To understand exactly why you get 12 values, it takes a bit of detective work. I'd say you actually have 3 potential problems in this code. The first one is what rafaelmenmell pointed out: if you use a for loop with index i, it's because you want to select the ith element of your vector at each occurrence of the loop.
But another potential problem, more an inefficiency, is that you're using ifelse(). If you use a for loop, a common way to write it would be:
for (i in 1:length(gender)) {
if(gender[i] == "male"){
print("M")
} else{
print("F")
}
}
In that case, at each occurrence of the loop, you'd check the current value of gender[i] and act in consequence. But ifelse() is a very useful function, because it already contains a loop inside!
ifelse(gender=="male", "M", "F")
#> [1] "M" "F" "M" "F" "M" "F"
So if you give the full vector to ifelse(), it will automatically loop over each element, so you don't need to write the loop yourself.
Finally a third problem, more on formatting, let's run your ifelse() command:
ifelse(gender=="male", print("M"), print("F"))
#> [1] "M"
#> [1] "F"
#> [1] "M" "F" "M" "F" "M" "F"
That is a confusing result, but it makes sense. When R first sees print("M"), it doesn't think, it immediately prints it. Similarly, it then runs into print("F"), so prints it. Only then does it look at the actual command and run the ifelse() loop. So in your case, you don't need the print() inside the loop, you want to first run the loop without printing anything, then print the result. One way to make it explicit would be:
print(ifelse(gender=="male", "M", "F"))
#> [1] "M" "F" "M" "F" "M" "F"
But note that when you run something in the R console, R assumes you want it printed (unless you tell it otherwise), so that print() is unnecessary.
One of the few cases where R doesn't automatically print a result is... in a for loop! So this code doesn't print anything:
for (i in 1:length(gender)) {
ifelse(gender=="male", "M", "F")
}
Now we can look at why you get 12 results (and why they're wrong but you can't see it in that particular case). You define your gender vector of length 6, then run a for loop. So your loop will be run 6 times. Each time it's run, there is a print("M"), and a print("F"). So you get 6 times the output
#> M
#> F
but what about the values inside the vector gender? Well, ifelse() does run on them, and computes the result, but since you're inside a for loop that result is not printed. So these values are actually completely ignored. You can see it with a different gender:
gender = c("male", "male","male", "female","female", "female")
length(gender)
for (i in 1:length(gender)) {
ifelse(gender=="male", print("M"), print("F"))
}
#> [1] "M"
#> [1] "F"
#> [1] "M"
#> [1] "F"
#> [1] "M"
#> [1] "F"
#> [1] "M"
#> [1] "F"
#> [1] "M"
#> [1] "F"
#> [1] "M"
#> [1] "F"
oops, your example just happened to have perfectly alternating male and female, so it's a complete coincidence that the result looked correct. You can make it even more obvious by changing the number of times the loop runs:
gender = c("female", "male", "male", "female","female", "female", "male", "female","female", "female")
length(gender)
for (i in 1:2) {
ifelse(gender=="male", print("M"), print("F"))
}
#> [1] "M"
#> [1] "F"
#> [1] "M"
#> [1] "F"
Technical note
Actually when I wrote
When R first sees print("M"), it doesn't think, it immediately prints it. Similarly, it then runs into print("F"), so prints it. Only then does it look at the actual command and run the ifelse() loop.
I lied a bit to simplify. In practice, R will first run the ifelse() loop, then will try to give you back the results, and notice that the result has a print(), so will print it. It doesn't change my above explanation, but it changes that if you have gender which has only "male", the ifelse() will never run into print("F") so you will only print "M".