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Data Reshaping in R Programming
  • Last Updated : 30 Jun, 2020

Generally, in R, data processing is done by taking data as an input from 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 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 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:




    # 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:




    # 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 method 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:




      # 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
      
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