从数据框中提取具有最小值或最大值的行

Question

Im working on a crash course for R at https://bioinformatics-core-shared-training.github.io/r-crash-course/crash-course.nb.html

The problem im facing is to extract rows that are min or max for a certain value.

For example, when running

df[df$tmp ==min(df$tmp),]

I get the correct row with the expected value.

However, when running the following code

df[min(df$tmp),]

I get something else completely.

Im wondering what is causing this discrepancy?

Answer 1

Assuming df$Tmp is numeric with no NAs, min(df$Tmp) should be returning a number. Assuming that number is an integer, i, df[min(df$Tmp),] will return the ith row of your data frame, assuming that your data frame has an ith row.

On the other hand, df[df$Tmp ==min(df$tmp),] will return the row(s) of df where df$Tmp is equal to the minimum value in that column.

Answer 2

df[df$Tmp ==min(df$tmp),] is the correct approach to get what you are looking for.

df[min(df$Tmp),] returns the row in df that is equal to min(df$Tmp) . It may result in an error in certain cases for eg when min(df$Tmp) is not an integer, or is negative, or if it is greater than the number of rows in df etc. Hope this makes sense.