语句位置的不同导致的错误？

	while(getline(input,line))

	{		

		istringstream stream(line);

		string word;

		bool firstword=true;

		while(stream>>word)

		{

			map<string,string>::const_iterator map_it=trans_map.find(word);

			if(map_it!=trans_map.end())

				word=map_it->second;  //运行成功

				if(firstword)

					firstword=false;

				else cout<<" ";

		       // word=map_it->second;	 //运行产生错误

			cout<<word;

		}

		cout<<endl;

	}