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Formatted input-output is available in a number of programming languages. In the most general case, the correspondence between data items and format items cannot be determined during compilation, and so it is determined dynamically during execution. However, in most pairs of data and format lists that occur in practice, determination of the correspondence is in fact possible during compilation. Although some commercial compilers make this determination, there is little published literature on the subject. In this paper, we briefly examine three areas in which compile-time determination of the data-format correspondence is useful: optimization, program validation, and automatic test data generation. A formalism for stating the problem is given, and a solution is discussed in terms of formal language theory. Using this formalism, an algorithm for determining the correspondence is given, and its application is illustrated by examples in both PL/I and Fortran.