def lda_filter_articles(ids, articles):
    """From sets of ids to set of sentences (str)
    :argument ids: dictionary (keys are topics and values are ids of articles
    :argument articles: list of str
    :returns a dictionary, keys are topics and values are sentences (str)
    """
    articles_per_topic = {}
    for topic in ids.keys():
        tmp_topic_articles = [article for i,article in enumerate(articles) if i in ids[topic]]
        articles_per_topic[topic] = ' '.join(tmp_topic_articles)
    return articles_per_topic