Programming Concepts

A string is a sequence of characters such as "Hello world" or "PASSWORD123". CIE pseudocode names four string operations you must know.

LENGTH

LENGTH(s) returns the number of characters in the string s.

Password ← "letmein"
OUTPUT LENGTH(Password)      // outputs 7

Name ← ""
OUTPUT LENGTH(Name)          // outputs 0 (empty string)

A common use is validation: rejecting a password that is too short.

INPUT Password
IF LENGTH(Password) < 8 THEN
   OUTPUT "Password must be at least 8 characters"
ENDIF

SUBSTRING

SUBSTRING(s, start, length) returns a piece of the string s, beginning at position start and length characters long.

In CIE pseudocode, strings are 1-indexed: the first character is at position 1.

Word ← "Revision"
OUTPUT SUBSTRING(Word, 1, 3)   // outputs "Rev"
OUTPUT SUBSTRING(Word, 4, 5)   // outputs "ision"
OUTPUT SUBSTRING(Word, 1, 1)   // outputs "R"

Useful for extracting parts of formatted data: e.g. taking the first three letters of a postcode, or splitting a date string "05/12/2024" into day, month and year.

UCASE (uppercase)

UCASE(s) returns the string s with every letter changed to uppercase.

Name ← "Sarah"
OUTPUT UCASE(Name)             // outputs "SARAH"

LCASE (lowercase)

LCASE(s) returns the string s with every letter changed to lowercase.

Name ← "SARAH"
OUTPUT LCASE(Name)             // outputs "sarah"

Putting them together

A common idiom: standardise an input before comparing it so that minor differences in case do not cause false rejections.

Example — A program asks the user to type "yes" to continue. Accept any combination of upper and lower case.

INPUT Answer
IF LCASE(Answer) = "yes" THEN
   OUTPUT "Continuing..."
ELSE
   OUTPUT "Stopped."
ENDIF

The user can type Yes, YES, yes, or even yEs, and the program will treat them all the same.

A complete worked exam answer

Example — The function LENGTH(x) returns the size of a string x. The function SUBSTRING(x, y, z) returns the piece of x that begins at index y and runs for z characters. Positions count from 1.

Write pseudocode that stores "Save my exams" in X, outputs its length, then extracts the word "exams" from the string and outputs it.

X ← "Save my exams"
OUTPUT LENGTH(X)                  // outputs 13

Y ← 9                             // "exams" starts at position 9
Z ← 5                             // "exams" is 5 characters long
OUTPUT SUBSTRING(X, Y, Z)         // outputs "exams"

Mark-scheme split: 1 mark for storing the string, 1 for calling LENGTH(X), 1 for outputting the length, 1 for using SUBSTRING, 1 for the correct parameters, 1 for outputting the substring.