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Software Engineering and Program Construction

Lexical analysis and the symbol table

  • 3 things to know
  • AS Unit 1

Overview

First stage of the compilation process, two things a lexical analyser removes from the source program and error can be detected during lexical analysis.

What you need to know

Answer each one before you reveal it.

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Process

What does a translator record about each identifier, and how is that record used by the stages that follow?

Process

What is the first stage of the compilation process?

Key term

State four items of information a symbol table entry may hold.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is the first stage of the compilation process?

    Model answer

    Lexical analysis, which reads the characters of the source program and groups them into the basic items of the language.

  2. 2

    What is a token?

    Model answer

    A token is a single meaningful item of the language, such as a keyword, an identifier, an operator, a number or a punctuation symbol, represented in a compact coded form.

  3. 3

    State two things a lexical analyser removes from the source program.

    Model answer

    It removes comments and unnecessary spacing such as spaces, tabs and blank lines, because neither affects the meaning of the program.

  4. 4

    Why does removing spacing and comments make the later stages easier?

    Model answer

    Later stages then work on a short, regular stream of tokens rather than on raw text whose layout can vary, so their rules can be about language structure alone.

  5. 5

    Explain how a lexical analyser distinguishes a keyword from a name the programmer has chosen.

    Model answer

    It compares each word it collects against the fixed list of words reserved by the language, and anything not on that list is treated as an identifier.

  6. 6

    What kind of error can be detected during lexical analysis?

    Model answer

    A character or symbol that is not part of the language at all, or a badly formed item such as a number containing a letter, can be reported at this stage.

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