Changing values in one column based on another in R - r

Question

So I am using R and trying to change values in a data frame in one column by comparing two columns together. I have something like
Median MyPrice
10 0
20 18
20 20
30 35
15 NA
And I would like to say something like
if(MyPrice == 0 & MyPrice < Median){MyPrice <- 1
}else if (MyPrice == Median){MyPrice <- 2
}else if (MyPrice > Median){MyPrice <- 3
}else {MyPrice <- 4}
To come up with
Median MyPrice
10 1
20 1
20 2
30 3
15 4
But there is always an error. I have also tried something like
for(i in MyPrice){if(MyPrice == 0 & MyPrice < Median){MyPrice <- 1
}else if (MyPrice == Median){MyPrice <- 2
}else if (MyPrice > Median){MyPrice <- 3
}else {MyPrice <- 4}
}
The for loop runs but it changes all values in MyPrice to 4. I also tried the ifelse() function but it seemed to have an issue taking that many arguments at once.
I would also not be opposed to a new column being added to the end of the data frame if a solution like that is easier.

Answer 1

You don't necessarily have to use a for loop. Start by setting every comparison to 4.
> x$Comp=4
> x$Comp[x$Median>x$MyPrice]=1 #if Median is higher, comparison = 1
> x$Comp[x$Median==x$MyPrice]=2 #if Median is equal to MyPrice, comparison = 2
> x$Comp[x$Median<x$MyPrice]=3 #if Median is lower, comparison = 3
> x
Median MyPrice Comp
1 10 0 1
2 20 18 1
3 20 20 2
4 30 35 3
5 15 NA 4

Answer 2

Given your first argument that if MyPrice == 0 & MyPrice < Median, your 2nd row where Median: 20 and MyPrice: 18 should also be 4. Here is a working nested ifelse statement with an NA handler after.
df <- as.data.frame(matrix(c(10,0,20,18,20,20,30,35,15,NA), byrow = T, ncol = 2))
colnames(df) <- c("Median","MyPrice")
df$NewPrice <- ifelse(df$MyPrice == 0 & df$MyPrice < df$Median, 1,
ifelse(df$MyPrice == df$Median, 2,
ifelse(df$MyPrice > df$Median, 3, 4)))
df$NewPrice[is.na(df$MyPrice)] <- 4
df
# Median MyPrice NewPrice
#1 10 0 1
#2 20 18 4
#3 20 20 2
#4 30 35 3
#5 15 NA 4

Answer 3

What about setting a new variable with all values in 4 and then, replace those cases where your conditions apply?
Simple, straight forward and easy to read :-)
#(Following the example from #Evans Friedland)
df <- as.data.frame(matrix(c(10,0,20,18,20,20,30,35,15,NA), byrow = T, ncol = 2))
colnames(df) <- c("Median","MyPrice")
df <- mutate(df, myNewPrice = 4) #set my new price to 4, then edit by following your conditions
df$myNewPrice<- replace (df$myNewPrice, df$MyPrice == 0 & df$MyPrice < df$Median, 1)
df$myNewPrice<- replace (df$myNewPrice, df$MyPrice == df$Median , 2)
df$myNewPrice<- replace (df$myNewPrice, df$MyPrice > df$Median , 3)
df$myNewPrice <- as.numeric (df$myNewPrice) #might, might not be needed.