Arrays and Strings in Pseudocode

Array and string questions are common because they test indexing and loop control.

Array basics

Array stores multiple values under one name.

Example:

A = [3, 5, 7, 9]

If 0-indexed:

• A[0] = 3
• A[1] = 5
• A[2] = 7
• A[3] = 9

If 1-indexed:

• A[1] = 3
• A[2] = 5
• A[3] = 7
• A[4] = 9

Always check indexing in the question.

Array sum

SET sum = 0
FOR i = 0 TO n-1
    SET sum = sum + A[i]
END FOR
DISPLAY sum

Maximum element

SET max = A[0]
FOR i = 1 TO n-1
    IF A[i] > max THEN
        SET max = A[i]
    END IF
END FOR
DISPLAY max

Count even numbers

SET count = 0
FOR i = 0 TO n-1
    IF A[i] % 2 = 0 THEN
        SET count = count + 1
    END IF
END FOR
DISPLAY count

Reverse array output

FOR i = n-1 DOWNTO 0
    DISPLAY A[i]
END FOR

String basics

String is a sequence of characters.

Example:

S = "CODE"

If 0-indexed:

• S[0] = C
• S[1] = O
• S[2] = D
• S[3] = E

Palindrome string

String is palindrome if it reads same forward and backward.

SET i = 0
SET j = length(S) - 1
SET flag = true
WHILE i < j
    IF S[i] != S[j] THEN
        SET flag = false
        BREAK
    END IF
    SET i = i + 1
    SET j = j - 1
END WHILE
DISPLAY flag

Examples:

• MADAM: palindrome
• LEVEL: palindrome
• CODE: not palindrome

Character frequency

Concept:

• Use count array/map.
• Increment count for each character.

Pseudocode:

FOR each character ch in S
    count[ch] = count[ch] + 1
END FOR

Common array/string traps

• Accessing A[n] in 0-indexed array is out of bounds.
• Loop i <= n is wrong for 0-indexed array of size n.
• In string comparison, case matters: A and a are different.
• Space may be counted as character if not explicitly ignored.