How to modify data frame using transform() function in R

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Similar as with() function in R, transform() function provides a way to work on data frame with simplified code inside its block. transform() is mostly used to modify a data frame in a concise and explicit manner. The basic form of usage is transform(df, new_var = operation)

Where

df is the object data frame,

new_var is the variable exist in the resulting data frame,

and operation upon the current variables in the data frame.

In the following example, all the elements of variable ‘age’ in data frame familymember are increased with number 1.

#create a data frame
familymember <- data.frame( name=c("Wilson","Dudu","Maomao","Miaomiao","Mico","Mia"),
                    age=c(32,20,22, 12,8,3))
#show data frame
familymember
#output
     name age
1   Wilson  32
2     Dudu  20
3   Maomao  22
4 Miaomiao  12
5     Mico   8
6      Mia   3
# Apply transform function to increase age with 1
familymember <- transform(familymember, age = age + 1) 
#show data frame again 
familymember
#output
      name age
1   Wilson  33
2     Dudu  21
3   Maomao  23
4 Miaomiao  13
5     Mico   9
6      Mia   4

In the next example of testing grades for students, two new variables, 'total_grade' and 'mean_grade' are created easily with the application of transform() block. 

#show first observations of original data frame
head(grade)
#output
 StudentID        Fullname Race Gender Country Age Math Physics
1         1     James Zhang    A   Male      US  23   73      70
2         2       Wilson Li    E Female      UK  26   95      76
3         3 Richard Nuan Ye    A   Male      UK  35   77      83
4         4       Mary Deng    E Female      US  21   60      99
5         5    Jason Wilson    A   Male      UK  19   77      89
6         6 Jennifer Hopkin    A Female      UK  43   79      64
  Chemistry       Date
1        87 10/31/2008
2        83  3/16/2008
3        92  5/22/2008
4        84  1/24/2009
5        93  7/30/2009
6        83   4/5/2009
#create two new variables
grade$total_grade <- grade$Math + grade$Physics + grade$Chemistry
grade$mean_grade <- (grade$Math + grade$Physics + grade$Chemistry)/3
#show first observations of data frame again
head(grade)
#output
 StudentID        Fullname Race Gender Country Age Math Physics
1         1     James Zhang    A   Male      US  23   73      70
2         2       Wilson Li    E Female      UK  26   95      76
3         3 Richard Nuan Ye    A   Male      UK  35   77      83
4         4       Mary Deng    E Female      US  21   60      99
5         5    Jason Wilson    A   Male      UK  19   77      89
6         6 Jennifer Hopkin    A Female      UK  43   79      64
  Chemistry       Date total_grade mean_grade
1        87 10/31/2008         230   76.66667
2        83  3/16/2008         254   84.66667
3        92  5/22/2008         252   84.00000
4        84  1/24/2009         243   81.00000
5        93  7/30/2009         259   86.33333
6        83   4/5/2009         226   75.33333

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