Find if a specific choice is in a Data Frame R - r

Question

I have a Data Frame object which contains a list of possible choices. For example, an analogy of this would be:
FirstName, SurName, Subject, Grade
Brian, Smith, History, 75
Jenny, Jackson, English, 60
How would I...
1) Check to see if a certain pupil-subject combination is in my Data Frame
2) And for those who are there, extract their grade (And potentially other relevant fields)
?
Thanks so much
The only solutions I've found so far include appending the values onto the end of the Data Frame and trying to see if it is unique or not? This seems a crude and ridiculous hack?

Answer 1

learn data subset (extraction) using base R.
To subset any data frame by its rows and column you use [ ]
Let df be your data frame.
FirstName SurName Subject Grade
1 Brian Smith History 75
2 Jenny Jackson English 60
3 Tom Brandon Physics 50
You can subset it by its rows and columns using
df[rows,columns]
Here rows and column can be :
1) Index (Number/Name)
Which means subset that give me that particular row and column like
df[2,3]
this will return second row and third column
[1] English
or
df[2,"Grade"]
returns
[1] 60
2) Range (Indices/List of Names)
Which means subset that give me these rows and columns like
df[1:2,2,drop=F]
Here drop=F to avoid flattening of result and output like a data.frame. It will give you this
SurName
1 Smith
2 Jackson
Range also supports all by leaving either rows or columns empty like
df[,3,drop=F]
this will return all rows for third column
Subject
1 History
2 English
3 Physics
or
df[1:2,c("Grade","Subject")]
Grade Subject
1 75 History
2 60 English
3) Logical
Which means you want to subset using a logical condition.
df[df$FirstName=="Brian",]
meaning give me rows where FirstName is Brian and all columns for it.
FirstName SurName Subject Grade
1 Brian Smith History 75
or
df[df$FirstName=="Brian",1:3]
give me rows where FirstName is Brian and give me only 1 to 3 columns.
or create complex logicals
df[df$FirstName=="Brian" & df$SurName==" Smith",1:3]
output
FirstName SurName Subject
1 Brian Smith History
or complex logical and extract column by name
df[df$FirstName=="Brian" & df$SurName==" Smith","Grade",drop=F]
Grade
1 75
or complex logical and extract multiple columns by name
df[df$FirstName=="Brian" & df$SurName==" Smith",c("Grade","Subject")]
Grade Subject
1 75 History
to use this in a function do
myfunc<-function(input_var1,input_var2,input_var3)
{
df[df$FirstName==input_var1 & df$SurName==input_var2 & df$Subject==input_var3,"Grade",drop=F]
}
run it like this
myfunc("Tom","Brandon","Physics")

Answer 2

I think you are looking for this:
result <- data[data$FirstName == "Brian" & data$Subject == "History", c("Grade") ]

Answer 3

Try subset:
con <- textConnection("FirstName,SurName,Subject,Grade\nBrian,Smith,History,75\nJenny,Jackson,English,60")
dat <- read.csv(con, stringsAsFactors=FALSE)
subset(dat, FirstName=="Brian" & SurName=="Smith" & Subject=="History", Grade)
Maybe aggregate can be helpful, too. The following code gives the mean of the grades for all pupil/subject combinations:
dat <- transform(dat, FullName=paste(FirstName, SurName), stringsAsFactors=FALSE)
aggregate(Grade ~ FullName+Subject, data=dat, FUN=mean)