How to count the number of consecutive identical lines

I have a file that looks like this:

2000
2000
2001
2001
2001
2001
2002
2002

I need a script to show me this:

2000 - 2
2001 - 4
2002 - 2

I prefer using sed or awk

>Solution :

This is precisely what uniq -c does. From man uniq:

DESCRIPTION

Filter adjacent matching lines from INPUT (or standard input), writing to OUTPUT (or standard output).

[ . . . ]

-c, –count
       prefix lines by the number of occurrences

So with your example, we get:

$ uniq -c file
      2 2000
      4 2001
      2 2002

You can also write a little script if you prefer for some reason. For instance, with awk:

$ awk '{ count[$0]++ } END{ for(line in count){ print line,count[line] }}' file 
2000 2
2001 4
2002 2