1. AI Programming
Week Two:
Procedures, the Stack & Debugging
Richard Price
rmp@cs.bham.ac.uk

2. Overview
Introduction
•
• Recap
• Procedures
• The Stack
2

3. Course Details
http://www.cs.bham.ac.uk/internal/courses/ai-prog-a/
• Lab sessions:
– Monday 3pm - 5pm.
– Friday 2pm-4pm.
• Assessment Deadline
– Mondays 5pm.
• Late submissions until:
– Wednesday 10am.
– 50% penalty.
3

4. Recap
• Variables & Expressions
– Print arrow
– Semicolon
– Declaration
– Assignment
– Data Types
– Comments.
4

5. List
• Lists:
[1 2 3] =>
** [1 2 3]
lvars mylist = [19 18 25];
mylist =>
mylist(1) =>
** 19
lvars listInAList = [1 [2 3]];
listInAList(2) =>
** [2 3];
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6. Procedures
• Programs can become very large.
• Procedures break down code into pieces.
– Reusable.
– Recognisable.
– Named mini-program.
6

7. Built in procedures
• Addition and division are built in
procedures.
• Other examples:
max(2,5) =>
** 5
min(2,5) =>
** 2
• In Xved, press Enter then type help max.
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8. Write your own procedures
• Note the brackets in max(2,5);
define myprocedure()
;;; code goes here
enddefine;
myprocedure();
define printTwo()
2 =>
enddefine;
printTwo();
** 2
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9. Variables & Procedures
• lvars declares local variables.
– Local to a procedure.
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10. Arguments & Procedues
• Procedures often take in arguments.
– +(oneNumber, anotherNumber);
– max(argument1, argument2);
define printTwice(valueToPrint);
valueToPrint =>
valueToPrint =>
enddefine;
printTwice(‘Rich’);
** Rich
** Rich
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11. Arguments & Procedures
• Write a procedure called addition that:
– Takes in two numbers as arguments.
– Adds them together.
– And prints the result.
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12. Returning results
• Procedures often return results:
– Max returns the maximum of two numbers.
define max(firstNumber, secondNumber) -> largerNumber;
…
enddefine;
max(5,25) =>
** 25
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13. Multiple Arguments & Results
• Procedures often return results:
– Max returns the maximum of two numbers.
define printAPlusBPlusC(a,b,c)
a + b + c =>
enddefine;
define returnAthenB(a,b) -> (result1, result2);
a -> result1;
b -> result2;
enddefine;
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14. Procedures in Procedures
• Procedures can be used within
procedures:
define minOfThree(number1, number2, number3) -> smallestOfThree;
lvars smallestOfTwo;
min(number1, number2) -> smallestOfTwo;
min(number3, smallestOfTwo) -> smallestOfThree;
enddefine;
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15. The Pop-11 Stack
• Almost everything in Pop-11 uses the
stack.
• If variables are just containers.
• Then the stack is a pile of boxes.
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16. Assignment and the Stack
• Assignment in Pop-11
– Uses the stack!
– lvars creates the x variable.
– 4 is place on the stack
– Assignment pops 4 off.
– Into the x variable.
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17. 17

18. Viewing the Stack
lvars myVariable;
5 -> myVariable;
myVariable =>
4;
5;
6;
=>
** 4 5 6
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19. Procedures & the stack
3;
11;
max();
=>
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