condition is met.
Examples
The following example set starts with a simple metric, which looks like this:
Now we use an if
condition that returns a value of 50 for any point that’s greater than 100, and a value of 25 otherwise. Here, the behavior of the two time series diverges: For one, the value alternates between 25 and 50. The other time series is always greater than 100 and displays a single orange line.
Finally, we look at an example that does not use an else
expression. For this case, we’ve limited the query to one time series, and we can see the interpolation that happens between the different true values.