R | 根据另一列中的条件更改列中的元素

Question

This might be a basic question so apologies beforehand. I have a database with following columns

Name   Subject   Marks   Check
Mark   English   45      P
Susan  English   23      F
Gerald Math      33      P

I want to add 10 to value in column 'Marks' if Check == 'F'. There are many such rows so I don't want to apply a for loop

Answer 1

In base R , we can use a simple arithmetic approach to update

df$Marks <- with(df, (Check == 'F') * 10 + Marks)

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Or create a logical condition in base R and do the assignment

i1 <- df$Check == 'F'
df$Marks[i1] <- df$Marks[i1] + 10

-output

df
#    Name Subject Marks Check
#1   Mark English    45     P
#2  Susan English    33     F
#3 Gerald    Math    33     P

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Or in data.table way, the assignment operation is more simple as the general syntax of data.table is [i, j, by] where i can be integer index or logical condition. Here, we specify the logical condition ( Check == 'F' ), specify the j ( Marks + 10 ) and assign ( := ) it to the same column. The original data.frame is converted to data.table with setDT

library(data.table)
setDT(df)[Check == 'F', Marks := Marks + 10]

The evaluation happens from left to right ie first it converts to data.table ( setDT ), evaluates the i , do the assignment on the expression ( Marks + 10 ) on the 'Marks' column

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Or with dplyr

library(dplyr)
df %>%
      mutate(Marks = case_when(Check == 'F' ~ Marks + 10L, TRUE ~ Marks))

data

df <- structure(list(Name = c("Mark", "Susan", "Gerald"), Subject = c("English", 
"English", "Math"), Marks = c(45L, 23L, 33L), Check = c("P", 
"F", "P")), class = "data.frame", row.names = c(NA, -3L))

Answer 2

You can also use ifelse() :

#Base R sol
df$Marks <- ifelse(df$Check=='F',df$Marks+10,df$Marks)

Output:

    Name Subject Marks Check
1   Mark English    45     P
2  Susan English    33     F
3 Gerald    Math    33     P