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Lab
2
Task for this week
▪ Implement Tokenizer
▪ Implement Parser
▪ The code structure/skeleton is available on Wattle
▪ This lab contains assessable items!
▪ Submission Guidelines
▪ The last slide contains information about the submission
▪ Read it carefully to avoid losing marks!
The main objective of this task is to implement a simple tokenizer. The input of the tokenizer is a string. Your
goal is to break down the character stream and return a sequence of tokens.
3
First, take a look at Token.java and
understand the types of tokens we want to
extract from a string. There are 7 types of
tokens:
INT: a decimal 32-bit unsigned integer
(e.g. 4869)
ADD: a single add character “+”
SUB: a single minus character “-”
MUL: a single star character “*”
DIV: a single slash character “/”
LBRA: a single left round bracket “(“
RBRA: a single right round bracket “)”
Task 1 – Tokenizer (0.5 marks)
The main tokenizer class, MyTokenizer.java, inherits abstract class Tokenizer.java which has three
methods:
hasNext() – check if there are still other tokens in the buffer
current() – return the current token
next() – find and extract a new token from buffer and store it in the current token
MyTokenizer.java implements Tokenizer.java and defines two additional private fields: _buffer
and currentToken. _buffer keeps the string that we want to tokenize, and currentToken keeps a Token
instance which is extracted by next() method.
Read the comments in the code to understand the details of how these functions must be implemented.
4
Task 1 – Tokenizer
Check the constructor of MyTokenizer, which will initially save the input string to _buffer and extract the first
token using next(). After, check the implementation of hasNext() and current() in MyTokenizer.
You job is to implement the missing part
of the next() method. Do not change
any other part of the code.
We implemented the (+) and (
-) tokens
for you. The other methods should be
implemented in a similar way (the
integer tokenizer may be a bit different).
White space is also removed for you.
Modify the next() implementation such
that it deals with the tokens “*”, “/”, “(“,
“)” and unsigned integer.
5
Task 1
– Tokenizer
Before implementing the actual method, please go and check the test cases in TokenizerTest.java.
Test cases are one of the best ways to define the requirements of the code (see below one example).
6
Task 1 – Tokenizer
7
Task 1 – Tokenizer
Do not forget to implement your code within the block indicated by the following comments:
‘YOUR CODE STARTS HERE’ and ‘YOUR CODE ENDS HERE’.
How to test your code?
The TokenizerTest.java file has 4 test cases, check each of them.
To assess your code, we will use 5 different test cases, each worth 0.5/5. We will test each method with
empty tokens, and evaluate different expressions.
Again: check the submission guidelines to avoid losing marks (you may get zero marks)!
The main objective of this part is to implement a simple parser. It is simple, but not easy, you need to dedicate
some time to understand the code and start implementing it. Besides that, to complete this task, you first need
to complete Task 1.
After you implement the tokeniser, copy the files Token.java, MyTokenizer.java, and Tokenizer.java
to your Parser project.
Now the goal of this task is to implement a parser for the following grammar:
8
Task 2 – Parser (1.5 marks)
::= | + | -
::= | * | /
::= | ( )
You can try to recreate it using this online parser:
https://web.stanford.edu/class/archive/cs/cs103/cs103.1156/tools/cfg/
Try to understand the grammar and implement the parser.
Go through Exp.java, AddExp.java, IntExp.java, SubExp.java, DivExp.java, MultExp.java files:
Check the implementations of show() and evaluate() methods:
- show() method is designed to return the content of parsed expression
- evaluate() method evaluates and executes the expression and return the result.
Go through ParserTest.java:
ParserTest is a JUnit test class, which defines the proper behaviour of the parser through a set of examples.
Read the code and try to understand what the requirements of the parser are.
Implement the following missing parts of Parser.java:
- parseExp() method
- parseTerm() method
- parseFactor() method
*It can be difficult to start, but I will give you some hints in the next slide! If you do not understand it, ask for help, your tutor
is there to help you understand it.
