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LRANGE, LINDEX and LLEN

LRANGE 12 minLRANGE reads a slice of a list by inclusive start and stop indexes, LINDEX reads one position, LLEN counts items, and negative indexes count back from the tail.

LRANGE, LINDEX and LLEN

Asha's activity feed has grown to hundreds of entries, but her profile only shows the latest five, a "see more" page shows the next five, and a badge says how many there are in total. You never want the whole list; you want a slice, one item, or a count.

The rule

Indexes start at 0 from the head and -1 from the tail; LRANGE includes both ends.

feed:asha holds, from head to tail: won, lost, won, joined, won.

Item won lost won joined won
From head 0 1 2 3 4
From tail -5 -4 -3 -2 -1 
127.0.0.1:6379> LRANGE feed:asha 0 1
1) "won"
2) "lost"
127.0.0.1:6379> LRANGE feed:asha -2 -1
1) "joined"
2) "won"
127.0.0.1:6379> LINDEX feed:asha 3
"joined"
127.0.0.1:6379> LINDEX feed:asha 9
(nil)
127.0.0.1:6379> LLEN feed:asha
(integer) 5
  • LRANGE key start stop returns the items from start to stop, both included. 0 -1 is the whole list.
  • An index past the end is clipped, not an error: LRANGE feed:asha 3 100 returns just joined, won. If start is already past the end, or after stop, the reply is an empty list.
  • LINDEX key index returns one item, or (nil) if there is nothing at that index.
  • LLEN key counts the items; a missing key has length 0.

Paging a feed. Page 1 is LRANGE feed:asha 0 4, page 2 is LRANGE feed:asha 5 9. In general, page n of size 5 is start = 5 × (n − 1) and stop = start + 4.

A note on speed. Reading near either end is instant. LINDEX in the middle of a very long list has to walk to that position, so lists are best used from their ends; for "is X in here?" a set is the right tool.

The shortcut

To count how many items a range returns, use stop − start + 1 (after turning negative indexes into positive ones and clipping to the list).

LRANGE feed:asha 1 -2Step 1 of 4

  1. Convert the negatives

    The list has 5 items, so -2 is index 5 − 2 = 3.

Drills

Answer fast — trust the rule.

quick drill

feed:asha is won, lost, won, joined, won. What does LINDEX feed:asha -2 return?

quick drill

Same list. How many items does LRANGE feed:asha 1 3 return?

quick drill

Same list. What does LRANGE feed:asha 4 10 return?

quick drill

Which command gets page 3 of a feed shown 10 items per page?

Remember

  • Indexes start at 0 from the head and -1 from the tail; LRANGE includes both ends.
  • Items in a range = stop − start + 1, after converting negatives.
  • Out-of-range stops are clipped, and LINDEX past the end is nil.

Practice — level up

0/1 solved

Write your answer, then compare it with a rubric and a model answer.

ExplainSlicing the feed

feed:asha holds, from head to tail: won, lost, won, joined, won. Predict the replies to LRANGE feed:asha 0 2, LINDEX feed:asha -1, LLEN feed:asha and LRANGE feed:asha 3 10, and explain the rule behind each.