R Conditional evaluation when using the pipe operator %>%

Question

When using the pipe operator %>% with packages such as dplyr, ggvis, dycharts, etc, how do I do a step conditionally? For example;

step_1 %>% step_2 %>%  if(condition) step_3 

These approaches don't seem to work:

step_1 %>% step_2  if(condition) %>% step_3  step_1 %>% step_2 %>% if(condition) step_3 

There is a long way:

if(condition) { step_1 %>% step_2  }else{ step_1 %>% step_2 %>% step_3 } 

Is there a better way without all the redundancy?

Answer 1

Here is a quick example that takes advantage of the . and ifelse:

X<-1 Y<-T  X %>% add(1) %>% { ifelse(Y ,add(.,1), . ) } 

In the ifelse, if Y is TRUE if will add 1, otherwise it will just return the last value of X. The . is a stand-in which tells the function where the output from the previous step of the chain goes, so I can use it on both branches.

Edit As @BenBolker pointed out, you might not want ifelse, so here is an if version.

X %>%  add(1) %>%   {if(Y) add(.,1) else .} 

Thanks to @Frank for pointing out that I should use { braces around my if and ifelse statements to continue the chain.