Accessing the values by their rowname and columnname,instead of numbers - r

Question

I have a table which has multiple columns and rows. I want to access the each value by its column name and rowname, and make a plot with these values.
The table looks like this with 101 columns:
IDs Exam1 Exam2 Exam3 Exam4 .... Exam100
Ellie 12 48 33 64
Kate 98 34 21 76
Joe 22 53 49 72
Van 77 40 12
Xavier 88 92
What I want is to be able to reach the marks for given row (IDs),and given column(exams) as:
table[Ellie,Exam3] --> 48
table[Ellie,Exam100] --> 64
table[Ellie,Exam2] --> (empty)
Then with these numbers, I want to see the distribution of how Ellie did comparing the rest of exams to Exam2,3 and 100.
I have almost figured out this part with R:
library(data.table)
library(ggplot2)
pdf("distirbution_given_row.pdf")
selectedvalues <- c(table[Ellie,Exam3] ,table[Ellie,Exam100])
library(plyr)
cdat <- ddply(selected values, "IDs", summarise, exams.mean=mean(exams))
selectedvaluesggplot <- ggplot(selectedvalues, aes(x=IDs, colour=exams)) + geom_density() + geom_vline(data=cdat, aes(xintercept=exams.mean, colour=IDs), linetype="dashed", size=1)
dev.off()
Which should generate the Ellie's marks for exams of interests versus the rest of the marks ( if it is a blank, then it should not be seen as zero. It is still a blank.)
Red: Marks for Exam3, 100 and 2 , Blue: The marks for the remaining 97 exams
(The code and the plot are taken as an example of ggplot2 from this link.)
All ideas are appreciated!

Answer 1

For accessing your data at least you can do the following:
df=data.frame(IDs=c("Ellie","Kate","Joe","Van","Xavier"),Exam1=c(12,98,22,77,NA),Exam2=c(NA,34,53,NA,NA),
Exam3=c(48,21,49,40,NA),Exam4=c(33,76,NA,12,88))
row.names(df)=df$IDs
df=df%>%select(-IDs)
> df['Joe','Exam2']
[1] 53
Now I prepared an example with random created numbers to illustrate a bit what you could do. First let us create an example data frame
df=as.data.frame(matrix(rnorm(505,50,10),ncol=101))
colnames(df)=c("IDs",paste0("Exam",as.character(1:100)))
df$IDs=c("Ellie","Kate","Joe","Van","Xavier")
To work with ggplot it is recomended to convert it to long format:
df0=df%>%gather(key="exams",value="score",-IDs)
From here on you can play with your variables as desired. For instance plotting the density of the score per ID:
ggplot(df0, aes(x=score,col=IDs)) + geom_density()
or selecting only Exams 2,3,100 and plotting density for different exams
df0=df0%>%filter(exams=="Exam2"|exams=="Exam3"|exams=="Exam100")
ggplot(df0, aes(x=score,col=exams)) + geom_density()

Answer 2

IIUC - you want to plot each IDs select exams with all else exams. Consider the following steps:
Reshape your data to long format even replace NAs with zero as needed.
Run by() to subset data by IDs and build mean aggregrate data and ggplots.
Within by, create a SelectValues indicator column on the select exams then graph with vertical line mean summation.
Data
txt = 'IDs Exam1 Exam2 Exam3 Exam4 Exam100
Ellie 12 NA 48 33 64
Kate 98 34 21 76 NA
Joe 22 53 49 NA 72
Van 77 NA 40 12 NA
Xavier NA NA NA 88 92'
exams_df <- read.table(text=txt, header = TRUE)
# ADD OTHER EXAM COLUMNS (SEEDED FOR REPRODUCIBILITY)
set.seed(444)
exams_df[paste0("Exam", seq(5:99))] <- replicate(99-4, sample(100, 5))
Reshape and Graph
library(ggplot2) # ONLY PACKAGE NEEDED
# FILL NA
exams_df[is.na(exams_df)] <- 0
# RESHAPE (BASE R VERSION)
exams_long_df <- reshape(exams_df,
timevar = "Exam",
times = names(exams_df)[grep("Exam", names(exams_df))],
v.names = "Score",
varying = names(exams_df)[grep("Exam", names(exams_df))],
new.row.names = 1:1000,
direction = "long")
# GRAPH BY EACH ID
by(exams_long_df, exams_long_df$IDs, FUN=function(df) {
df$SelectValues <- ifelse(df$Exam %in% c("Exam1", "Exam3", "Exam100"), "Select Exams", "All Else")
cdat <- aggregate(Score ~ SelectValues, df, FUN=mean)
ggplot(df, aes(Score, colour=SelectValues)) +
geom_density() + xlim(-50, 120) +
labs(title=paste(df$IDs[[1]], "Density Plot of Scores"), x ="Exam Score", y = "Density") +
geom_vline(data=cdat, aes(xintercept=Score, colour=SelectValues), linetype="dashed", size=1)
})
Output