0478 Pseudocode

Run and edit Cambridge IGCSE Computer Science 0478 pseudocode directly in VS Code — desktop and web.

⚠️ How files work — read this before using OPENFILE / READFILE / WRITEFILE

File behaviour depends on where your program is open, because OPENFILE needs a real folder to resolve a filename against. This matters most on iPad / web, where there is often no filesystem at all.

Where a file is read from, in priority order:

1. Embedded data — a //<<FILE … //>> block in your program (see below). Always used if present, on every platform.
2. Session memory — a file your program wrote earlier in this VS Code window (kept until you reload the window; never saved to disk if the document is unsaved).
3. The real folder on disk — only when your program file is saved inside a folder (desktop, or a web repo such as github.dev).

Where a file is written:

• Saved in a folder → written to a real file next to your program.
• Unsaved / untitled document → kept in session memory only (lost on window reload).

The common failure: running file code from an unsaved tab with no folder open (typical on iPad). There is no folder to read a provided data file from, so READFILE reports “file not found.” Fix it by embedding the data (below) or by saving your program into a folder that also contains the data file.

Embedding input data (works everywhere, including iPad)

When a question gives you a data file to read, paste it into your program as a comment block. The extension loads it as if it were a real file — no filesystem needed:

//<<FILE numbers.txt
//10
//25
//7
//>>

DECLARE n : INTEGER
OPENFILE "numbers.txt" FOR READ
READFILE "numbers.txt", n        // reads 10
OUTPUT n
CLOSEFILE "numbers.txt"

• Open with //<<FILE <name>, write each data line prefixed with //, close with //>>.
• The text after the // is taken verbatim (//10 → 10, // 10 → 10).
• Filenames are case-sensitive and must match exactly between the block and your OPENFILE — "Numbers.txt" ≠ "numbers.txt". (Note: this is unlike variable names, which are case-insensitive.)
• Use a plain name like data.txt; don't use folder paths.

When a run uses embedded data or session memory, the terminal prints a [files] … note so you always know where the data came from.

Quick start

1. Create a file with the .pseudo or .0478 extension.
2. Write your pseudocode. Syntax highlighting and error checking activate automatically.
3. Open the Command Palette (Cmd+Shift+P / Ctrl+Shift+P), type 0478, and run 0478: Run Pseudocode File.
4. A terminal opens. INPUT statements pause and wait for you to type a value — press Enter to submit.

Debugging

Open a .pseudo / .0478 file and press F5 to start debugging — no launch.json needed. Execution pauses at the first statement.

• Breakpoints — click the gutter next to any line.
• Step — Step Over (F10), Step Into (F11), Step Out (Shift+F11), Continue (F5).
• Variables — the VARIABLES panel shows every variable in the current scope; arrays expand to their elements.
• Watch — add an expression to the WATCH panel to track its value as you step.
• Debug Console — type an expression (e.g. Score + 10, Grid[1,1], LENGTH(Name)) to evaluate it in the current scope.
• Hover — hover over a variable in the editor while paused to see its value.

OUTPUT goes to the Debug Console; INPUT prompts via an input box.

Example

DECLARE Name   : STRING
DECLARE Score  : INTEGER

OUTPUT "Enter your name: "
INPUT Name
OUTPUT "Enter score (0-100): "
INPUT Score

IF Score >= 70
  THEN
    OUTPUT "Well done, ", Name, "!"
  ELSE
    OUTPUT "Keep practising, ", Name
ENDIF

Language features

Strict typing

Types are enforced at runtime, matching exam marking:

• INTEGER → REAL widening is allowed; REAL → INTEGER is not.
• CHAR satisfies a STRING parameter; the reverse is not allowed.
• Array indices are bounds-checked; out-of-range access is a runtime error.

Syntax reference (quick)

// Variables
DECLARE Counter : INTEGER
DECLARE Grid    : ARRAY[1:3, 1:3] OF CHAR
CONSTANT Pi     ← 3.14159

// Assignment  (← or ASCII alias <-)
Counter ← 0
Grid[1,1] ← 'X'

// Operators
//   Arithmetic : + - * / ^ (power)
//   Integer    : DIV(a,b)  MOD(a,b)
//   Comparison : = <> < <= > >=
//   Boolean    : AND  OR  NOT

// Loops
FOR i ← 1 TO 10 STEP 2
    OUTPUT i
NEXT i

WHILE Count > 0 DO
    Count ← Count - 1
ENDWHILE

REPEAT
    INPUT Answer
UNTIL Answer = "yes"

// Procedure
PROCEDURE Greet(Name : STRING)
    OUTPUT "Hello, ", Name
ENDPROCEDURE

CALL Greet("World")

// Function
FUNCTION Square(n : INTEGER) RETURNS INTEGER
    RETURN n * n
ENDFUNCTION

OUTPUT Square(7)

Assignment operator

The spec uses ←. This extension also accepts the ASCII alias <- for easier typing — both work identically.