Lex/Flex tool overview in Compiler Design - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary purpose of Lex/Flex?

Lex/Flex is a tool used in compiler design. What is its main function?

A To parse tokens into a syntax tree

B To optimize machine code for faster execution

C To generate lexical analyzers that break input text into tokens

D To generate executable binary files from source code

💡 Hint

Think about the first step in processing source code in a compiler.

Lex/Flex generates lexical analyzers, also called scanners, which read input text and split it into meaningful pieces called tokens. This is the first step in compiling source code.

📋 Factual

intermediate

Which language is used to write Lex/Flex specifications?

Lex/Flex specifications combine patterns and actions. What language are the actions written in?

A C or C++

B Java

C Python

D Assembly

💡 Hint

Lex/Flex was originally designed to work with a specific programming language.

Lex and Flex generate C code for lexical analyzers, so the actions in the specification are written in C or C++.

🔍 Analysis

advanced

What happens if a Lex/Flex pattern matches multiple rules?

If two or more patterns in a Lex/Flex specification match the same input text, how does Lex/Flex decide which action to execute?

A It chooses the rule that appears first in the specification

B It raises a syntax error and stops

C It chooses the rule with the shortest pattern

D It chooses the rule that matches the longest input text

💡 Hint

Lex/Flex always tries to match as much input as possible.

Lex/Flex uses the longest match rule: it selects the pattern that matches the longest portion of the input text. If there is a tie, it chooses the rule that appears first.

❓ Comparison

advanced

How does Flex differ from the original Lex tool?

Flex is a modern alternative to Lex. Which of the following is a key difference?

A Flex is open source and faster than the original Lex

B Flex only supports Java actions, Lex supports C

C Flex cannot generate scanners for multiple languages

D Flex requires manual memory management unlike Lex

💡 Hint

Consider licensing and performance improvements.

Flex is an open-source tool that is generally faster and more flexible than the original proprietary Lex tool.

❓ Reasoning

expert

What is the output of this Lex/Flex specification snippet?

Given the following Lex/Flex rules, what will be the output when the input text is "abc123"? %{ #include %} %% [a-z]+ { printf("Word\n"); } [0-9]+ { printf("Number\n"); } . { /* ignore other characters */ } %% int main() { yylex(); return 0; }

Compiler Design

%{
#include <stdio.h>
%}

%%
[a-z]+    { printf("Word\n"); }
[0-9]+    { printf("Number\n"); }
.        { /* ignore other characters */ }
%%

int main() {
  yylex();
  return 0;
}

A

Number
Number
Word

B

Word
Number

C

Number
Word

D

Word
Word
Number

💡 Hint

Lex/Flex matches the longest possible token at each step.

The input "abc123" is scanned left to right. The pattern [a-z]+ matches "abc" first, printing "Word". Then [0-9]+ matches "123", printing "Number".