Thank you for you reply. That's right for your understand. If the input sentence is "a b c d", the length of output sentence("e f g h") is the same as the input sentence, which is 4. And a--e, b--f, c--g, d--h, for example:
the input: I like eating --- the length is 3
the output: 我 喜欢 吃 --- the length is 4 [It's wrong]
the output: 我 爱 吃 --- the length is 3 [It's right, the same length]
If add function"valid(src, tgt)", will it lead to a case which have not output for the input sentence? I need all input sentences and every input matches one output sentence.