Symbols

1
Symbols
2
Outline

• Prerequisites
• Basic Scheme functions
• Boolean expressions
• Conditionals
• Objectives
• Symbols
• Strings
• Reference
• HTDP Section 5

3
Background

• We frequently need to reason about ideas rather
  than numbers
• Most languages provide the ability to express
  non-numeric ideas
• Scheme allows the following representations of
  ideas
• Symbols
• Strings
• Characters (later)
• Images (later)

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Symbols

• Sequences of legal characters preceded by a
  single forward quotation mark
• Legal characters are the upper and lower case
  letters, numbers, and some punctuation marks
• - _ _at_ ? are safe to use
• Do not use symbols with special meaning
• ' " \ gt lt ltspacegt etc.
• e.g. 'red 'perfect 'on-top-of 'where?

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Manipulating Symbols

• Scheme provides for comparing two symbols
• symbol? symbol symbol -gt boolean
• determines whether two symbols are identical
• For example, a telephone answering system
• auto-switch symbol -gt (dial ltnumbergt)
• switch incoming calls to an extension
• (define (dial n)
• (display n))
• (define (auto-switch choice)
• (cond (symbol? choice 'one) (dial 12)
• (symbol? choice 'two) (dial 8)
• (symbol? choice 'three) (dial 0)
• else (display "say one two or three")))

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Strings

• For now, strings can be treated like special
  symbols
• 0 or more printable characters between quotes
  "..."
• Can include spaces
• Scheme provides for comparing two symbols
• string? string string -gt boolean
• determines whether two strings are
• identical
• Strings are really compound data objects (next
  section)

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Demo
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Actual Code

• auto-switch symbol -gt (dial ltnumbergt)
• switch incoming calls to an extension
• (define (auto-switch choice)
• (cond (symbol? choice 'one) (dial 12)
• (symbol? choice 'two) (dial 8)
• (symbol? choice 'three) (dial 0)
• else (display "say one two or three")))
• (define (dial n)
• (display n))

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Questions?
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Summary

• You should now know
• Symbols
• Strings

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