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ITEC 320

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ITEC 320

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  1. ITEC 320 Lecture 11 Application Part Deux Look Ahead

  2. Review • In-class coding • What did you learn?

  3. Outline • Look ahead

  4. Input • What are the problems with ADA input? • What would need to be added to solve this?

  5. Example EOL: Boolean; c: Character; -- Caution: This example ignores EOL begin lookAhead(c, EOL); put(c); -- Output is 'H' lookAhead(c, EOL); put(c); -- Output is 'H' get(c); lookAhead(c, EOL); put(c); -- Output is 'e’ end example; Assumes input Hello

  6. Motivation • Allows you to figure out how to read next bit of input lookAhead(c, EOL); -- Use get to input if Ada.Characters.Handling.Is_Digit(c) then Ada.Integer.Text_io.get(n); -- Get an integer else Ada.Text_io.get(c); -- Get a character end if;

  7. Motivation • Allows multiple portions of code to access the same input • Loop conditionals loop LookAhead(c, EOL); exit when is_Something(c); -- do something with c get(c); end loop; For example, when reading 123+456 This can be used to help a compiler How else could this be accomplished?

  8. Issue • What happens when you hit the end of line? • Value returned depends on which way the wind blows EOL: Boolean; c: Character; begin loop lookAhead(c, EOL); if EOL then – End of line Ada.text_IO.Skip_line; else if is_digit(c) then -- do a numeric get else -- do a character get end if; end if; end loop;

  9. Review • EOL false • Doesn’t advance input stream • Will return same value until some form of get is called • EOL true • What it returns doesn’t matter

  10. Longer examples • Web examples

  11. White space • What is white space? • Code to skip white space?

  12. Others • Java • java.io.PushbackInputStream(InputStream) - constructs a stream that can be used to push back one byte • java.io.PushbackInputStream(InputStream, int size) - constructs a stream that can be used to push back size bytes • void unread(int b) pushes back the byte b which will be input again by the next read. Only one byte can be pushed back at a time. • C++ • unget and putback • C • ungetc

  13. Example • Given a string, compute recursively (no loops) the number of times lowercase "hi" appears in the string.

  14. Summary • Application coding • Other languages