Strings: Revision Notes (Computer Science Principles 3.4)

Recap

A string is an ordered sequence of characters: letters, digits, spaces, punctuation.
"" is the empty string (length 0). "7" is a string; 7 is a number.
Concatenation joins strings end to end and makes a new string. Order matters.
A substring is a part of an existing string. The original does not change.
In this pack: positions start at 1, substring(text, start, length) and concat(a, b). On the exam, use the definition the question supplies.

Key relationships

Task	Expression	Value
Join two strings	`concat("sun", "shine")`	`"sunshine"`
Join three strings	`concat(concat("a", "b"), "c")`	`"abc"`
First character	`substring("Mina", 1, 1)`	`"M"`
Last two characters of “2011”	`substring("2011", 3, 2)`	`"11"`
Grow a string in a loop	`result ← concat(result, "ab")`	`"ababab"` after three passes
Empty string	`""`	length 0

Assumptions

Strings are not changed by substring or concat; they return new strings.
Substring requests in the examples stay inside the string.
A question may use different names or a different counting convention. Follow the question.

Method

Write the string with each character numbered above it.
Work out each substring by marking its characters.
Evaluate inner concat calls first.
Count the characters in your answer and compare with the expected length.

Mistakes to avoid

Reversing the order of a concat.
Treating the third argument as an end position.
Ignoring spaces inside quotes.
Believing substring edits the original.
Treating "12" as the number 12.

Worked mini-example

s ← "banana"
part ← substring(s, 2, 3)
DISPLAY(concat(part, "!"))

Output: ana!

"banana" is b-a-n-a-n-a. Three characters from position 2 are a, n, a, so part is "ana", and joining "!" gives "ana!".

A second mini-example: a short code from a name

name ← "Okafor"
tag ← concat(substring(name, 1, 2), substring(name, 5, 2))
DISPLAY(tag)

Output: Okor

Positions in "Okafor": O-1, k-2, a-3, f-4, o-5, r-6. Two characters from position 1 are "Ok"; two characters from position 5 are "or". Joined: "Okor", four characters, which equals 2 + 2 ✓.

Quick self-check

What is concat("go", "al")?
What is substring("sunshine", 4, 3)?
Are "tea" and "eat" the same string?

Answers

"goal".
"shi" (positions 4, 5 and 6 are s, h, i).
No. The same characters in a different order are different strings.

Where marks are won and lost

String questions are usually short evaluation tasks: given a string variable and one or two procedures defined in the question, state the final string. Marks go to candidates who track positions carefully and who remember that concatenation order matters. A second pattern asks which of four code segments builds a required string. Check the lengths first: if the target has 9 characters, a correct answer must produce 9. In the Create task, strings often appear as messages or labels in your own procedure.

Back to the full study guide for worked examples and explanations.