This is to avoid the I/O system being used recursively. By far the best course is to use the subroutine form for any procedure with side-effects. 9.4 Arguments of External Procedures Arguments can pass information into a procedure or out from it, or in both directions. There are two restrictions on complex values: firstly they cannot be compared at all to ones of double precision type. Both values must be within the range 1 to LEN, where LEN is the length of the parent string, and the length of the resulting substring must not be less than one. The reference pages for the various collider types linked above have further information about their properties and uses. Another widely available option is the detection of syntax which does not conform to the Fortran Standard: this helps to ensure program portability. Note the use of the lexical comparison functions LGE and LLE in the definition of DIGIT which make it completely independent of the local character code. Most Fortran systems allow procedures to be written in other languages and linked with Fortran modules into an executable program. If the procedure interface is similar to that of a Fortran subroutine or function this presents no problem. The next example, which computes the area of a triangle, shows both forms of function in action. Von Kruger and Hans are members of the Thule Society.
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