Some programmers complain about the use of global variables in a program. One complaint is that it is difficult to see what information is being passed to a function unless all that information is passed as parameters. Sometimes global variables are very useful however, and this problem need not be crippling. A way to make this clear is to write global variables in capital letters only, while writing the rest of the variables in mainly small letters..

{ int local integer;

This allows global variables to be spotted easily. Another reason for restricting the use of global variables is that it is easier to debug a program if only local variables are used. The reason is that once a function capsule is tested and sealed it can be guaranteed to work in all cases, provided it is not affected by any other functions from outside. Global variables punch holes in
the sealed function capsules because they allow bugs from other functions to creep into tried and tested ones. An alert and careful programmer can usually control this without difficulty.

The following guidelines may help the reader to decide whether to use
local or global data:

• Always think of using a local variable first. Is it impractical? Yes, if it means passing dozens of parameters to functions, or reproducing a lot of variables. Global variables will sometimes tidy up a program.

• Local variables make the flow of data in a program clearer and they reduce the amount of memory used by the program when they are not in use.

• The preference in this book is to use local variables for all work, except where a program centres around a single data structure. If a data structure is the main reason for a program’s existence, it is nearly always defined globally.

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