AS Level11Paper 2 — Problem-solving and Programming

Programming

Writing real, structured programs: procedures, functions, parameters and scope.

YOU SHOULD BE ABLE TO

Write programs using sequence, selection and iteration.
Write and call procedures and functions with parameters.
Explain the difference between passing by value and by reference.
Explain local and global scope.
Use built-in functions for string handling and arithmetic.
Write maintainable code with meaningful names and comments.

RUN THE PSEUDOCODE

Write the pseudocode your syllabus uses and press Run. Assignment is the arrow — type <- and it works the same as ←. Both syllabuses are accepted, so OUTPUT and PRINT both work; where a word differs from the one the exam prints, the runner says so.

PROGRAM

DECLARE Name : STRING
DECLARE Age : INTEGER

Name <- "Bat"
Age <- 14

OUTPUT "Hello ", Name
OUTPUT "Next year you are ", Age + 1

INPUT — ONE VALUE PER LINE

OUTPUT

Press Run to see what it does

TRACE TABLE

Total ← 0
FOR Count ← 1 TO N
    Total ← Total + Count
NEXT Count
OUTPUT Total

STARTN = 5

Count	Total	OUTPUT

One row per pass through the loop. Leave a cell blank if it does not change on that pass.

//By value or by reference

BYVALUE passes a COPY, so changes inside the procedure do not affect the caller's variable. BYREF passes the original, so changes DO affect it. Use BYREF when the procedure must give something back through a parameter.

PROCEDURE Double(BYVALUE n : INTEGER)
   n ← n * 2        // caller's variable unchanged
ENDPROCEDURE

PROCEDURE Double(BYREF n : INTEGER)
   n ← n * 2        // caller's variable IS changed
ENDPROCEDURE

//Procedure or function?

A FUNCTION returns a value and is used inside an expression: x ← Square(5). A PROCEDURE performs an action and is CALLed on its own line: CALL PrintHeader(). Choosing the wrong one loses marks even if the logic is right.

//String handling functions

Know the standard ones and that positions usually start at 1 in pseudocode.

LENGTH("Hello")        = 5
SUBSTRING("Hello",1,3) = "Hel"
UCASE("hi")            = "HI"
LCASE("HI")            = "hi"

CHECK YOURSELF

1.A procedure changes a parameter passed BYVALUE. What happens to the caller's variable?

2.Which should you use when the code must give a value back for use in an expression?

Data Types and Structures Software Development

RUN THE PSEUDOCODE

PROGRAM

DECLARE Name : STRING
DECLARE Age : INTEGER

Name <- "Bat"
Age <- 14

OUTPUT "Hello ", Name
OUTPUT "Next year you are ", Age + 1

INPUT — ONE VALUE PER LINE

OUTPUT

Press Run to see what it does

Count

Total

OUTPUT

//By value or by reference

PROCEDURE Double(BYVALUE n : INTEGER)
   n ← n * 2        // caller's variable unchanged
ENDPROCEDURE

PROCEDURE Double(BYREF n : INTEGER)
   n ← n * 2        // caller's variable IS changed
ENDPROCEDURE