Using Span to Extract Multiple Spans from a List

Questions
1

Let l1 be a list of 100 elements. Assume I’m interested in taking several spans from this list, which I wish to have as a single list, say elements 5 through 25, 33 through 55, and 81 through 95.
So I can use span to extract all three spans separately:

p1 = l1[[5;;25]]
p2 = l1[[33;;55]]
p3 = l1[[81;;95]]

I could then combine these into one list. I’m just wondering if there’s a simpler way to do that in one step.

I tried these options:

l2 = l1[[5 ;; 25, 33 ;; 55, 81 ;; 95]]

and

l2 = l1[[{5 ;; 25, 33 ;; 55, 81 ;; 95}]]

and

l2 = l1[[5 ;; 25 && 33 ;; 55 && 81 ;; 95]]

None of these worked though. I got different error messages along the lines of:
Part::pspec: “Part specification {5;;25,33;;55} is neither a machine-sized integer nor a list of machine-sized integers.”

Any suggestions?

2

Hey Michael,

You may quickly define a small function to handle multiple span in one line:

In[1]:= f[data_List, {span__List}] :=Map[data[[ #[[1]] ;; #[[2]] ]] &, {span}]

In[2]:= f[Range[100], {{3, 5}, {7, 9}}]

Out[2]= {{3, 4, 5}, {7, 8, 9}}

This function can handle any number of span functions you want to use. The following argument uses BlankSequence function so it can handle multiple inputs

{span__List}

which is very handy in this type of question.