How to compare two columns and write the result into a third - using R

Question

X1 is the column of the home team.

  goals
    X1 X2
1    1  1
2    4  0
3    0  0
4    2  1
5    1  1
6    1  1
7    1  1
8    0  0
...

If I type the nested elseif, I get the correct result in the console.

> ifelse(goals$X1 > goals$X2, goals$winner <- "h",ifelse(goals$X1 < 
   goals$X2, goals$winner <- "d",goals$winner <- "a"))

  [1] "a" "h" "a" "h" "a" "a" "a" "a" "h" "h" "h" "h" "h" "a" "h" "a" "a" "h"
 [19] "h" "a" "h" "h" "h" "d" "h" "h" "h" "h" "a" "a" "a" "d" "d" "h" "d" "h"
 [37] "h" "h" "a" "a" "h" "h" "a" "h" "h" "h" "h" "h" "h" "h" "a" "h" "h" "h"
 [55] "h" "a" "h" "h" "h" "h" "h" "a" "h" "h" "h" "h" "d" "h" "h" "a" "h" "a"
 [73] "d" "h" "d" "h" "h" "h" "h" "h" "h" "a" "d" "h" "d" "h" "d" "d" "d" "a"
 [91] "h" "h" "h" "d" "a" "h" "h" "h" "h" "h" "h" "h" "d" "h" "h" "a" "d" "h"
[109] "h" "a" "d" "h" "h" "h" "h" "h" "a" "d" "d" "h" "h" "h" "h" "h" "h" "h"
[127] "h" "a" "h" "h" "h" "h" "h" "a" "h" "a"

However, if I check the data frame it's not correct because I have only 'a''s.

> goals
    X1 X2 winner
1    1  1      a
2    4  0      a
3    0  0      a
4    2  1      a
5    1  1      a
6    1  1      a
7    1  1      a
8    0  0      a
...

Where am I wrong?

Solution

You are not using ifelse in a correct fashion:

ifelse(test_expression,x,y)

Here, test_expression must be a logical vector (or an object that can be coerced to logical). The return value is a vector with the same length as test_expression.

This returned vector has element from x if the corresponding value of test_expression is TRUE or from y if the corresponding value of test_expression is FALSE.You should not try to assign the value inside ifelse here, rather entire ifelse should be assigned to a new object entirely.

goals$winner <- ifelse(goals$x1 > goals$x2, "h", ifelse(goals$x1 < goals$x2, "d", "a"))

Search for ?ifelse in R console for better understanding.

You can avoid ifelse statement with subsetting:

df <- data.frame(x1 = c(1,4,0,2,1,1,1,0),x2=c(1,0,0,1,1,1,1,0))
df$winner <- NULL
df[df$x1 > df$x2,"winner"] <- "h"
df[df$x1 < df$x2,"winner"] <- "d"
df[df$x1 == df$x2,"winner"] <- "a"

Answer:

df
  x1 x2 winner
1  1  1      a
2  4  0      h
3  0  0      a
4  2  1      h
5  1  1      a
6  1  1      a
7  1  1      a
8  0  0      a
> 

Comments

[Ronak Shah]

goals$winner <- ifelse(goals$X1 > goals$X2, "h", ifelse(goals$X1 < goals$X2, "d", "a"))

[JohnnyS]

Thanks that solved it. Together with Kabirs explanation I was able to learn a lot!