Using an if() statement within a for() loop

Question

A seemingly simple R task has got me stumped… I have a Number field and a TrueFalse field in a data.frame as seen here.

#Make Data
TrueFalse <- rep(c("TRUE","TRUE", "FALSE"),10)
Number <- seq(1, length(TrueFalse),3)
Table <- as.data.frame(cbind(TrueFalse, Number))

look at the head()

head(Table)

I am trying to create a new column that contains values from the Numbers column if the TrueFalse field is TRUE. In a meta code:

For each row, NewField ==Number if TrueFalse = True, else nothing (ie. Leave as NA)

My working code is below. While searches have been productive, I am still missing a few steps. Thanks in advance for any help!

#for() loop using if()
NewField <- rep("NA", nrow(Table))
for (i in 1:nrow(NewField)){
    if(Table$TrueFalse[i] == "TRUE")
        {NewField[i] <- Table$Number[i]}
    }

Hong Ooi · Accepted Answer · 2013-10-26 05:32:32Z

2

First you don't want to cbind your columns before converting to a data frame. cbind will force the result to whatever data type is compatible with all your columns, so in this case, it'll turn your Number variable into character. Probably not what you want, if not now, then eventually.

Second, R has a logical data type, so you should use it. Especially if you have values called "TRUE" and "FALSE".

Thus:

TrueFalse <- rep(c(TRUE, TRUE, FALSE),10)
Table <- data.frame(TrueFalse, Number)

Table$NewField <- ifelse(Table$TrueFalse, Table$Number, -1)

answered Oct 26, 2013 at 5:32

Hong Ooi

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Simon · Accepted Answer · 2013-10-26 05:29:22Z

2

You can do this in one line. For example:

Table$NewField <- ifelse(Table$TrueFalse=="TRUE",Table$Number,-1)

Also, if you had used TRUE and FALSE instead of "TRUE" and "FALSE", you wouldn't need to use the equality test either.

answered Oct 26, 2013 at 5:29

Simon

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