2.3c String manipulation and scope — Edexcel 4CP0

String Functions

Strings can be manipulated using built-in functions. These are commonly tested in Edexcel 4CP0 Paper 1 exams.

Function	Description	Example	Result
LENGTH(s)	Returns the number of characters in string s	LENGTH("Hello")	5
SUBSTRING(s, start, len)	Returns len characters from s starting at position start (0-indexed)	SUBSTRING("Computer", 0, 4)	"Comp"
UPPER(s)	Converts all characters to upper case	UPPER("hello")	"HELLO"
LOWER(s)	Converts all characters to lower case	LOWER("HELLO")	"hello"

String Concatenation

Strings can be joined together using the & operator (not +):

SET firstName TO "Alice"
SET lastName TO "Smith"
SET fullName TO firstName & " " & lastName
SEND fullName TO DISPLAY    # outputs "Alice Smith"

# LENGTH example
SEND LENGTH(fullName) TO DISPLAY  # outputs 11

SUBSTRING Example

⚠️ Note: SUBSTRING, UPPER, and LOWER are not listed in the Edexcel 4CP0 Appendix 5 pseudocode reference. Only LENGTH and RANDOM appear there. These functions are included here as additional context — exams may not require them, and you will not be penalised for not knowing them.

SET word TO "Computer"
# Extract "Comp" — start at 0, take 4 characters
SEND SUBSTRING(word, 0, 4) TO DISPLAY   # "Comp"

# Extract "ter" — start at 5, take 3 characters
SEND SUBSTRING(word, 5, 3) TO DISPLAY   # "ter"

Local Variables

A local variable is declared inside a procedure or function and only exists while that procedure/function is running. It cannot be accessed from outside the subprogram.

PROCEDURE calculateArea()
BEGIN PROCEDURE
    SET width TO 5     // local variable — only exists here
    SET height TO 10   // local variable
    SET area TO width * height
    SEND area TO DISPLAY
END PROCEDURE

// width, height and area do NOT exist here

Global Variables

A global variable is declared in the main program and can be accessed from anywhere — including inside procedures and functions. Overusing global variables is considered poor practice as it makes programs harder to debug.

SET highScore TO 0   // global variable — accessible everywhere

PROCEDURE updateScore(newScore)
BEGIN PROCEDURE
    IF newScore > highScore THEN
        SET highScore TO newScore  // accessing global
    END IF
END PROCEDURE

Local vs Global Variables

Feature	Local Variable	Global Variable
Where declared	Inside a procedure/function	In the main program body
Accessible from	Only within that procedure/function	Anywhere in the program
Lifetime	Created when subprogram runs; destroyed on exit	Exists for the whole program
Risk	Low — cannot accidentally affect other parts	Higher — any part of program can change it

📝 Exam Tip: SUBSTRING is 0-indexed. SUBSTRING("Hello", 1, 3) returns "ell" — starts at position 1 (not 0) and takes 3 characters. Trace through carefully.

⚠️ Common Mistakes

Forgetting SUBSTRING is 0-indexed — position 0 is the first character
Using + to add numbers stored as strings — "5" + "3" = "53" not 8; cast to integer first
Thinking local variables persist after the subprogram ends — they are destroyed