NAG Library Chapter Introduction

X04 – Input/Output Utilities

This chapter contains utility routines concerned with input and output to or from an external file.

Output from NAG Library routines to an external file falls into two categories.

(a)	Error messages which are always associated with an error exit from a routine, that is, with a nonzero value of IFAIL as specified in Section 6 of the routine document.
(b)	Advisory messages which include output of final results, output of intermediate results to monitor the course of a computation, and various warning or informative messages.

Each category of output is written to its own Fortran output unit – the error message unit or the advisory message unit. In practice these may be the same unit number. Default unit numbers are provided for each implementation of the Library (see the Users' Note for your implementation); they may be changed by users. Output of error messages may be controlled by the setting of IFAIL (see Section 3.3.2 in the Essential Introduction). Output of advisory messages may usually be controlled by the setting of some other parameter (e.g., MSGLVL) (or in some routines also by IFAIL). An alternative mechanism for completely suppressing output is to set the relevant unit number $<0$.

At present only formatted records are output from the Library. All formatted output to an external file from within the Library is performed by X04BAF. Similarly, all formatted input from an external file is performed by X04BBF.

When the library is being called from another language, such as C or Visual Basic, the routines X04ACF and X04ADF may be especially useful: X04ACF connects a file to a FORTRAN unit; X04ADF disconnects a file from a FORTRAN unit.

Routines are provided to allow formatted output of

(a)	general matrices stored in a two-dimensional array (real, complex and integer data types);
(b)	triangular matrices stored in a packed one-dimensional array (real and complex data types);
(c)	band matrices stored in a packed two-dimensional array (real and complex data types).

Routines in (b) and (c) allow printing of matrices stored in formats used in particular by Chapters F06 and F07 of the Library.

By appropriate choice of parameters you can specify titles, labels, maximum output record length, and the format of individual matrix elements. All output is directed to the advisory messages unit, which may be altered by a call to X04ABF.

Apart from the obvious utility of the matrix printing routines, users of the Library may need to call routines in Chapter X04 for the following purposes.

If the default error message unit (given in the Users' Note for your implementation) is not satisfactory, it may be changed to a new value NERR by the statement

CALL X04AAF(1,NERR)

Similarly the advisory message unit may be changed to a new value NADV by the statement

CALL X04ABF(1,NADV)

Note that both X04AAF and X04ABF use a Fortran SAVE statement to retain the value of the unit number and so neither routine is safe to use in a multithreaded environment.

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