文件扩展名更改程序的功能：在命令行中输入renew dir old new，应用程序的名字为renew.exe,后面的dir，old和new是三个命令行参数，dir代表一个全路径名，old和new代表文件扩展名，在路径为dir中搜索所有扩展名为old的文件，如果找到将扩展名改为new而文件名不变。在读入参数的过程中检验路径是否合法，若不合法则输出提示并退出，如果用户输入的参数不符合要求也输出提示并且退出程序；在修改文件扩展的过程中如果遇到重名的文件，提示用户是否覆盖，若选择覆盖则删除原文件，更改扩展名，如果不覆盖则不修改；最后输出被修改的文件名，并且统计有多少个文件被修改。-File extensions to change procedure functions: at the command line, type renew dir old new, the application for the first name renew.exe, behind the dir, old and new are three command-line parameters, dir on behalf of a full-path name, old and new representative of the file name extension, in the path for dir in old english all extension documents, if found will be replaced by new extensions and file name unchanged. Read into the parameter in the process of testing the legality of the path, if it is not legitimate is the output prompt and exit, if the user input parameters do not meet the output requirements and prompt exit procedures　modify the file extension at the process of the documents if they are duplicate names, suggesting Whether or not covered by the user, if you choose to delete the original coverage of the document, change the extension, if no coverage is not modified　Finally the output file name is modified, and statistics the number of documents have been modified.