Setting multiple if statements correctly

Question

I have difficulty to set proper nested if statement in a user-defined function.

My sample data is like this

test <- data.frame(x=rev(0:10),y=10:20)

if_state <- function(x,y) {
  if (x==min(x) && y==max(y)) {
    "good"
  } else if (max(x)/2==y[which(y==15)]/3) {  # to find when x=5 and y=5 condition if it is true set class to "y==5"
    "y==5"
  }
    NA
}

   > test
    x  y
1  10 10
2   9 11
3   8 12
4   7 13
5   6 14
6   5 15
7   4 16
8   3 17
9   2 18
10  1 19
11  0 20

library(dplyr)
test %>%
  mutate(class = if_state(x,y))

    x  y class
1  10 10    NA
2   9 11    NA
3   8 12    NA
4   7 13    NA
5   6 14    NA
6   5 15    NA
7   4 16    NA
8   3 17    NA
9   2 18    NA
10  1 19    NA
11  0 20    NA

I don't know why the if statement is not working correctly? The question is what is the base R function that work same as dplyr's case_when ? please see the comments below.

So the expected output

    x  y class
1  10 10    NA
2   9 11    NA
3   8 12    NA
4   7 13    NA
5   6 14    NA
6   5 15    y==5
7   4 16    NA
8   3 17    NA
9   2 18    NA
10  1 19    NA
11  0 20    good

After the if statement, you are returning NA. You need to explicitly return, e.g. return("good") — Jack Brookes
– Jack Brookes, Commented Apr 30, 2018 at 18:45

cmaher · Accepted Answer · 2018-04-30 19:28:06Z

3

R functions return the last value evaluated evaluated during their invocation, even without an explicit call to return (see this answer for more detail); so, where NA is the last value evaluated in your if_state function (as it's outside the if-else if control flow, and so will always be evaluated), it will always return NA, even when the if and else if conditions are true. For your function to work as you expect, you need to move NA into an else statement:

if_state <- function(x,y) {
  if (x == min(x) && y == max(y)) {
    "good"
  } else if (max(x)/2 == y[which(y == 15)]/3) {
    "y==5"
  } else {
    NA 
  }
}

Note that when using dplyr, testing for multiple conditions to determine a return value is often more succinctly accomplished with case_when:

test %>% mutate(class = case_when(
  x == min(x) && y == max(y) ~ "good",
  max(x)/2 == y[which(y == 15)]/3 ~ "y == 5",
  TRUE ~ NA_character_
))

Edit: based on OP's clarification and eipi10's help, here is the final function:

if_state = function(x, y) {
  case_when(x == min(x) && y == max(y) ~ "good", 
            x == max(x)/2 & y/3 == 5 ~ "y==5", 
            TRUE ~ NA_character_)
}

edited Apr 30, 2018 at 19:28

answered Apr 30, 2018 at 18:44

cmaher

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15 Comments

eipi10 Over a year ago

It looks like max(x)/2 == y[which(y == 15)]/3 is always TRUE, so the result will be "y==5" for any rows that don't satisfy the first condition. Maybe the OP actually wanted something like x==max(x)/2 & y/3==5 ~ "y==5"?

Alexander Over a year ago

@cmaher Thank you for explicit answer. wheh I run the your new if_state I am getting all class values to be y==5 ??

eipi10 Over a year ago

See my comment above.

eipi10 Over a year ago

max(x)/2 returns 5 for every row in the data frame. y[which(y == 15)]/3 returns 5 for every row in the data frame. So the condition being evaluated is 5==5 which is always TRUE.

eipi10 Over a year ago

Wrap @cmaher's answer in a function call (and I've changed the second condition as well):

if_state = function(x,y) {case_when(   x == min(x) && y == max(y) ~ "good",   x == max(x)/2 & y/3 == 5 ~ "y==5",   TRUE ~ NA_character_ )}

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