[SOLVED] Conditional string concatenation in same column in R

Issue

I am new to R and have a very large irregular column in a data frame like this:

x <- data.frame(section = c("BOOK I: Introduction", "Page one: presentation", "Page two: acknowledgments", "MAGAZINE II: Considerations", "Page one: characters", "Page two: index", "BOOK III: General Principles", "BOOK III: General Principles", "Page one: invitation"))

section
BOOK I: Introduction
Page one: presentation
Page two: acknowledgments
MAGAZINE II: Considerations 
Page one: characters
Page two: index
BOOK III: General principles
BOOK III: General principles
Page one: invitation

I need to concatenate this column to look like this:

section
BOOK I: Introduction 
BOOK I: Introduction / Page one: presentation
BOOK I: Introduction / Page two: acknowledgments
MAGAZINE II: Considerations
MAGAZINE II: Considerations / Page one: characters
MAGAZINE II: Considerations / Page two: index
BOOK III: General Principles
BOOK III: General Principles
BOOK III: General Principles / Page one: invitation

Basically the goal is to extract the value of the upper string based in a condition and then concatenate with the lower actualizing the value with a regex expression, but I really don’t know how to do it.

Thanks in advance.

Solution

You can do:

unlist(lapply(split(x$section, cumsum(grepl('^[A-Z]{3}', x$section))), 
              function(y) {
                  if(length(y) == 1) return(y)
                  else c(y[1], paste(y[1], y[-1], sep = " / "))
                }), use.names = FALSE)
#> [1] "BOOK I: Introduction"                               
#> [2] "BOOK I: Introduction / Page one: presentation"      
#> [3] "BOOK I: Introduction / Page two: acknowledgments"   
#> [4] "MAGAZINE II: Considerations"                        
#> [5] "MAGAZINE II: Considerations / Page one: characters" 
#> [6] "MAGAZINE II: Considerations / Page two: index"      
#> [7] "BOOK III: General Principles"                       
#> [8] "BOOK III: General Principles"                       
#> [9] "BOOK III: General Principles / Page one: invitation"

Answered By – Allan Cameron

Answer Checked By – David Marino (BugsFixing Volunteer)