Multiplying column value by another value matching column name R - r

Question

I have a data frame which looks like this:
Value1 = c("1","2","1","3")
Letter = c("A","B","B","A")
A = c("2","2","0","1")
B = c("1","1","1","0")
data <- data.frame(Value1,Letter,A,B)
data
Value1 Letter A B
1 1 A 2 1
2 2 B 2 1
3 1 B 0 1
4 3 A 1 0
I'm trying to add a new column which is the multiplication of column Value1, by column A or B depending on what is in the Letter column. The expected result would be:
Value1 Letter A B Results
1 1 A 2 1 2
2 2 B 2 1 2
3 1 B 0 1 1
4 3 A 1 0 3
I'm trying to use the match() function, but without success.
Thanks!

Answer 1

With base R:
data <- type.convert(data, as.is = TRUE)
data$Results <- ifelse(data$Letter == 'A', data$A * data$Value1, data$B * data$Value1)
Output
Value1 Letter A B Results
1 1 A 2 1 2
2 2 B 2 1 2
3 1 B 0 1 1
4 3 A 1 0 3
Another option would be to pivot to long form, do the calculation, then pivot back to wide format.
library(tidyverse)
data %>%
type.convert(as.is = TRUE) %>%
pivot_longer(c(A, B)) %>%
mutate(Results = ifelse(Letter == name, value * Value1, NA_integer_)) %>%
pivot_wider(names_from = "name", values_from = "value") %>%
group_by(Value1, Letter) %>%
summarise_all(discard, is.na)
Output
Value1 Letter Results A B
<int> <chr> <int> <int> <int>
1 1 A 2 2 1
2 1 B 1 0 1
3 2 B 2 2 1
4 3 A 3 1 0

Answer 2

Use case_when or ifelse
library(dplyr)
data <- data %>%
type.convert(as.is = TRUE) %>%
mutate(Results = case_when(Letter == 'A' ~ A * Value1,
TRUE ~ B * Value1))
-output
data
Value1 Letter A B Results
1 1 A 2 1 2
2 2 B 2 1 2
3 1 B 0 1 1
4 3 A 1 0 3
Or use get with rowwise
data <- data %>%
type.convert(as.is = TRUE) %>%
rowwise %>%
mutate(Result = get(Letter) * Value1) %>%
# or may also use
# mutate(Result = cur_data()[[Letter]] * Value1) %>%
ungroup
-output
data
# A tibble: 4 × 5
Value1 Letter A B Result
<int> <chr> <int> <int> <int>
1 1 A 2 1 2
2 2 B 2 1 2
3 1 B 0 1 1
4 3 A 1 0 3
In base R, we may use row/column indexing as vectorized option
data <- type.convert(data, as.is = TRUE)
nm1 <- unique(data$Letter)
data$Results <-data[nm1][cbind(seq_len(nrow(data)),
match(data$Letter, nm1))] * data$Value1