Groovy way to select block of lines from a list

Question

In my Groovy program, I have a list of lines and want to select a contiguous block of lines from the list. The first line of the desired block contains a particular string, and the last line (which I can include or not - it doesn't matter) contains a (different) marker string.

Working code is below but surely there is a "groovier" way to do this in a line or two - can anyone suggest how?

(Actually my list of lines is from an HTTP GET, but I just define a URL object and do url.openStream().readLines() to get the lines.)

lines = ["line1", "line2", "line3", "line4", "line5", "line6"]
println extract_block("2", "5", lines)

def extract_block(start, end, lines) {
    def block = null
    for (line in lines) {
        if (null == block && line.contains(start)) { block = [] }
        if (null != block && line.contains(end)) { break }
        if (null != block) { block << line }
    }
    return block
}

This prints

["line2", "line3", "line4"]

which includes the first line (containing "2") and skips the final line (containing "5").

Answer 1

Regular expressions are designed for this kind of problem. Here's an example that shows it works both where the file contents

(NB: the String.find method is in groovy 1.6.1 and above, prior to that you'd need to modify the syntax slightly)

def BEGIN_MARKER = "2"
def END_MARKER = "5"
def BEGIN_LINE = /.*$BEGIN_MARKER.*\n/
def MIDDLE_LINES = /(.*\n)*?/
def LOOKAHEAD_FOR_END_LINE_OR_EOF = /(?=.*$END_MARKER.*|\Z)/

def FIND_LINES = "(?m)" +
                 BEGIN_LINE +
                 MIDDLE_LINES +
                 LOOKAHEAD_FOR_END_LINE_OR_EOF    

def fileContents = """
line1 ipsum
line2 lorem
line3 ipsum
line4 lorem
line5 ipsum
line6 lorem
"""

// prints:
// line2 lorem
// line3 ipsum
// line4 lorem
println fileContents.find(FIND_LINES)

def noEndMarkerFileContents = """
line1 ipsum
line2 lorem
line3 ipsum
line4 lorem
line6 lorem
"""

// prints:
// line2 lorem
// line3 ipsum
// line4 lorem
// line6 lorem        
println noEndMarkerFileContents.find(FIND_LINES)

Answer 2

Alternatively, if you don't want to use regular expressions, you can make it a little more groovy using findAll:

lines = ["line1", "line2", "line3", "line4", "line5", "line6"]

def extract_block(start, end, lines) {
    def inBlock = false
    return lines.findAll { line ->        
        if (line.contains(start)) inBlock = true
        if (line.contains(end)) inBlock = false            
        return inBlock
    }
}

assert ["line2", "line3", "line4"] == extract_block("2", "5", lines)

Answer 3

A shorter but somewhat less expressive version of Ted's "groovier" answer:

def extract_block(start, end, lines) {
    def inBlock = false
    return lines.findAll { inBlock = (it.contains(start) || inBlock) && !it.contains(end) } 
}