extracting value of variable from dataframe

Question

I have one issue in selecting a value of one variable conditional on the value of another variable in a dataframe.

Dilutionfactor=c(1,3,9,27,80)
Log10Dilutionfactor=log10(Dilutionfactor)
Protection=c(100,81.25,40,10.52,0)
RM=as.data.frame(cbind(Dilutionfactor,Log10Dilutionfactor,Protection))

Now i want to know the value of Log10Dilutionfactor condition on the value of Protection is equal to either 50 (if it appear) or the value immediately just below 50. when i used subset(RM,Protection<= 50)it gives three rows and when I tried RM[grepl(RM$Protection<=50,Log10Dilutionfactor),] it gives 0 values with warning message. I really appreciate if someone help me.

If you cbind before you coerce to a data.frame, you're creating a matrix (which can only hold one type) before changing to data.frame (which can hold multiple). Just use data.frame(...) instead. — alistaire
– alistaire, Commented May 12, 2017 at 5:58

Erdem Akkas · Accepted Answer · 2017-05-12 06:06:21Z

1

You can use 2 subset:

subset(RM,Protection==max(subset(RM,Protection<= 50)$Protection))$Log10Dilutionfactor
# [1] 0.954243

edited May 12, 2017 at 6:06

answered May 12, 2017 at 5:59

Erdem Akkas

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2 Comments

R.P. Tamil Selvan Over a year ago

This one is easiest in solving my issue, thanking you

R.P. Tamil Selvan Over a year ago

Yes it is not a problem with the data provided, i rechecked it and found correct. that's why i deleted the comment

ikop · Accepted Answer · 2017-05-12 06:03:23Z

1

You could use

with(RM, Log10Dilutionfactor[which(Protection == max(Protection[Protection <= 50]))])
# [1] 0.9542425

answered May 12, 2017 at 6:03

ikop

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Comments

Larusson · Accepted Answer · 2017-05-12 06:03:40Z

1

or find the index value of protection that is closest to 50

 index = which(abs(RM$Protection-50)<=min(abs(RM$Protection-50)))

and then look it up in what ever column you want. e.g for Dilutionfactor

 RM$Dilutionfactor[index]

answered May 12, 2017 at 6:03

Larusson

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