Video 4

This section is optional. We consider here the conditional operators (if/else) and loops, which can be avoided working in R.

4.1. Conditions

a=1
b=2

if (a==b) {
  print("a equals to b")
} else {
  print("a is not equal to b")
}

## use if in-a-line
ifelse(a==b, "equal", "different")

4.2. Loops

Using loops in R is not recommended. You should avoid them, when possible, because of very slow execution. R is interpretable language, not precompiled (opposite to C).

4.2.1. FOR

Runs the same code for each values of the ‘iterator’.

# load some data
Shop = read.table("http://edu.modas.lu/data/txt/shop.txt",header=T,sep="\t")

# print all information for the first client
for (i in 1:ncol(Shop) ){
  print(Shop[1,i])
}

4.2.2. WHILE loop

Similar to FOR, but without iterator (we have to introduce it)

# print all information for the first client
i=1;
while (i <= ncol(Shop)){
  print(Shop[1,i])
  i=i+1
}

4.2.3. REPEAT loop

i=1
repeat {
  print(i)
  i=i+1
  if (i>10) break
}

Keywords break (stop loop) and next (switch to the next iteration) help to control the workflow.


4.3. Custom functions

Let us write own function to print vectors.

printVector = function(x, name=""){
  print(paste("Vector",name,"with",length(x),"elements:")) # print header
  if (length(x)>0) # if vector is not empty
    for (i in 1:length(x)) # for each element
      print(paste(name,"[",i,"] =",as.character(x[i]))) # print value
}

printVector(Shop$Payment, "Payment")

You can run scripts saved on a web page using source():

source("http://r.modas.lu/plotPCA.r")

plotPCA

Exercises (optional)

These three tasks can be dome without using loops (by matrix operations). So, consider it only as an example.

  1. Create a matrix 8x8. Fill it with 0. Using “for” loop change elements of the main diagonal to 1.

matrix, for

##      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
## [1,]    1    0    0    0    0    0    0    0
## [2,]    0    1    0    0    0    0    0    0
## [3,]    0    0    1    0    0    0    0    0
## [4,]    0    0    0    1    0    0    0    0
## [5,]    0    0    0    0    1    0    0    0
## [6,]    0    0    0    0    0    1    0    0
## [7,]    0    0    0    0    0    0    1    0
## [8,]    0    0    0    0    0    0    0    1
  1. Fill a matrix 8x8 with “1” to get a chess-board, where 0 codes a white cell and 1 - a black cell. Note, that black cells appear only if the sum of indexes is an even number (1+1=2, 1+3=4, 2+2=4, etc).

matrix,for, if

##      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
## [1,]    1    0    1    0    1    0    1    0
## [2,]    0    1    0    1    0    1    0    1
## [3,]    1    0    1    0    1    0    1    0
## [4,]    0    1    0    1    0    1    0    1
## [5,]    1    0    1    0    1    0    1    0
## [6,]    0    1    0    1    0    1    0    1
## [7,]    1    0    1    0    1    0    1    0
## [8,]    0    1    0    1    0    1    0    1
  1. Calculate the sum of the first n=1000 members of the series: s = (4/1) - (4/3) + (4/5) - (4/7) + (4/9) - (4/11) + (4/13) - (4/15) + ... Can you guess the value when n –> oo ?

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By Petr Nazarov