Why must a programming language have an unambiguous syntax?

Every valid program must have exactly one meaning, because the translator has to choose one sequence of machine instructions and the programmer must be able to predict what will run.

Give an example of a sentence made ambiguous by a word having two meanings.

The sentence she left the bank can mean that she left a financial institution or that she left the edge of a river.

Give an example of a sentence that is ambiguous because of the way its words group.

The sentence I saw the man with the telescope can mean that a telescope was used to see him or that he was the one carrying it.

Why would the expression 6 − 2 × 3 be ambiguous without a rule of precedence?

Taken strictly from left to right it gives 12, while doing the multiplication first gives 0, so a rule is needed to fix which reading is intended.

WJEC A-Level Computer Science 601-5345-3

Hardware: Architecture, Memory and Storage

Topic overview Components of a computer system and how they connect Von Neumann architecture and the CPU The fetch-execute cycle Memory and caching Parallel processing Input and output devices and their uses Secondary storage, fragmentation and defragmentation

Networks and Data Transmission

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Logic Gates and Boolean Algebra

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Data Representation and Data Types

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Database Systems

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The Operating System

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Algorithms and Programs

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Systems Analysis

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Practical Programming

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Data Security and Integrity

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Computing, Society and the Law

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Advanced Data Structures and Logic

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Recursion, Sorting and Searching

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Programming Paradigms and Translators

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System Design and Human-Computer Interaction

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Software Engineering Tools and Professional Practice

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Advanced Architecture and Assembly Language

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Communication Networks and File Organisation

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Number Representation and Floating Point

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Databases and Distributed Systems

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Operating System Management

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Software Systems and Applications

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Data Security, Cryptography and Attack

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Programming Paradigms and Translators

Ambiguity in natural language and unambiguous notation

3 things to know
A2 Unit 3

Overview

Lexical ambiguity, sentence made ambiguous by a word having two meanings and structural ambiguity.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Evaluation

How is a stack used to evaluate an expression written in reverse Polish notation?

Key term

What is meant by lexical ambiguity?

Key term

What is meant by structural ambiguity?

Questions from this subtopic

Each one with the model answer underneath.

1
Why must a programming language have an unambiguous syntax?
Model answer
Every valid program must have exactly one meaning, because the translator has to choose one sequence of machine instructions and the programmer must be able to predict what will run.
2
What is meant by lexical ambiguity?
Model answer
Lexical ambiguity arises when a single word carries more than one meaning, so a statement containing it can be read in more than one way.
3
Give an example of a sentence made ambiguous by a word having two meanings.
Model answer
The sentence she left the bank can mean that she left a financial institution or that she left the edge of a river.
4
What is meant by structural ambiguity?
Model answer
Structural ambiguity arises when the individual words are clear but can be grouped in more than one way, and each grouping gives a different meaning.
5
Give an example of a sentence that is ambiguous because of the way its words group.
Model answer
The sentence I saw the man with the telescope can mean that a telescope was used to see him or that he was the one carrying it.
6
Why would the expression 6 − 2 × 3 be ambiguous without a rule of precedence?
Model answer
Taken strictly from left to right it gives 12, while doing the multiplication first gives 0, so a rule is needed to fix which reading is intended.

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