Lex/Flex tool overview in Compiler Design - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the section where definitions are placed in a Lex/Flex file.

Compiler Design

%[1]%

Drag options to blanks, or click blank then click option'

Arules

Bdefinitions

Cuser code

Doptions

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the definitions section with the rules section.

The definitions section in a Lex/Flex file is where you define macros and patterns used later in the rules section.

2fill in blank

medium

Complete the code to write a rule that matches the word 'if' in Lex/Flex.

Compiler Design

"[1]"    { return IF; }

Drag options to blanks, or click blank then click option'

Awhile

Belse

Cif

Dfor

Attempts:

3 left

💡 Hint

Common Mistakes

Using single quotes instead of double quotes.

Writing the keyword without quotes.

In Lex/Flex, rules are written as patterns in quotes. To match the keyword if, you write it as "if".

3fill in blank

hard

Fix the error in the Lex/Flex rule to correctly match an identifier (a letter followed by letters or digits).

Compiler Design

[1]    { return IDENTIFIER; }

Drag options to blanks, or click blank then click option'

A[a-zA-Z][a-zA-Z0-9]*

B[0-9]+

C"[a-zA-Z]+"

D[^a-zA-Z0-9]

Attempts:

3 left

💡 Hint

Common Mistakes

Using only digits in the pattern.

Including quotes around the pattern incorrectly.

The correct pattern for an identifier is a letter followed by zero or more letters or digits, written as [a-zA-Z][a-zA-Z0-9]*.

4fill in blank

hard

Fill both blanks to create a rule that matches whitespace characters and ignores them.

Compiler Design

[1]    { [2]; }

Drag options to blanks, or click blank then click option'

A[ \t\n]+

Breturn

C/* ignore */

D/* skip */

Attempts:

3 left

💡 Hint

Common Mistakes

Returning a token instead of skipping.

Not matching all whitespace characters.

The pattern [ \t\n]+ matches one or more whitespace characters (space, tab, newline). The action /* skip */ indicates to ignore these characters.

5fill in blank

hard

Fill all three blanks to define a Lex/Flex rule that matches a number and converts it to an integer.

Compiler Design

"[1]"    { yylval = atoi(yytext); return [2]; [3] }

Drag options to blanks, or click blank then click option'

A[0-9]+

BNUMBER

C/* convert to int */

Dreturn

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect pattern for numbers.

Forgetting to return the token.

Not converting the text to integer.

The pattern [0-9]+ matches one or more digits. The action converts the matched text to an integer using atoi, returns the token NUMBER, and includes a comment for clarity.