Data Frame

A data frame is a combination of different vectors. 

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x = c(1, 2, 3)  
y = c("a", "b", "c")  
z = c(TRUE, FALSE, TRUE)  
co = data.frame(x, y, z)

Column Slice

  • Numeric Indexing
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> co[2]  
  y  
1 a  
2 b  
3 c
  • Name Indexing
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> co["y"]  
  y  
1 a  
2 b  
3 c

Row Slice

  • Numeric Indexing
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> co[2,]  
  x y     z  
2 2 b FALSE

To retrieve more than one rows at one time 

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> co[c(2,1),]  
  x y     z  
2 2 b FALSE  
1 1 a  TRUE
  • Name Index

Row Slice also could be retrieved by name indexing.

  • Logical Indexing
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> L  
[1] FALSE TRUE FALSE  
> co[L,"x"]  
[1] 2

Subset

subset() function could return subsets of vectors, matrices or data frames which meet conditions. 

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> co1 <- subset(co, select = y)  
> co1  
  y  
1 a  
2 b  
3 c

Select indicating columns to select in a data frame

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> co2 <- subset(co, x > 1, select = y)  
> co2  
  y  
2 b  
3 c

subset logical expression indicating elements or row to keep

Statistic of a data set

To get the number of row and columns, nrow(), ncol(), NROW(), NCOL() could be used.
nrow() and ncol() could count for vector, array or data frame, NROW() NCOL() count for 1-column matrix.