The structure of data frames

Data frame construction 

# First create a few vectors from which we construct the data frame:
population  <- c(55619400,1885400,5424800,3125000)
area.sq.m   <- c(50301,5460,30090,8023)
GVA.cap     <- c(28096,20000,24800,19900)
# then we use 'data.frame' to construct the data frame:
UK <- data.frame(population,area.sq.m,GVA.cap)
UK

  population area.sq.m GVA.cap
1 55619400   50301     28096  
2  1885400    5460     20000  
3  5424800   30090     24800  
4  3125000    8023     19900  

names(UK)
names(UK) <- c("Population","Area","GVA")
UK

[1] "population" "area.sq.m"  "GVA.cap"   

  Population Area  GVA  
1 55619400   50301 28096
2  1885400    5460 20000
3  5424800   30090 24800
4  3125000    8023 19900

row.names(UK)

[1] "1" "2" "3" "4"

row.names(UK) <- c("England",
                   "Northern Ireland",
                   "Scotland",
                   "Wales")
UK

                 Population Area  GVA  
England          55619400   50301 28096
Northern Ireland  1885400    5460 20000
Scotland          5424800   30090 24800
Wales             3125000    8023 19900

# It is also possible to set the names and row names in the data frame explicitly, when this
# appears more convenient:
UK <- data.frame(
           Population = c(55619400,1885400,5424800,3125000),
           Area = c(50301,5460,30090,8023),
           GVA = c(28096,20000,24800,19900),
           row.names = c("England",
                         "Northern Ireland",
                         "Scotland",
                         "Wales"))
UK

                 Population Area  GVA  
England          55619400   50301 28096
Northern Ireland  1885400    5460 20000
Scotland          5424800   30090 24800
Wales             3125000    8023 19900

nrow(UK)

[1] 4

ncol(UK)

[1] 3

dim(UK)

[1] 4 3

In what follows we treat the data frame ‘UK’ as a list: 

# Here we get the variable 'Population':
UK$Population

[1] 55619400  1885400  5424800  3125000

# Analoguously, one can use the double bracket-operator ('[[]]')
# to get the variable 'Population':
UK[["Population"]]

[1] 55619400  1885400  5424800  3125000

# Also the single bracket-operator works as with lists.
# We get a data frame of the first two variables in
# the data frame
UK[1:2]

                 Population Area 
England          55619400   50301
Northern Ireland  1885400    5460
Scotland          5424800   30090
Wales             3125000    8023

# Now we get a data frame with the variables named 'Population' and
# 'GVA'
UK[c("Population","GVA")]

                 Population GVA  
England          55619400   28096
Northern Ireland  1885400   20000
Scotland          5424800   24800
Wales             3125000   19900

In the next few lines show the selection of rows and columns of a data frame 

# We select the first two rows of the
# data frame 'UK' by just using their numbers:
UK[1:2,]

                 Population Area  GVA  
England          55619400   50301 28096
Northern Ireland  1885400    5460 20000

# By referring to row names, we select Scotland and Wales:
UK[c("Scotland","Wales"),]

         Population Area  GVA  
Scotland 5424800    30090 24800
Wales    3125000     8023 19900

# As in a previous example, we select the first two columns ...
UK[,1:2]

                 Population Area 
England          55619400   50301
Northern Ireland  1885400    5460
Scotland          5424800   30090
Wales             3125000    8023

# and the variables named 'Population' and 'GVA'
UK[,c("Population","GVA")]

                 Population GVA  
England          55619400   28096
Northern Ireland  1885400   20000
Scotland          5424800   24800
Wales             3125000   19900