Programming 💻

➡️ Sequence 8.1.4(a)

The order of the steps matters. A wrong order gives wrong results and can add steps the task doesn't need. A classic: adding marks until a rogue value (999) is typed. If the 999 is added to the total and counted before the loop ends, every output is wrong. Moving the input to after the addition, starting the counter at −1, and working out the average once, after the loop, fixes all three.

Load both versions into the runner below with test data 25, 27, 23, 999 and compare the trace tables.

🔀 Selection: IF 8.1.4(b)

Selection lets a program take different routes — pick out the largest value, items over a price, everyone in a group. Python has no THEN or ENDIF, just a colon and indentation; VB uses Then … End If; Java uses .

🎛️ Selection: CASE 8.1.4(b)

CASE handles many choices on one value. Python uses if … elif … else (or match in Python 3.10+), VB uses Select Case … Case Else, Java uses switch with default — and needs break to stop each case running into the next.

🔁 Iteration: three kinds of loop 8.1.4(c)

➕ Totalling and counting 8.1.4(d)

Totalling keeps a running total that values are added to; counting keeps track of how many times something happens. Totalling looks the same in all three languages; Java has a shortcut for counting:

🔤 String handling 8.1.4(e)

A string holds text, from the empty string up to a maximum length set by the language. Each character has a position number — and here's the trap: pseudocode strings start at position 1, but Python, VB and Java start at 0. You need four methods (usually provided as library routines):

• 📏 length — the number of characters ("Computer Science" is 16: the space counts)
• ✂️ substring — extract part of a string, e.g. "Science"
• 🔠 upper — "COMPUTER SCIENCE" · lower — "computer science"

Read the substring parameters carefully: pseudocode and VB take (start, length); Python slices take [start : end]; Java's substring(start, end) has an exclusive end — the position after the last character.

🧮 Arithmetic, logical and Boolean operators 8.1.4(f)

Group expressions with brackets to make the order clear, e.g. ((X + Y) * 3) / PI. Logical operators decide the path through a program; Boolean operators combine them into bigger conditions:

🪆 Nested statements 8.1.5

One construct can sit inside another — selection inside a loop, a loop inside a loop. Nesting cuts down the code you write and makes it easier to test. A big example: highest, lowest and average marks for each subject (5 tests), each student (6 subjects) and the whole class (20 students) needs three nested loops:

FOR Student ← 1 TO ClassSize                 // outer loop
  // set StudentHigh ← 0, StudentLow ← 100, StudentTotal ← 0
  FOR Subject ← 1 TO NumberOfSubjects        // middle loop
    // set SubjectHigh ← 0, SubjectLow ← 100, SubjectTotal ← 0
    FOR Test ← 1 TO NumberOfTests            // inner loop
      INPUT Mark
      // update SubjectHigh, SubjectLow, SubjectTotal
    NEXT Test
    // output the subject results; update the student results
  NEXT Subject
  // output the student results; update the class results
NEXT Student
// output the class results

Using constants for ClassSize, NumberOfSubjects and NumberOfTests means you can shrink them to 2 or 3 for testing by changing one line each. Build it inside-out: get the inner loop working first (Activity 8.11), then wrap the middle loop around it (8.12), then the outer loop (8.13).

🧩 Procedures, functions and parameters 8.1.6

When the same group of statements is needed again and again, write it once as a subroutine and call it whenever it's needed.

Names differ by language: a procedure is a void function in Python, a Sub (subroutine) in VB and a method in Java; a function is a fruitful function in Python, a Function in VB and a method with a return in Java.

A procedure without parameters

🎚️ Passing a parameter 8.1.6

An argument changes what the procedure does — here, how many stars it outputs. The call must match the definition: the same number of arguments as parameters, in the same order, of the right types. (For IGCSE, at most two parameters.)

Call it with a value, CALL Stars(7), or with a variable, MyNumber ← 7 then CALL Stars(MyNumber) — the value 7 is copied into the parameter Number.

↩️ Functions return a value 8.1.6

RETURN gives back the value — usually the last statement. Both the parameter and the return value have data types in the header. Because a function produces a value, its call goes in an expression, on the right-hand side: MyTemp ← Celsius(MyTemp). A procedure call is a standalone statement.

The header (first line) of a procedure or function contains its name, any parameters and their data types, and — for a function — the data type of the return value.

🌍 Local and global variables 8.1.6

In this program the main program's Number1, Number2 and Answer are global. The procedure declares its own local Number1, Number2, Number3 and Answer — so changing them inside Test doesn't touch the globals, and the main program can't see Number3 at all:

Output: the main program shows 50, 100 and 150; Test shows 10, 20 and 30; back in the main program they're still 50, 100 and 150; the last line is an error — Number3 is undefined there. Load "Local and global variables" into the runner above and step through it.

