Open Watcom FORTRAN 77 Language Reference

10 Input/Output
10.1 Introduction
FORTRAN 77 provides a means of communicating information or data between a FORTRAN program
and the computing environment. The computing environment may include a number of devices which are
capable of the recording, retrieval, display, and input of data. Disk and magnetic tape devices are capable
of storing large amounts of data. Other devices such as printers and display terminals can be used to
present a visual (i.e., human-readable) representation of the data. Yet other devices such as terminal
keyboards and card-readers make possible the entry of new data into the computing environment.
For the purposes of our discussion, data is any information which can be processed by an executing
FORTRAN program. Some examples of data are names, addresses, telephone numbers, credit card
balances, flight trajectories, bus schedules, athletic records, etc. In computing, such information is usually
well-organized in order to make it useful for processing.
To use an example, consider the entries in a telephone book. There are essentially three pieces of data
listed for each entry; a name, an address, and a number.
Smith J 32 Arthur St--------------------------555-3208
Smith JW 512 King St--------------------------555-9229
Smith Jack 255-113 Queen St N-----------------555-0572
Each entry is a record. The organization of the book is clear. The name is always listed first, the address
second, and the number last. The records are sorted, for our convenience, by name (within each city or
geographical location). The length of each record is the same. This fixed length does sometimes lead to
problems since entries which have a long name or address won’t fit in a record. The phone company
solved this by continuing the information in subsequent records. We might have solved this problem by
increasing the length of a record with the disadvantage of wasting a lot of printing space. Alternatively, we
could have used a variable length record. This solves the problem of wasted space but creates a severe
problem when trying to display the records in nice orderly columns. The telephone book itself is a
collection of records or a file.
We have introduced much of the terminology of data processing: "data", "records", "fixed and variable
record sizes", "files", "sorted", etc.
10.2 Reading and Writing
FORTRAN provides a mechanism called "reading" for transferring data into the environment of an
executing program. The READ statement is used to do this. Similarly "writing" is the mechanism for
transferring data out of an executing program. The WRITE and PRINT statements are used to do this.
Other statements provide additional functions such as positioning to a certain record in a file, establishing
which files are to be processed by the program, or making inquiries about files.
Reading and Writing 215