off_bottom_of_page |
% boolean |
/regular_proc load |
% “false” proc |
(a random string) |
|
/off_bottom_proc load |
% “true” proc |
exch pop exch |
% get rid of random string |
ifelse |
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Another interesting example that illustrates the fact that ifelse is just an ordinary operator is Example 7.11, which redefines the ifelse operator to get a little extra information while it is executing, for debugging purposes.
Example 7.11: Redefining ifelse for Debugging
% redefine “ifelse” to trace which clause is being executed: /orig_ifelse /ifelse load def
/ifelse { %def
2 index dup (condition: ) print == { %ifelse
1 index == }{ %else
dup == } orig_ifelse orig_ifelse
} bind def
NESTED CONDITIONALS AND ELSE CLAUSES
Fairly often, in a complicated program, you need to have conditionals that have more conditionals on each of the two result clauses. When these nest more than one level deep, it can become tricky to maintain them without leaving things on the operand stack or introducing bugs of some sort.
The first issue is simply balancing the curly braces correctly, so that there are no basic syntactic errors in the program. Using the template technique for this is a good approach, and one that can be expanded as necessary later (see Example 7.12).
Chapter 7: BUILDING CONDITIONAL STATEMENTS |
87 |