9
Task 2 – Parser
10
Task 2
– Parser
Note that here I give some tips to start
implementing parseExp().
Try to understand all classes and check
the ParserTest.java. It may help you
understand even more the code.
Ask your tutor if you need help. It is
important that you understand how to
implement a parser!
It is time to code!
11
Task 2 – Parser
Do not forget to implement your code within the block indicated by the following comments:
‘YOUR CODE STARTS HERE’ and ‘YOUR CODE ENDS HERE’.
How to test your code?
The ParserTest.java file has 7 test cases, check each of them.
To assess your code, we will use 13 different test cases, each worth 1.5/13. We will test each method
and evaluate different expressions.
Again: check the submission guidelines to avoid losing marks (you may get zero marks)!
12
Task 2 – Parser
Note that we evaluate the correctness of the code based on the methods: show() and evaluate().
13
Submission Guidelines
▪ Assignment deadline: see the deadline on Wattle (always!)
▪ Submission mode: via Wattle (Lab Tokenizer and Parsing)
Submission format (IMPORTANT):
▪ Upload only your final version of MyTokenizer.java (for task 1) and Parser.java (for task 2) to Wattle
▪ Each test case must run for at most 1000ms, otherwise it will fail (zero marks).
▪ Do not change the file names
▪ Do not upload any other files (only the specified files are needed)
▪ Do not upload a folder (your submission should be only two java files).
▪ The answers will be marked by an automated marker.
▪ Do not change the structure of the source code including class name, package structure, etc.
▪ You are only allowed to edit the designated code segment indicated in the comments.
▪ Do not import packages outside of the standard java SE package. The list of available packages can be found here:
https://docs.oracle.com/en/java/javase/12/docs/api/index.html
▪ Any violation of the submission format will result in zero marks
▪ Reference: see lecture slides / https://web.stanford.edu/class/archive/cs/cs103/cs103.1156/tools/cfg/
Lab
2
Task for this week
▪ Implement Tokenizer
▪ Implement Parser
▪ The code structure/skeleton is available on Wattle
▪ This lab contains assessable items!
▪ Submission Guidelines
▪ The last slide contains information about the submission
▪ Read it carefully to avoid losing marks!
The main objective of this task is to implement a simple tokenizer. The input of the tokenizer is a string. Your
goal is to break down the character stream and return a sequence of tokens.
3
First, take a look at Token.java and
understand the types of tokens we want to
extract from a string. There are 7 types of
tokens:
INT: a decimal 32-bit unsigned integer
(e.g. 4869)
ADD: a single add character “+”
SUB: a single minus character “-”
MUL: a single star character “*”
DIV: a single slash character “/”
LBRA: a single left round bracket “(“
RBRA: a single right round bracket “)”
Task 1 – Tokenizer (0.5 marks)
The main tokenizer class, MyTokenizer.java, inherits abstract class Tokenizer.java which has three
methods:
hasNext() – check if there are still other tokens in the buffer
current() – return the current token
next() – find and extract a new token from buffer and store it in the current token
MyTokenizer.java implements Tokenizer.java and defines two additional private fields: _buffer
and currentToken. _buffer keeps the string that we want to tokenize, and currentToken keeps a Token
instance which is extracted by next() method.
Read the comments in the code to understand the details of how these functions must be implemented.
4
Task 1 – Tokenizer
Check the constructor of MyTokenizer, which will initially save the input string to _buffer and extract the first
token using next(). After, check the implementation of hasNext() and current() in MyTokenizer.
You job is to implement the missing part
of the next() method. Do not change
any other part of the code.
We implemented the (+) and (
-) tokens
for you. The other methods should be
implemented in a similar way (the
integer tokenizer may be a bit different).
White space is also removed for you.
Modify the next() implementation such
that it deals with the tokens “*”, “/”, “(“,
“)” and unsigned integer.
