The Ultimate R Cheat Sheet - Data Management (Page 4 of 4) in pdf

Loops and automation



v1=vector(length=20) initializes an empty vector with 20 elements. This is often required as an

initial statement to subsequently write results of a loop into an output vector or output table.



m1=matrix(nrow=20, ncol=10) similarly initializes an empty matrix with 20 rows and 10

columns.



for (i in 1:10) {

one or more operations with v1[i] or m1[,i] }



for (i in 1:10) { for (j in 1:20) { one or more operations with m1[j,i] }}



Example for an application, where a for-loop is used to calculate cumulative values. Copy and paste

the code below into R to see what it does.

dat=round(rnorm(10)+2)

cum=vector(length=10)

cum[1]=dat[1]

for (i in 2:length(dat)) { cum[i]=cum[i-1]+dat[i] }

cbind(dat,cum)



Example for an application where a for-loop allows automatic data processing of multiple files in a

directory. This batch-converts DBF format files into CSV format files. With similar code, you could

merge a large number of files into one master file, or do manipulations or analysis on multiple files

consecutively.

library(foreign)

setwd("C:/your path/")

a<-list.files(); a

for (name in a) { dat1=read.dbf(name)

write.csv(dat1, paste(name,".csv"), row.names=F, quote=F) }

Handy built-in functions



paste("hello", "world") joins vectors after converting them to characters. The sep="" option

can place any character string or nothing between values (a single space is the default)



substr("Year 1998",6,9) extracts characters from start to stop position from vector



tolower("Year 1998") convert to lowercase - handy to correct inconsistencies in data entry.



toupper("Year 1998") convert to uppercase



nchar("Year 1998") number of characters in a string, allows you to substring the last four digits

of a variable regardless of length, for example: substr(VAR1,nchar(VAR1)-3,nchar(VAR1))



Plenty of math functions, of course: log(VAR1), log10(VAR1), log(VAR1,2), exp(VAR1),

sqrt(VAR1), abs(VAR1), round(VAR1,2)

Programming custom functions



You can program your own functions, if something is missing, or if you want to utilize a bunch of code

over and over to make similar calculations. Here is a clever example for calculating the statistical

mode of a variable, which is missing from the built-in R functions.

mode=function(input){ freq=table(as.vector(input))

descending_freq=sort(-freq)

mode=names(descending_freq)[1]

as.numeric(mode)

}

VAR1=c(1,3,3,2,3,2,2,3,5,3)

mode(VAR1)

More information, help, and on-line resources



Adding a question mark before a command or functions brings up a help file. E.g. ?paste. Be sure to

check out the example code at the end of the help file, which often helps to understand the syntax.



More information and R resources can be found with the search engine

The Ultimate R Cheat Sheet - Data Management Page 4

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