A program creation system is described which generates a data manipulation program based on input-output examples. The created program may include a collection of subprograms together with a collection of corresponding selection conditions. When a new input item is received, a program execution module uses the selection conditions to select one of the subprograms. The program execution module then applies the selected subprogram to generate a new output item. The program creation system generates the program using a three-part approach, involving: generating sets of subprograms for the respective input-output examples; grouping the sets of programs into partitions and choosing representative subprograms for the partitions; and determining the selection conditions. A user interaction module provides various mechanisms which allow a user to interact with the program creation system and thereby improve the performance of the created program.