5
Task 1
– Tokenizer
Before implementing the actual method, please go and check the test cases in TokenizerTest.java.
Test cases are one of the best ways to define the requirements of the code (see below one example).
6
Task 1 – Tokenizer
7
Task 1 – Tokenizer
Do not forget to implement your code within the block indicated by the following comments:
‘YOUR CODE STARTS HERE’ and ‘YOUR CODE ENDS HERE’.
How to test your code?
The TokenizerTest.java file has 4 test cases, check each of them.
To assess your code, we will use 5 different test cases, each worth 0.5/5. We will test each method with
empty tokens, and evaluate different expressions.
Again: check the submission guidelines to avoid losing marks (you may get zero marks)!
The main objective of this part is to implement a simple parser. It is simple, but not easy, you need to dedicate
some time to understand the code and start implementing it. Besides that, to complete this task, you first need
to complete Task 1.
After you implement the tokeniser, copy the files Token.java, MyTokenizer.java, and Tokenizer.java
to your Parser project.
Now the goal of this task is to implement a parser for the following grammar:
8
Task 2 – Parser (1.5 marks)
You can try to recreate it using this online parser:
https://web.stanford.edu/class/archive/cs/cs103/cs103.1156/tools/cfg/
Try to understand the grammar and implement the parser.
Go through Exp.java, AddExp.java, IntExp.java, SubExp.java, DivExp.java, MultExp.java files:
Check the implementations of show() and evaluate() methods:
- show() method is designed to return the content of parsed expression
- evaluate() method evaluates and executes the expression and return the result.
Go through ParserTest.java:
ParserTest is a JUnit test class, which defines the proper behaviour of the parser through a set of examples.
Read the code and try to understand what the requirements of the parser are.
Implement the following missing parts of Parser.java:
- parseExp() method
- parseTerm() method
- parseFactor() method
*It can be difficult to start, but I will give you some hints in the next slide! If you do not understand it, ask for help, your tutor
is there to help you understand it.
9
Task 2 – Parser
10
Task 2
– Parser
Note that here I give some tips to start
implementing parseExp().
Try to understand all classes and check
the ParserTest.java. It may help you
understand even more the code.
Ask your tutor if you need help. It is
important that you understand how to
implement a parser!
It is time to code!
11
Task 2 – Parser
Do not forget to implement your code within the block indicated by the following comments:
‘YOUR CODE STARTS HERE’ and ‘YOUR CODE ENDS HERE’.
How to test your code?
The ParserTest.java file has 7 test cases, check each of them.
To assess your code, we will use 13 different test cases, each worth 1.5/13. We will test each method
and evaluate different expressions.
Again: check the submission guidelines to avoid losing marks (you may get zero marks)!
12
Task 2 – Parser
Note that we evaluate the correctness of the code based on the methods: show() and evaluate().
13
Submission Guidelines
▪ Assignment deadline: see the deadline on Wattle (always!)
▪ Submission mode: via Wattle (Lab Tokenizer and Parsing)
Submission format (IMPORTANT):
▪ Upload only your final version of MyTokenizer.java (for task 1) and Parser.java (for task 2) to Wattle
▪ Each test case must run for at most 1000ms, otherwise it will fail (zero marks).
▪ Do not change the file names
▪ Do not upload any other files (only the specified files are needed)
▪ Do not upload a folder (your submission should be only two java files).
▪ The answers will be marked by an automated marker.
▪ Do not change the structure of the source code including class name, package structure, etc.
▪ You are only allowed to edit the designated code segment indicated in the comments.
▪ Do not import packages outside of the standard java SE package. The list of available packages can be found here:
https://docs.oracle.com/en/java/javase/12/docs/api/index.html
▪ Any violation of the submission format will result in zero marks
▪ Reference: see lecture slides / https://web.stanford.edu/class/archive/cs/cs103/cs103.1156/tools/cfg/