Data Reshaping in R Programming

Generally, in R Programming Language, data processing is done by taking data as an input from a data frame where the data is organized into rows and columns. Data frames are mostly used since extracting data is much simpler and hence easier. But sometimes we need to reshape the format of the data frame from the one we receive. Hence, in R, we can split, merge and reshape the data frame using various functions.

The various forms of reshaping data in a data frame are:

• Transpose of a Matrix
• Joining Rows and Columns
• Merging of Data Frames
• Melting and Casting

Why R – Data Reshaping is Important?

While doing an analysis or using an analytic function, the resultant data obtained because of the experiment or study is generally different. The obtained data usually has one or more columns that correspond or identify a row followed by a number of columns that represent the measured values. We can say that these columns that identify a row can be the composite key of a column in a database.

Transpose of a Matrix

We can easily calculate the transpose of a matrix in R language with the help of t() function. The t() function takes a matrix or data frame as an input and gives the transpose of that matrix or data frame as it’s output.

Syntax:

t(Matrix/ Data frame)

Example:

R

# R program to find the transpose of a matrix

first <- matrix(c(1:12), nrow=4, byrow=TRUE)
print("Original Matrix")
first

first <- t(first)
print("Transpose of the Matrix")
first

Output:

[1] "Original Matrix"
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    5    6
[3,]    7    8    9
[4,]   10   11   12
[1] "Transpose of the Matrix"
     [,1] [,2] [,3] [,4]
[1,]    1    4    7   10
[2,]    2    5    8   11
[3,]    3    6    9   12
 

Joining Rows and Columns in Data Frame

In R, we can join two vectors or merge two data frames using functions. There are basically two functions that perform these tasks:

cbind():

We can combine vectors, matrix or data frames by columns using cbind() function.

Syntax: cbind(x1, x2, x3)

where x1, x2 and x3 can be vectors or matrices or data frames. 

rbind():

We can combine vectors, matrix or data frames by rows using rbind() function.

Syntax: rbind(x1, x2, x3)

where x1, x2 and x3 can be vectors or matrices or data frames.

Example:

R

# Cbind and Rbind function in R
name <- c("Shaoni", "esha", "soumitra", "soumi")
age <- c(24, 53, 62, 29)
address <- c("puducherry", "kolkata", "delhi", "bangalore")

# Cbind function
info <- cbind(name, age, address)
print("Combining vectors into data frame using cbind ")
print(info)

# creating new data frame
newd <- data.frame(name=c("sounak", "bhabani"),
                   age=c("28", "87"),
                   address=c("bangalore", "kolkata"))

# Rbind function
new.info <- rbind(info, newd)
print("Combining data frames using rbind ")
print(new.info)

Output:

[1] "Combining vectors into data frame using cbind "
     name       age  address     
[1,] "Shaoni"   "24" "puducherry"
[2,] "esha"     "53" "kolkata"   
[3,] "soumitra" "62" "delhi"     
[4,] "soumi"    "29" "bangalore" 
[1] "Combining data frames using rbind "
      name age    address
1   Shaoni  24 puducherry
2     esha  53    kolkata
3 soumitra  62      delhi
4    soumi  29  bangalore
5   sounak  28  bangalore
6  bhabani  87    kolkata

Merging two Data Frames

In R, we can merge two data frames using the merge() function provided both the data frames should have the same column names. We may merge the two data frames based on a key value.

Syntax: merge(dfA, dfB, …)

Example:

R

# Merging two data frames in R
d1 <- data.frame(name=c("shaoni", "soumi", "arjun"),
                 ID=c("111", "112", "113"))

d2 <- data.frame(name=c("sounak", "esha"),
                 ID=c("114", "115"))

total <- merge(d1, d2, all=TRUE)
print(total)

Output:

    name  ID
1  arjun 113
2 shaoni 111
3  soumi 112
4   esha 115
5 sounak 114

Melting and Casting

Data reshaping involves many steps in order to obtain desired or required format. One of the popular methods is melting the data which converts each row into a unique id-variable combination and then casting it. The two functions used for this process:

melt():

 It is used to convert a data frame into a molten data frame.

Syntax: melt(data, …, na.rm=FALSE, value.name=”value”)

where, 

data: data to be melted 
… : arguments 
na.rm: converts explicit missings into implicit missings 
value.name: storing values

dcast():

It is used to aggregate the molten data frame into a new form.

Syntax: melt(data, formula, fun.aggregate)

where, 

data: data to be melted 
formula: formula that defines how to cast 
fun.aggregate: used if there is a data aggregation

Example:

R

# melt and cast
library(MASS)
library(reshape)
a <- data.frame(id=c("1", "1", "2", "2"),
                points=c("1", "2", "1", "2"),
                x1=c("5", "3", "6", "2"),
                x2=c("6", "5", "1", "4"))

print("Melting")
m <- melt(a, id=c("id", "point"))
print(m)

print("Casting")
idmn <- dcast(a, id~variable, mean)
print(idmn)

Output:

Melting
id   points   variable   value
1     1         x1          5
1     2         x1          3
2     1         x1          6
2     2         x1          2
1     1         x2          6
1     2         x2          5
2     1         x2          1
2     2         x2          4
Casting
id    x1     x2
1     4      5.5
2     4      2.5