r - How to create vector with for loops and ifelse - r

Question

I'm having a problem with nested for loops and ifelse statements. This is my dataframe abund:
Species Total C1 C2 C3 C4
1 Blue 223 73 30 70 50
2 Black 221 17 50 56 98
3 Yellow 227 29 99 74 25
4 Green 236 41 97 68 30
5 Red 224 82 55 21 66
6 Orange 284 69 48 73 94
7 Black 154 9 63 20 62
8 Red 171 70 58 13 30
9 Blue 177 57 27 8 85
10 Orange 197 88 61 18 30
11 Orange 112 60 8 31 13
I would like to add together some of abund’s columns but only if they match the correct species I’ve specified in the vector colors.
colors <- c("Black", "Red", "Blue")
So, if the Species in abund matches the species in color then add columns C2 through C4 together in a new vector minus. If the species in abund does not match the species in color then add a 0 to the new vector minus.
I'm having trouble with my code and hope it's just a small matter of defining a range, but I'm not sure. This is my code so far:
# Use for loop to create vector of sums for select species or 0 for species not selected
for( i in abund$Species)
{
for( j in colors)
{
minus <- ifelse(i == j, sum(abund[abund$Species == i,
"C2"]:abund[abund$Species == i, "C4"]), 0)
}
}
Which returns this: There were 12 warnings (use warnings() to see them)
and this "vector": minus [1] 0
This is my target:
minus
[1] 150 204 0 0 142 0 145 101 120 0 0
Thank you for your time and help with this.

Answer 1

This is probably better done without any loops.
# Create the vector
minus <- rep(0, nrow(abund))
# Identify the "colors" cases
inColors <- abund[["Species"]] %in% colors
# Set the values
minus[inColors] <- rowSums(abund[inColors, c("C2","C3","C4")])
Also, for what it is worth there are quite a few problems with your original code. First, your first for loop isn't doing what you think. In each round, the value of i is being set to the next value in abund$Species, so first it is Blue then Black then Yellow, etc. As a result, then you index using abund[abund$Species == i, ], you may return multiple rows (ex. Blue will give you 1 and 9, since both those rows Species == "Blue").
Second when you make the statement abund[abund$Species == i, "C2"]:abund[abund$Species == i, "C4"] you are not indexing the columns C2 C3 and C4 you are making a sequence starting at the value in C2 and ending at the value in C4. For example, when i == "Yellow" it returns 99:25 or 99, 98, 97, ... , 26, 25. The reason you were getting those warnings was a combination of this problem and the last one. For example, when i == "Blue", you were trying to make a sequence starting at both 30 and 27 and ending at both 50 and 85. The warning was saying that it was just using the first number in your start and finish and giving you 30:50.
Finally, you were constantly over writing your value of minus rather than adding to it. You need to first create minus as above and index into it for the assignment like this minus[i] <- newValue.

Answer 2

Note that ifelse is vectorized so you usually don't need any for loops when using it.
I like Barker's answer best, but if you wanted to do this with ifelse this is the way:
abund$minus = with(abund, ifelse(
Species %in% colors, # if the species matches
C2 + C3 + C4, # add the columns
0 # otherwise 0
))
Even though this is just one line and Barker's is 3, on large data it will be slightly more efficient to avoid ifelse.
However, ifelse statements can be nested and are often easier to work with when conditions get complicated - so there are definitely good times to use them. On small to medium sized data the speed difference will be negligible so just use whichever you think of first.

Answer 3

# Create a column called minus with the length of the number of existing rows.
# The default value is zero.
abund$minus <- integer(nrow(abund))
# Perform sum of C2 to C4 only in those rows where Species is in the colors vector
abund$minus[abund$Species %in% colors] <- rowSums(abund[abund$Species %in% colors,5:7])