CODEWITHSUNDEEP
rdataframer-faqdollar-sign
this.is.not.a.nick
this.is.not.a.nick

Reputation: 2701

Dollar sign before a variable

I have this sample code to create a new data frame 'new_data' from the existing data frame 'my_data'.

new_data = NULL
n = 10 #this number correspond to the number of rows in my_data
conditions = c("Bas_A", "Bas_T", "Oper_A", "Oper_T") # the vector characters correspond to the target column names in my_data
for (cond in conditions){
    for (i in 1:n){
        new_data <- rbind(new_data, c(cond, my_data$cond[i]))
    }
}

The problem is that my_data$cond (where cond is a variable, and not the column name) is not accepted.

How can I call a column of a data frame by using, after the dollar sign, a variable value?

Upvotes: 30

Views: 60502

Answers (3)

Jossie Calderon
Jossie Calderon

Reputation: 1415

$ works on columns, not individual column objects. It's a form of vectorization. The code

corrections$BookDate = as.Date(corrections$BookDate, format = "%m/%d/%Y")

converts the contents of the BookDate column of the corrections table from strings to Date objects. It performs it in one operation, assignment.

Do the following and it will fix your problem:

new_data <- rbind(new_data, c(cond, my_data$cond))

Upvotes: 1

rankthefirst
rankthefirst

Reputation: 1488

I guess you need get().

For example,
get(x,list), where list is the list and x is the variable(can be a string), which equals list$x.

But in get(x,list), x can be a variable while using $, x cannot be a variable.

Upvotes: 5

Sven Hohenstein
Sven Hohenstein

Reputation: 81753

To access a column, use:

my_data[ , cond]

or

my_data[[cond]]

The ith row can be accessed with:

my_data[i, ]

Combine both to obtain the desired value:

my_data[i, cond]

or 

my_data[[cond]][i]

Upvotes: 42

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