Calculation with multiple column conditional on data being there - r

Question

I am trying to solve is how to calculate the weighted score for each class each month.
Each class has multiple students and the weight (contribution) of a student's score varies through time.
To be included in the calculation a student must have both score and weight.
I am a bit lost and none of the approaches I have used have worked.
Student Class Jan_18_score Feb_18_score Jan_18_Weight Feb_18_Weight
Adam 1 3 2 150 153
Char 1 5 7 30 60
Fred 1 -7 8 NA 80
Greg 1 2 NA 80 40
Ed 2 1 2 60 80
Mick 2 NA 6 80 30
Dave 3 5 NA 40 25
Nick 3 8 8 12 45
Tim 3 -2 7 23 40
George 3 5 3 65 NA
Tom 3 NA 8 78 50
The overall goal is to calculate the weighted score for each class each month.
Taking Class 1 (first 4 rows) as an example and looking at Jan_18.
-The observations of Adam, Char and Greg are valid since they have both scores and weights. Their scores and weights should be included
- Fred does not have a Jan_18_weight, therefore both his Jan_18_score and Jan_18_weight are excluded from the calculation.
The following calculation should then occur:
= [(3*150)+(5*30)+(2*80)]/ [150+30+80]
= 2.92307
This calculation would be repeated for each class and each month.
A new dataframe something like the following should be the output
Class Jan_18_Weight_Score Feb_18_Weight_Score
1 2.92307 etc
2 etc etc
3 etc etc
There are many columns and many rows.
Any help is appreciated.

Answer 1

Here's a way with tidyverse. The main trick is to replace NA with 0 in "weights" columns and then use weighted.mean() with na.rm = T to ignore NA scores. To do so, you can gather the scores and weights into a single column and then group by Class and month_abb (a calculated field for grouping) and then use weighted.mean().
df %>%
mutate_at(vars(ends_with("Weight")), ~replace_na(., 0)) %>%
gather(month, value, -Student, -Class) %>%
group_by(Class, month_abb = paste0(substr(month, 1, 3), "_Weight_Score")) %>%
summarize(
weight_score = weighted.mean(value[grepl("score", month)], value[grepl("Weight", month)], na.rm = T)
) %>%
ungroup() %>%
spread(month_abb, weight_score)
# A tibble: 3 x 3
Class Feb_Weight_Score Jan_Weight_Score
<int> <dbl> <dbl>
1 1 4.66 2.92
2 2 3.09 1
3 3 7.70 4.11
Data -
df <- structure(list(Student = c("Adam", "Char", "Fred", "Greg", "Ed",
"Mick", "Dave", "Nick", "Tim", "George", "Tom"), Class = c(1L,
1L, 1L, 1L, 2L, 2L, 3L, 3L, 3L, 3L, 3L), Jan_18_score = c(3L,
5L, -7L, 2L, 1L, NA, 5L, 8L, -2L, 5L, NA), Feb_18_score = c(2L,
7L, 8L, NA, 2L, 6L, NA, 8L, 7L, 3L, 8L), Jan_18_Weight = c(150L,
30L, NA, 80L, 60L, 80L, 40L, 12L, 23L, 65L, 78L), Feb_18_Weight = c(153L,
60L, 80L, 40L, 80L, 30L, 25L, 45L, 40L, NA, 50L)), class = "data.frame", row.names = c(NA,
-11L))

Answer 2

Maybe this could be solved in a much better way but here is one Base R option where we perform aggregation twice and then combine the results.
#Separate score and weight columns
score_cols <- grep("score$", names(df))
weight_cols <- grep("Weight$", names(df))
#Replace NA's in corresponding score and weight columns to 0
inds <- is.na(df[score_cols]) | is.na(df[weight_cols])
df[score_cols][inds] <- 0
df[weight_cols][inds] <- 0
#Find sum of weight columns for each class
df1 <- aggregate(.~Class, cbind(df["Class"], df[weight_cols]), sum)
#find sum of multiplication of score and weight columns for each class
df2 <- aggregate(.~Class, cbind(df["Class"], df[score_cols] * df[weight_cols]), sum)
#Get the ratio between two dataframes.
cbind(df1[1], df2[-1]/df1[-1])
# Class Jan_18_score Feb_18_score
#1 1 2.92 4.66
#2 2 1.00 3.09
#3 3 4.11 7.70