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Substrings and string methods

A substring is a continuous section of a string. Python can also return changed copies of strings using methods such as .upper(), .lower(), .replace() and .strip(" ").

Strings are immutable: these operations return a new string. They do not silently alter the original variable.

1. Slice from start up to stop

The pattern text[start:stop] includes start but excludes stop.

For code = "CS2026":

code[0:2] gives CS;
code[2:6] gives 2026;
code[:2] also gives CS;
code[2:] gives everything from index 2 onward.

The excluded stop position prevents adjacent slices from overlapping.

Fill in the blanks3 marks

For animal = tiger, animal[1:4] gives gap 1, animal[:2] gives gap 2, and animal[2:] gives gap 3.

er
ig
ige
ti
ger
tiger

Multiple choice1 mark

Why does word[1:4] contain three characters?

AIndex 1 is skipped.
BIndex 4 is included twice.
CThe stop position 4 is excluded.
DPython begins slices at index 1.

2. Methods return useful results

Expression	Result	Purpose
`name.upper()`	uppercase copy	normalise case
`name.lower()`	lowercase copy	normalise case
`name.strip(" ")`	copy without surrounding spaces	clean input
`name.replace("-", " " )`	copy with replacements	substitute text
`name.find("x")`	first position, or `-1`	search safely
`name.isalpha()`	Boolean	test whether all characters are letters

A method call uses a dot after the string. Save the returned value if it will be needed later.

Worked example

raw = "  Ada-Lovelace  "
clean = raw.strip(" ")
display = clean.replace("-", " ").upper()
print(display)

Written answer2 marks

Explain why raw still contains its surrounding spaces after this program has run, while display does not.

Use the idea that string methods return new strings.

Students type their answer here.

3. Build a short identifier

For a supplied first_name and year, create identifier from the first three lowercase letters of the name followed by the year. With Charlotte and 26, the result is cha26.

Coding task4 marks

first_name = "Charlotte"
year = "26"
# Create and output identifier.

4. Position and not-found behaviour

find() returns the first matching position. If the text is absent, it returns -1; this lets a program make a decision later. By contrast, index() raises an exception when no match exists.

Example: "banana".find("na") is 2 because the first na begins at index 2.

Fill in the blanks3 marks

planet.find("net") returns the gap 1 where the substring begins. If it is absent, find() returns gap 2. The method that instead raises an exception when absent is gap 3.

-1
0
first position
index()
length

5. Independent manipulation

Input a phrase. Store a cleaned lowercase copy in cleaned, store its first four characters in prefix, then output only prefix.

It will be tested with COMPUTING and Data . Both contain at least four non-space characters.

Coding task5 marks

# Input phrase, clean it, select the prefix, then output it.

Multiple choice1 mark

Which expression produces a lowercase copy of title without surrounding spaces?

Atitle.lower.strip
Btitle.strip(" ").lower()
Clower(strip(title))
Dtitle = strip.lower()

Route forward

You can now manipulate strings within the Pearson PLS scope. Later series revisit formatting, validation and more substantial string processing. Next, decisions allow programs to respond differently to data.