An automated method and apparatus are used for creating data processing application programs in programmer oriented languages such as COBOL from atomic or fundamental program building blocks, i.e., individual programming language statements, based on information provided by the user at the terminal. Host/peripheral input/ output subroutines are selected from a library of existing I/O subroutines, based on the type of I/O indicated by the user; and, the application program can incorporate existing subroutines, tables, files, etc. so as to avoid having to create code which has already been created. The system also provides for a level of validity checks and interactive editing of the application program as it is being created at the terminal. A user may thus change some aspect of the application program, or correct a logic error which has been detected by the system, interactively without having to start over from scratch after compiling the source program to detect errors.