一列的值基于R中另一列的值

Question

I have to find value minimum value in one column and based on that value I have to find the value corresponding in another column.

my data set is K and i have column name as X, Y:

> K
   X  Y
1  2  3
2  4  5
3  6  7
4  8  9
5 10 11

The columns have these values and i find the minimum value of column Y using

apply(K[c(2)],2,min) # this gives me 3.

now i have to relate it to column X which i am finding difficult.

I am totally new to R and i am still learning. Also i don't know any other libraries other than, library(readr) .

Answer 1

Assuming that you want to get the entry in X which is placed in the same row as the minimum of Y , you can try:

# create example data.frame:
K <- data.frame(X = seq(2, 10, 2), Y = seq(3, 11, 2))

# find index of minimum entry in column Y:
idx <- match(min(K$Y), K$Y) # gives you the first entry of the minimum
idx2 <- which(min(K$Y) %in% K$Y) # gives you all indices of the minimum

# output the corresponding element (or elements) in column X:
K$X[idx]
K$X[idx2]

Regarding the second part of your question, Datacamp has a free introductory course to R covering the very basics.

Answer 2

No need for apply . If you know there is only one value of Y that is the lowest or you need the first record of the lowest Y, you can use which.min . Otherwise use min as this is more robust. In your example it doesn't matter, but test both methods on your full data.frame to see if there is a difference.

K[K$Y == min(K$Y), ]
  X Y
1 2 3

K[which.min(K$Y), ]
  X Y
1 2 3

Answer 3

Really depends on what you mean by "relate". Let's say you want to find all rows where X is less than the minimum of row Y:

 K[K$X<min(K$Y),]

Will result in:

  X Y
1 1 2

The command is a subset command, it asks to give all the columns of any row where X is smaller than the minimum of the entire column of Y.

Answer 4

I had the same problem, and there's a "tidier" way to do this with a mutate and filter:

    K %>% mutate(minY=min(Y)) %>% filter(Y=minY).

This can be used with group_by as well if you need the minimum record for each group.

Answer 5

If you are willing to explore other libraries (which eventually you will...), data.table is the one worth exploring. with data.table -

k <- data.table(
  X = c(2, 4, 6, 8, 10),
  Y = c(3, 5, 7, 9, 11)
)

will give you -

Description:data.table [5 × 2]
X         Y
<dbl>   <dbl>
2          3            
4          5            
6          7            
8          9            
10         11           
5 rows

Now to find minimum of Y and corresponding X value, you just need this -

k[, .(min_Y = min(Y), corr_X = X[which.min(Y)])]

which will give you -

Description:data.table [1 × 2]
min_Y corr_X
<dbl> <dbl>
  3     2