I am relatively new to R. I am iterating over a vector in R by using for() loop. However, based on a certain condition, I need to skip some values in the vector. The first thought that comes to mind is to change the loop index within the loop. I have tried that but somehow its not changing it. There must be some what to achieve this in R.
Thanks in advance. Sami
You can change the loop index within a for loop, but it will not affect the execution of the loop; see the Details section of ?"for":
The ‘seq’ in a ‘for’ loop is evaluated at the start of the loop;
changing it subsequently does not affect the loop. If ‘seq’ has
length zero the body of the loop is skipped. Otherwise the
variable ‘var’ is assigned in turn the value of each element of
‘seq’. You can assign to ‘var’ within the body of the loop, but
this will not affect the next iteration. When the loop terminates,
‘var’ remains as a variable containing its latest value.
Use a while loop instead and index it manually:
i <- 1
while(i < 100) {
# do stuff
if(condition) {
i <- i+3
} else {
i <- i+1
}
}
Look at
?"next"
The next command will skip the rest of the current iteration of the loop and begin the next one. That may accomplish what you want.
Without an example it is hard to see what you want to do, but you can always use an if-statement inside a for-loop:
foo <- 1:10*5
for (i in seq(length(foo)))
{
if (foo[i] != 15) print(foo[i])
}
In R, local alterations in the index variable are "corrected" with the next pass:
for (i in 1:10){
if ( i==5 ) {i<-10000; print(i)} else{print(i)}
}
#-----
[1] 1
[1] 2
[1] 3
[1] 4
[1] 10000
[1] 6
[1] 7
[1] 8
[1] 9
[1] 10
Since you have some criterion for skipping you should apply the criterion to the loop vector inside the for-parentheses. E.g:
for( i in (1:10)[-c(3,4,6,8,9)] ) {
print(i)}
#----
[1] 1
[1] 2
[1] 5
[1] 7
[1] 10
seq argument to the for loop assumes you a-priori know which elements to skip. If that's the case, you may be able to use a vectorized solution and skip the loop completely.Well the for loop and the while can be used different, Here is how you can use them for different results
The while loop
i <- 1
ten <- 10
while(i < ten) {
if(i>5) {
ten <- ten-1
}
print(i)
i<-i+1
}
Result:
[1] 1
[1] 2
[1] 3
[1] 4
[1] 5
[1] 6
[1] 7
> ten
[1] 8
The for loop
tin <- 10
for (l in 1:tin) {
if(l>5) {
tin <- tin-1
}
print(l)
}
Result:
[1] 1
[1] 2
[1] 3
[1] 4
[1] 5
[1] 6
[1] 7
[1] 8
[1] 9
[1] 10
> tin
[1] 5
Conclusion: The for loop index when initiated, can't be changed but the while loop can be changed within the loop.