Sample LA: Program style

Write and structure program code in ways that makes it easier to understand.

Although it is early in the semester, you have already started to learn some basic approaches to making your code more readable. Here are a few.

• Put all of the parameters to a procedure on the same line, or all on separate lines.
  • Similarly, when writing an if statement, put the test, consequent, and alternate all on the same line or all on separate lines.
• Use Ctrl-i to reindent the text.
• When defining a procedure, place the lambda and the parameters on a separate line from the define, and start the body on a separate line from the from the lambda and the parameters.
• Follow “The Zen of Booleans”: Avoid explicitly using #t and #f in expressions if you can write them more clearly with and or or.
• Avoid repeated code.
• Avoid unnecessary code.
• Choose descriptive names for variables and procedures.

a. Using those principles, as well as others you have learned, clean up the following code.

(define rab (lambda
(int
str)
(if (if (equal? #t (integer? (string->number (substring int str))))
#t #f) (string->number (substring int str))
(if (if (equal? (integer?
(string->number (substring int 0 str))) #f) #f
#t
) (string->number (substring int 0
str)) #f))))

b. In English, describe what rab does.

Here are some sample calls to help get you started.

> (rab "876xy12" 0)
#f
> (rab "876xy12" 1)
8
> (rab "876xy12" 2)
87
> (rab "876xy12" 3)
876
> (rab "876xy12" 4)
#f
> (rab "876xy12" 5)
12
> (rab "876xy12" 6)
2
> (rab "876xy12" 7)
#f
> (rab "876xy12" 8)
. . substring: starting index is out of range
  starting index: 8
    valid range: [0, 7]
      string: "876xy12"