Hi everyone,
I hope this is the right place for my question. If not, please, feel free to
ignore it ;) and I'm sorry for any inconvenience made :(
I'm writing a simple program for enumerating triangles in directed graphs
for my project. First, for each input arc (e.g. a b, b c, c a, note: a tab
symbol serves as a delimiter) I want my map function output the following
pairs ([a, to_b], [b, from_a], [a_b, -1]):
public void map(LongWritable key, Text value,
OutputCollector<Text, Text> output,
Reporter reporter) throws IOException {
String line = value.toString();
String [] tokens = line.split(" ");
output.collect(new Text(tokens[0]), new Text("to_"+tokens[1]));
output.collect(new Text(tokens[1]), new Text("from_"+tokens[0]));
output.collect(new Text(tokens[0]+"_"+tokens[1]), new Text("-1"));
}
Now my reduce function is supposed to cross join all pairs that have both
to_'s and from_'s and to simply propogate any other pairs whose keys contain
"_".
public void reduce(Text key, Iterator<Text> values,
OutputCollector<Text, Text> output,
Reporter reporter) throws IOException {
String key_s = key.toString();
if (key_s.indexOf("_")>0)
output.collect(key, new Text("completed"));
else {
HashMap <String, ArrayList<String>> lists = new HashMap
<String, ArrayList<String>> ();
while (values.hasNext()) {
String line = values.next().toString();
String[] tokens = line.split("_");
if (!lists.containsKey(tokens[0])) {
lists.put(tokens[0], new ArrayList<String>());
}
lists.get(tokens[0]).add(tokens[1]);
}
for (String t : lists.get("to"))
for (String f : lists.get("from"))
output.collect(new Text(t+"_"+f), key);
}
}
And this is where the most exciting stuff happens. tokens[1] yields an
ArrayOutOfBounds exception. If you scroll up, you can see that by this point
the iterator should give values like "to_a", "from_b", "to_b", etc... when I
just output these values, everything looks ok and I have "to_a", "from_b".
But split() don't work at all, moreover line.length() is always 1 and
indexOf("*") returns -1! The very same indexOf WORKS PERFECTLY for keys...
where we have pairs whose keys contain "_"* and look like "a_b", "b_c"
I'm really puzzled with all this. MapReduce is supposed to save lives making
everything simple. Instead I spent several hours to just spot this...
I'd really appreciate your help, guys!!! Thanks in advance!
