IGCSE2.1.3Paper 2 — Algorithms, Programming and Logic

Pseudocode and Trace Tables

The exact style of pseudocode Cambridge uses in exams, and how to trace an algorithm by hand.

YOU SHOULD BE ABLE TO

Read and write algorithms in Cambridge pseudocode.
Use sequence, selection and iteration.
Use totalling, counting, maximum, minimum and average.
Complete a trace table for a given algorithm.
Identify and correct errors in an algorithm.

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.

//This is not Python — the exam has its own style

Assignment uses a left arrow, keywords are capitalised, and every block is explicitly closed. You must be able to both READ and WRITE it.

DECLARE Count : INTEGER
DECLARE Name  : STRING

Count ← 0
OUTPUT "What is your name?"
INPUT Name

IF Count > 10 THEN
   OUTPUT "Too many"
ELSE
   OUTPUT "Fine"
ENDIF

CASE OF Grade
   'A' : OUTPUT "Excellent"
   'B' : OUTPUT "Good"
   OTHERWISE OUTPUT "See me"
ENDCASE

FOR i ← 1 TO 10
   OUTPUT i
NEXT i

WHILE Count < 5 DO
   Count ← Count + 1
ENDWHILE

REPEAT
   Count ← Count - 1
UNTIL Count = 0

//The five standard routines

Almost every Paper 2 question is built from these. Learn the shape of each.

TOTAL     Total ← 0            (before the loop!)
          Total ← Total + Number

COUNT     Count ← 0
          Count ← Count + 1

MAXIMUM   Max ← first value      (NOT 0)
          IF Number > Max THEN Max ← Number

MINIMUM   Min ← first value
          IF Number < Min THEN Min ← Number

AVERAGE   Average ← Total / Count

//Trace tables — work mechanically

One column per variable, plus one for OUTPUT. Add a row every time any variable changes. Do not try to guess the answer: the marks are for the working, and guessing is how people go wrong.

X ← 10
WHILE X > 6 DO
   X ← X - 2
   OUTPUT X
ENDWHILE

  X  | OUTPUT
 10  |
  8  | 8
  6  | 6      <- loop now stops, 6 is not > 6

IN THE EXAM

Assignment is ← , not =. Using = for assignment can lose the mark.
Always close blocks: ENDIF, ENDWHILE, NEXT, ENDCASE.
Initialise totals and counters to 0 BEFORE the loop — this is the single most common lost mark.
Seed a maximum with the first value, not 0, or negative data breaks it.

CHECK YOURSELF

1.In Cambridge pseudocode, which symbol assigns a value?

2.Where must a total be set to 0?

3.Trace: X ← 8, then WHILE X > 4 DO X ← X - 2, OUTPUT X. What is printed?

4.Which loop is guaranteed to run at least once?

Sub-systems and Decomposition Programming Concepts

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

//This is not Python — the exam has its own style

Assignment uses a left arrow, keywords are capitalised, and every block is explicitly closed. You must be able to both READ and WRITE it.

DECLARE Count : INTEGER
DECLARE Name  : STRING

Count ← 0
OUTPUT "What is your name?"
INPUT Name

IF Count > 10 THEN
   OUTPUT "Too many"
ELSE
   OUTPUT "Fine"
ENDIF

CASE OF Grade
   'A' : OUTPUT "Excellent"
   'B' : OUTPUT "Good"
   OTHERWISE OUTPUT "See me"
ENDCASE

FOR i ← 1 TO 10
   OUTPUT i
NEXT i

WHILE Count < 5 DO
   Count ← Count + 1
ENDWHILE

REPEAT
   Count ← Count - 1
UNTIL Count = 0

//The five standard routines

Almost every Paper 2 question is built from these. Learn the shape of each.

TOTAL     Total ← 0            (before the loop!)
          Total ← Total + Number

COUNT     Count ← 0
          Count ← Count + 1

MAXIMUM   Max ← first value      (NOT 0)
          IF Number > Max THEN Max ← Number

MINIMUM   Min ← first value
          IF Number < Min THEN Min ← Number

AVERAGE   Average ← Total / Count

//Trace tables — work mechanically

One column per variable, plus one for OUTPUT. Add a row every time any variable changes. Do not try to guess the answer: the marks are for the working, and guessing is how people go wrong.

X ← 10
WHILE X > 6 DO
   X ← X - 2
   OUTPUT X
ENDWHILE

  X  | OUTPUT
 10  |
  8  | 8
  6  | 6      <- loop now stops, 6 is not > 6

CHECK YOURSELF

1.In Cambridge pseudocode, which symbol assigns a value?

2.Where must a total be set to 0?

3.Trace: X ← 8, then WHILE X > 4 DO X ← X - 2, OUTPUT X. What is printed?

4.Which loop is guaranteed to run at least once?