R: find largest common substring starting at the beginning

Question

I\'ve got 2 vectors:

word1 <- \"bestelling\"   
word2 <- \"bestelbon\"

Now I want to find the largest common substring that starts at the

Answer 1

why not add another! and hack at it so the answer is different than everyone elses!

largestStartSubstr<-function(word1, word2){ 
    word1vec<-unlist(strsplit(word1, "", fixed=TRUE))
    word2vec<-unlist(strsplit(word2, "", fixed=TRUE))
    indexes<-intersect(1:nchar(word1), 1:nchar(word2))
    bools<-word1vec[indexes]==word2vec[indexes]
    if(bools[1]==FALSE){
        ""
    }else{
        lastChar<-match(1,c(0,diff(cumsum(!bools))))-1
        if(is.na(lastChar)){
            lastChar<-indexes[length(indexes)]
        }
        substr(word1, 1,lastChar)
    }
}

word1 <- "bestselling"
word2<- "bestsel"

largestStartSubstr(word1, word2)
#[1] "bestsel"

word1 <- "bestselling"
word2<- "sel"

largestStartSubstr(word1, word2)
#[1] ""

Answer 2

fun <- function(words) {
  #extract substrings from length 1 to length of shortest word
  subs <- sapply(seq_len(min(nchar(words))), 
                 function(x, words) substring(words, 1, x), 
                 words=words)
  #max length for which substrings are equal
  neqal <- max(cumsum(apply(subs, 2, function(x) length(unique(x)) == 1L)))
  #return substring
  substring(words[1], 1, neqal)
}

words1 <- c("bestelling", "bestelbon")
fun(words1)
#[1] "bestel"

words2 <- c("bestelling", "stel")
fun(words2)
#[1] ""

Answer 3

A little messy, but it's what I came up with:

largest_subset <- Vectorize(function(word1,word2) {
    substr(word1, 1, sum(substring(word1, 1, 1:nchar(word1))==substring(word2, 1, 1:nchar(word2))))
})

It produces a warning message if the words are not the same length, but have no fear. It checks to see if each substring from the first character of each word to every position produces a match between the two words. You can then count how many values came out to be true, and capture the substring up to that character. I vectorized it so you can apply it to word vectors.

> word1 <- c("tester","doesitwork","yupyppp","blanks")
> word2 <- c("testover","doesit","yupsuredoes","")
> largest_subset(word1,word2)
    tester doesitwork    yupyppp     blanks 
    "test"   "doesit"      "yup"         "" 

Answer 4

This will work for an arbitrary vector of words

words <- c('bestelling', 'bestelbon')
words.split <- strsplit(words, '')
words.split <- lapply(words.split, `length<-`, max(nchar(words)))
words.mat <- do.call(rbind, words.split)
common.substr.length <- which.max(apply(words.mat, 2, function(col) !length(unique(col)) == 1)) - 1
substr(words[1], 1, common.substr.length)
# [1] "bestel"

Answer 5

Here's another function that seems to work.

foo <- function(word1, word2) {
    s1 <- substring(word1, 1, 1:nchar(word1))
    s2 <- substring(word2, 1, 1:nchar(word2))
    if(length(w <- which(s1 %in% s2))) s2[max(w)] else character(1)
}

foo("bestelling", "bestelbon")
# [1] "bestel"
foo("bestelling", "stel")
# [1] ""
foo("bestelbon", "bestieboop")
# [1] "best"
foo("stel", "steal")
# [1] "ste"