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In python, I can slice the last four items from a five-item list (pull values from it) like so: mylist[1:] (note, 0-based indexing). In R, it seems that not having something after the colon is an error. In both languages, I can put the last argument as the length of the list, but that's not always convenient (e.g. inline slicing: colnames(iris)[2:length(colnames(iris))]).
Is there any such syntax in R?
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To produce a vector slice between two indexes, we can use the colon operator ":". This can be convenient for situations involving large vectors. More information for the colon operator is available in the R documentation.
Python also allows you to index from the end of the list using a negative number, where [-1] returns the last element. This is super-useful since it means you don't have to programmatically find out the length of the iterable in order to work with elements at the end of it.
There are various ways to slice data frame rows in R :Using Numeric Indexing. Using Name Indexing. Indexing using logical vectors.
The slicing operation doesn't raise an error if both your start and stop indices are larger than the sequence length. This is in contrast to simple indexing—if you index an element that is out of bounds, Python will throw an index out of bounds error. However, with slicing it simply returns an empty sequence.
Well this is confusing coming from a python background, but mylist[-1] seems to do the trick. The negative in this case can be read as "except," i.e. take everything except column 1. So colnames(iris)[-1] works to grab all but the first item.
Oh, and to exclude more items, treat it as a range that is excluded, so colnames(iris)[-2:-4] would keep only the first and all items after (and including) the fifth one.
For others coming from python, check out this nice slideshow comparing R to python.
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