add two columns (row wise) in apache pig - apache-pig

Question

I am trying to learn Apache Pig. I am sorry if its a lame question.
I have three columns sitename, upcount and downcount.
When I use describe res, I get: res: {sitename: chararray,upcount: int,downcount: int}
What I am trying to do is find site-up percentage of upcount by upcount/(upcount+downcount)
I am unable to figure how do I achieve it. I tried following:
res_sum = foreach res generate sitename, upcount+downcount;
But it gave following error:
Pig Stack Trace
---------------
ERROR 1066: Unable to open iterator for alias res_sum
org.apache.pig.impl.logicalLayer.FrontendException: ERROR 1066: Unable to open iterator for alias res_sum
at org.apache.pig.PigServer.openIterator(PigServer.java:935)
at org.apache.pig.tools.grunt.GruntParser.processDump(GruntParser.java:754)
at org.apache.pig.tools.pigscript.parser.PigScriptParser.parse(PigScriptParser.java:376)
at org.apache.pig.tools.grunt.GruntParser.parseStopOnError(GruntParser.java:230)
at org.apache.pig.tools.grunt.GruntParser.parseStopOnError(GruntParser.java:205)
at org.apache.pig.tools.grunt.Grunt.run(Grunt.java:66)
at org.apache.pig.Main.run(Main.java:565)
at org.apache.pig.Main.main(Main.java:177)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.hadoop.util.RunJar.run(RunJar.java:221)
at org.apache.hadoop.util.RunJar.main(RunJar.java:136)
Caused by: java.io.IOException: Job terminated with anomalous status FAILED
at org.apache.pig.PigServer.openIterator(PigServer.java:927)
... 13 more
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Answer 1

Try
res_sum = foreach res generate sitename, (upcount + downcount) as ud_sum;
if it doesn't work - show your entire script