R:If, else and loops - Medical University of South …

1 R:If, else and loopsPresenter: Georgiana OnicescuJanuary 19, 2012 Presenter: Georgiana OnicescuR:ifelse,where,looping 1/ 17 ContentsVectorsMatricesIf else statementsFor loopsLeaving the loop: stop, break, next commandsOther loops :while and repeatAvoiding the loops : apply functionPresenter: Georgiana OnicescuR:ifelse,where,looping 2/ 17 VectorsA one column list of elements (a scalar is also a vector) - aone dimensional of creating = c(2,3,4,5,6,7,8,9,10,11,12,13,14,15) = c(2:15) = (c( , , C )) = seq(2,15,by=1) = c(a, b, c, d)Presenter: Georgiana OnicescuR:ifelse,where,looping 3/ 17 MatricesA combination of combine by column or by of creating matrices:vec1 = c(2,3,4,5,6,7,8,9,10,11,12,13,14,15)vec2 = c( (22:35))vec3 = seq(42,55,by=1)vecb = c( aa , bb , cc , dd , ee , ff , gg , hh , ii , jj , kk , ll , mm , nn )matr1 = cbind(vec1, vec2, vec3)matr2 = rbind(vec1, vec2, vec3)matr3 = cbind(vec1, vec2, vec3, vecb)matr4 = matrix(vecb,7,2)matr5=matrix(vec1,ncol=2 ,byrow=TRUE)Presenter: Georgiana OnicescuR:ifelse,where,looping 4/ 17If else statementsif ( statement1 )statement2elsestatement3If statements can be nested:if ( statement1 )statement2else if ( statement3 )statement4else if ( statement5 )statement6elsestatement8 Presenter: Georgiana OnicescuR:ifelse,where,looping 5/ 17If else statements.

2 ExampleExample: Calculating median of a single samplex=c(2,7,1,8,9)med=function(x){odd= length(x)%%2if (odd==0){med=(sort(x)[length(x)/2]+sort( x)[1+length(x)/2])/2} else med=sort(x)[ceiling(length(x)/2)]}m=med( x)m7 Note:ceiling takes a single numeric argument and returns thesmallest integer not less than the corresponding elements ofthe : Georgiana OnicescuR:ifelse,where,looping 6/ 17 Ifelse statementsSyntax: ifelse(test, truevalue, falsevalue)ifelse operates on vectors therefore avoiding loopsExample:y=log(c(3, ,2,4))ifelse(y<0,NA,y)Presenter: Georgiana OnicescuR:ifelse,where,looping 7/ 17 For loopfor(i in 1:n){execute commands}Example: Calculating factorials for an integer x:x!=x*(x-1)*(x-2)*(x-3)*..2*1fac=function(x){f=1if (x<2){f=1} else {for (i in 2:x) f=f*i}return (f)}Presenter: Georgiana OnicescuR:ifelse,where,looping 8/ 17 For loopThe for loop can iterate any :for (i in 1:2) print(i)12for (i in (1:4)-2) print(i)-1012library(MASS)data(phones)for(var in names(phones)) print(var) year calls for(func in c(sin,cos)) print(func(pi)) : Georgiana OnicescuR:ifelse,where,looping 9/ 17 Leaving the loop:stopstop on condition and print error message:a = 1:10b = NULLfor (i in seq(along=a)){if (a[i]<5||a[i]>8){b = c(b,a[i])} else {stop( values need to be<5 )} }Error: values of a need to be<5b1 2 3 4 Presenter: Georgiana OnicescuR:ifelse,where,looping 10/ 17 Leaving the loop:breakbreak:stop on condition without printing the error messagerm(b)a = 1.

3 10b = NULLfor (i in seq(along=a)){if (a[i]<5||a[i]>8){b = c(b,a[i]){ else {break( values of a need to be<5 )}}b1 2 3 4 Presenter: Georgiana OnicescuR:ifelse,where,looping 11/ 17 Leaving the loop:nextnextrm(b)a = 1:10b = NULLfor (i in seq(along=a)){if (a[i]<5||a[i]>8){b =c(b,a[i]){ else {next}}b1 2 3 4 9 10 Presenter: Georgiana OnicescuR:ifelse,where,looping 12/ 17 Other loops : WhileWhen using while, we need to set up an indicator variable andchange its value within each :Calculation of the factorialx=4f=1t=xwhile (t>1){f=f*tt=t-1}f24 Presenter: Georgiana OnicescuR:ifelse,where,looping 13/ 17 Other loops :RepeatRepeat:We must have a logical escape clause that leads to abreak commandx=4f=1t=xrepeat{if (t<2) breakf=f*tt=t-1}f24 Presenter: Georgiana OnicescuR:ifelse,where,looping 14/ 17 Avoiding the loops :applySyntax: apply(X, MARGIN, FUN, ARGs)X is an array, matrix or ; MARGIN: 1 for rows, 2for columns, c(1,2) for both; FUN: one or more functions;ARGs: possible arguments for functionExamples:matrix = matrix(c(1:10, 11:20), nrow = 10, ncol = 2)apply(matrix, 1, mean)apply(matrix, 2, mean)apply(matrix, 1:2, function(x) x*2)Presenter: Georgiana OnicescuR:ifelse,where,looping 15/ 17 ReferencesMichael J.}}}}

4 Crawley. The R ,2007 Johns Hopkins BST Statistical Computing: bcaffo/statcomp/ : Georgiana OnicescuR:ifelse,where,looping 16/ 17 QUESTIONS?Source:Paul graphics, Second : Georgiana OnicescuR:ifelse,where,looping 17/ 17