📚 Library routines 8.1.7

An IDE comes with a standard library of ready-made, fully tested functions and procedures, like the string handling routines. In Java some must be imported first (import java.lang.String;). You need these four:

• ➗ MOD — the remainder of a division · DIV — the quotient (whole-number part)
• 🎯 ROUND — a value rounded to a given number of decimal places
• 🎲 RANDOM — a random number between 0 and 1

Python's divmod(x, y) gives both answers at once (DIV first, then MOD). Java's Math.round only rounds to a whole number — multiply by 100, round, divide by 100.0 for 2 decimal places.

🛠️ Creating a maintainable program 8.1.8

Programs are often updated later by someone else — who may have nothing but the source code. (Even you, years later, will have forgotten how it works!) A maintainable program:

• 🏷️ uses meaningful identifiers for variables, constants, arrays, procedures and functions
• 🧩 is divided into modules — procedures and functions — one for each task
• 💬 is fully commented using the language's comment feature

🗃️ Arrays 8.2

Arrays let you store, search and sort lists (names in alphabetical order, temperatures to search). The first index can be 0 or 1, but most languages start at 0. A one-dimensional array is a list. When it's declared in pseudocode, give its name, first index, last index and data type:

DECLARE MyList : ARRAY[0:9] OF INTEGER
MyList[4] ← 27        // store 27 at index 4 — the FIFTH element
OUTPUT MyList[1]      // display the element at index 1

Fill a whole array with a loop, using the loop counter as the index:

🔲 Two-dimensional arrays 8.2

A two-dimensional array is a table of rows and columns. MyTable has 10 rows and 3 columns — 30 elements — and its first element is at [0, 0]. The declaration gives the first and last row index, the first and last column index, and the data type. Populating it needs two nested loops, one for each index:

📄 File handling 8.3

Data in RAM is lost when the computer is switched off. Saving it to a file stores it permanently, so it can be used again later by the same program, by a different program, or sent to other computers. Every file is identified by its filename. The steps are always: open it (for READ, WRITE or APPEND), read or write a line, and close it.

You'll see two pseudocode styles: OPEN MyFile FOR WRITE with WRITEFILE, TextLine, and the Cambridge syllabus form OPENFILE MyFile FOR WRITE with WRITEFILE MyFile, TextLine. The runner accepts both.

✏️ Practice hub 26 activities · every example

Every activity from the coursebook chapter — auto-marked where there's a single right answer (many checked by the pseudocode interpreter itself), with model programs in pseudocode, Python, Visual Basic and Java for the writing tasks. Fresh questions every time you hit 🔀 New questions.

📘 Worked examples

Every type of worked example in the chapter, step by step. Hit 🔀 Another example for a new one.

🃏 Key-term flashcards Tap to flip

🧠 Mini-quiz

📝 Exam-style questions Try, then peek

OUTPUT "Please enter your password "
INPUT Password
IF LENGTH(Password) = 8 AND Password = UCASE(Password)
  THEN
    OUTPUT "Password meets the rules"
  ELSE
    OUTPUT "Password must be 8 characters, all letters in upper case"
ENDIF

// (b) bubble sort on the family name, swapping whole rows
Last ← 10
REPEAT
  Swapped ← FALSE
  FOR Row ← 1 TO Last - 1
    IF Names[Row, 2] > Names[Row + 1, 2]
      THEN
        FOR Column ← 1 TO 2
          Temp ← Names[Row, Column]
          Names[Row, Column] ← Names[Row + 1, Column]
          Names[Row + 1, Column] ← Temp
        NEXT Column
        Swapped ← TRUE
    ENDIF
  NEXT Row
  Last ← Last - 1
UNTIL NOT Swapped OR Last = 1
// (a) display the names
FOR Row ← 1 TO 10
  OUTPUT Names[Row, 1], " ", Names[Row, 2]
NEXT Row

OPENFILE "Message.txt" FOR READ
READFILE "Message.txt", LineOfText
OUTPUT LineOfText
CLOSEFILE "Message.txt"

OUTPUT "Please enter a number from 0 to 100 "
INPUT Number
IF Number < 0 OR Number > 100
  THEN
    OUTPUT "Error: the number must be from 0 to 100"
ENDIF

🚀 Challenge projects Beyond the activities

• 📞 Staff directory. Three parallel arrays StaffName, StaffPhone and StaffOffice. Write procedures to list phone numbers in alphabetical order of name, list staff grouped by office, show everything for one member of staff (name as a parameter) and update their details. Use local and global variables, comments — then test it with a trace table.
• 🏛️ Town guild membership. Ask questions about age (18–65), years lived in town and family, savings, a clean record and supporters. Issue a unique membership number if every rule is met, or list every reason for failure if not — remembering that five supporters can excuse one rule (except age).