R: Loop should return numeric element from string

Question

I have a question how to write a loop in r which goes checks if a certain expression occurs in a string . So I want to check if the the expression “i-sty” occurs in my variable for each i between 1:200 and, if this is true, it should give the corresponding i.

For example if we have “4-sty” the loop should give me 4 and if there is no “i-sty” in the variable it should give me . for the observation.

I used

for (i in 1:200){
  datafram$height <- ifelse(grepl("i-sty", dataframe$Description), i, ".")
}

But it did not work. I literally only receive points. Attached I show a picture of the string variable. enter image description here

"i-sty" is just a string with the letter i in it. To you use a regex pattern with your variable i, you need to paste together a string, e.g., grepl(paste0(i, "-sty"), ...). I'd also recommend using NA rather than "." for the "else" result - that way the resulting height variable can be numeric. — Gregor Thomas
– Gregor Thomas, Commented Jun 1, 2020 at 14:05
Welcome to Stack Overflow! Please make your example reproducible, read How to Ask and stackoverflow.com/questions/5963269/… — jogo
– jogo, Commented Jun 1, 2020 at 14:19
x <- c("6-sty xxx", "4-sty yyyy", NA, "sty zzz", "32-sty xyz"); as.numeric(sub("^([0-9]+)-sty.*", "\\1", x)) — jogo
– jogo, Commented Jun 1, 2020 at 17:40

Gregor Thomas · Accepted Answer · 2020-06-01 14:11:37Z

1

"i-sty" is just a string with the letter i in it. To you use a regex pattern with your variable i, you need to paste together a string, e.g., grepl(paste0(i, "-sty"), ...). I'd also recommend using NA rather than "." for the "else" result - that way the resulting height variable can be numeric.

for (i in 1:200){
  dataframe$height <- ifelse(grepl("i-sty", dataframe$Description), i, ".")
}

The above works syntactically, but not logically. You also have a problem that you are overwriting height each time through the loop - when i is 2, you erase the results from when i is 1, when i is 3, you erase the results from when i is 2... I think a better approach would be to extract the match, which is easy using stringr (but also possible in base). As a benefit, with the right pattern we can skip the loop entirely:

library(stringr)

dataframe$height = str_match(string = dataframe$Description, pattern = "[0-9]+-sty")[, 2]
# might want to wrap in `as.numeric`

You use both datafram and dataframe. I've assumed dataframe is correct.

answered Jun 1, 2020 at 14:11

Gregor Thomas

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1 Comment

web_1920 Over a year ago

Thank you so much for your help. The code you proposed works very well with [, 1] for me.

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