← IGCSE Computer Science Paper 2

SUBSTRING: extracting part of a string

Returns part of a string: length characters taken from the start position. SUBSTRING("Happy Days", 1, 5) gives "Happy". The result is always a string.

Part of: IGCSE Computer Science 0478 — 8.1a Data Types and Variables — Module 35

Understanding SUBSTRING: extracting part of a string in IGCSE Computer Science Paper 2

What it does: The start is 2 and the length is 4, so the call extracts four characters from position 2 and the result is assigned to Part with ←. Start 1 and length 1 give just the first character, and OUTPUT displays the string that SUBSTRING returns. Start and length must be positive whole numbers (integers). 2.5 is a REAL, so it is not valid. The variable should be declared as STRING (or CHAR for one character). The assigned value must match the variable's data type.

Common mistakes: The start and length are swapped. This takes 1 character from position 5, which gives "y". To get "Happy" it must start at 1 and take 5 characters: SUBSTRING(Word, 1, 5). Positions in Cambridge pseudocode start at 1, so a start of 0 is not valid. The first character is taken with SUBSTRING(Name, 1, 1).

Class: _____________________ · Name: _____________________ · Date: _________

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This module's key terms are also in a printable crossword (Modules 337–346) and word search (Modules 337–357).

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Learning

Duo

What does SUBSTRING("Happy Days", 1, 5) return?

"Days""Happy"

Matches

SUBSTRING("Happy Days", 5)

↓

The length argument is missing. SUBSTRING needs three arguments: the string, the start and the length.

Applying

Meanings

Term

SUBSTRING(<string>, <start>, <length>)

Meaning

Returns part of a string: length characters taken from the start position. SUBSTRING("Happy Days", 1, 5) gives "Happy". The result is always a string.

Practice

DECLARE Word : STRING

Testing

Gaps

Part ← ("Happy Days", 1, 5)

Check Answer

Spots

3Word ← "Happy Days"

4Part ← SUBSTRING(Word, 5, 1)

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Quick Questions

1. What does SUBSTRING("Happy Days", 1, 5) return?

A. "Happy" ✓

B. "Days"

Start at position 1 and take 5 characters: H, a, p, p, y. That gives "Happy".

2. In SUBSTRING("Computer", 1, 3), what does the last number (3) mean?

A. How many characters to take ✓

B. The position where the extract stops

The third argument is the length: the number of characters returned. It is not an end position.

3. Which call returns "i" from the string "Science"?

A. SUBSTRING("Science", 1, 3)

B. SUBSTRING("Science", 3, 1) ✓

Start 3, length 1 takes just the character at position 3, which is "i". Start 1, length 3 gives "Sci".

4. What does SUBSTRING("Pizza", 2, 1) return?

A. "iz"

B. "i" ✓

A length of 1 takes one character only, the one at the start position (position 2 is "i").

5. What is the data type of the result of SUBSTRING("Pizza", 2, 1)?

A. STRING, even though it is one character ✓

B. CHAR, because it is one character

SUBSTRING always returns a string, whatever the length. The result is a STRING.

Match the Pairs

SUBSTRING("Happy Days", 1, 5) → Returns "Happy": 5 characters taken from position 1

SUBSTRING(Word, 3, 1) → Returns one character: the character at position 3 of Word, as a string

Take the 4 characters of Name that begin at position 2 and store them in Part → Part ← SUBSTRING(Name, 2, 4)

Output the first character of Name by extracting one character from the start → OUTPUT SUBSTRING(Name, 1, 1)

SUBSTRING("Happy Days", 5) → The length argument is missing. SUBSTRING needs three arguments: the string, the start and the length.

Letter: INTEGER Letter ← SUBSTRING(Word, 1, 1) → SUBSTRING always returns a string, so it cannot be assigned to an INTEGER variable.

Key Term

SUBSTRING(<string>, <start>, <length>)

Returns part of a string: length characters taken from the start position. SUBSTRING("Happy Days", 1, 5) gives "Happy". The result is always a string.

Fill in the Blanks

1. Part ← SUBSTRING("Happy Days", 1, 5)

Hint: Type the function name that extracts part of a string.

2. OUTPUT SUBSTRING("Happy Days", 7, 4)

Hint: Type the start position of the first character wanted. Positions start at 1.

3. Name ← "Happy Days" Part ← SUBSTRING(Name, 1, 5)

Hint: Type the length: how many characters to take from the start position.

4. Letter ← SUBSTRING("Happy Days", 7, 1)

Hint: Type the length needed to take exactly one character.

5. DECLARE Part : STRING Part ← SUBSTRING("Happy Days", 1, 5)

Hint: Type the data type of the variable that stores the SUBSTRING result.

Spot the Error

Question 1

A DECLARE Word : STRING

B DECLARE Part : STRING

C Word ← "Happy Days"

D Part ← SUBSTRING(Word, 5, 1)

E OUTPUT Part

Line D — The start and length are swapped. This takes 1 character from position 5, which gives "y". To get "Happy" it must start at 1 and take 5 characters: SUBSTRING(Word, 1, 5).

Question 2

A DECLARE Letter : STRING

B Name ← "Cambridge"

C Letter ← SUBSTRING(Name, 0, 1)

D OUTPUT Letter

Line C — Positions in Cambridge pseudocode start at 1, so a start of 0 is not valid. The first character is taken with SUBSTRING(Name, 1, 1).

Question 3

A Text ← "IGCSE 0478"

B Code ← SUBSTRING[Text, 7, 4]

C OUTPUT Code

Line B — SUBSTRING is a function, so its arguments go inside round brackets ( ), not square brackets. It should be SUBSTRING(Text, 7, 4).

Question 4

A Word ← "Computer"

B Part ← SUBSTRING(Word, 1, -3)

C OUTPUT Part

Line B — The length must be a positive integer because you cannot take a negative number of characters. It should be SUBSTRING(Word, 1, 3), which gives "Com".

Question 5

A DECLARE Word : STRING

B DECLARE Part : INTEGER

C Word ← "Happy Days"

D Part ← SUBSTRING(Word, 7, 4)

E OUTPUT Part

Line B — SUBSTRING always returns a string, so the variable that stores the result must be declared as STRING, not INTEGER.

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