How to replace a value in a data frame based on a conditional If statement

Question

I tried replacing all of the times that B appears with b.

Consider the code below, to do the following:

junk <- data.frame(x <- rep(LETTERS[1:4], 3), y <- letters[1:12])

colnames(junk) <- c("alpha", "value")

this provides:

   alpha value
1   A    a
2   B    b
3   C    c
4   D    d
5   A    e
6   B    f
7   C    g
8   D    h
9   A    i
10  B    j
11  C    k
12  D    l

An initial attempt was to use a for and if statements like so:

for(i in junk$alpha) if(i %in% "B") junk$alpha <- "b"

but this replaces all of the values of junk$alpha with b.

 I know why this is doing this but I can't seem to get it to replace only those cases of junk$alpha where the original value was B.

Answer 1

It's easier to convert alpha to characters and then make the change:

junk$alpha <- as.character(junk$alpha)

junk$alpha[junk$alpha == "B"] <- "b"

junk$alpha <- as.factor(junk$alpha)

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Answer 2

Hi,

You can see the below example. You will get the idea.

Let's create some data that we can use in the examples below.

data <- data.frame(num1 = 1:5,                # Example data
                   num2 = 3:7,
                   char = letters[1:5],
                   fac = c("gr1", "gr2", "gr1", "gr3", "gr2"))
data$char <- as.character(data$char)          # Convert to character
data                                          # Print example data
#   num1 num2 char fac
# 1    1    3    a gr1
# 2    2    4    b gr2
# 3    3    5    c gr1
# 4    4    6    d gr3
# 5    5    7    e gr2

data$num1[data$num1 == 1] <- 99               # Replace 1 by 99
data                                          # Print updated data
#   num1 num2 char fac
# 1   99    3    a gr1
# 2    2    4    b gr2
# 3    3    5    c gr1
# 4    4    6    d gr3
# 5    5    7    e gr2