3.2.8 - String operations and conversions (Python)

Length and character position

A string is a sequence of characters with a length and indexed positions.

word = "PLANET"
print(len(word))
print(word[0])
print(word.find("N"))

The outputs are 6, P and 3. len() counts characters; square brackets obtain the character at the stated zero-based index; .find() finds the first occurrence's index. The indexes for PLANET are 0:P, 1:L, 2:A, 3:N, 4:E, 5:T. The final valid index is length minus one. Python .find() returns -1 if the text is absent; direct indexing outside the valid range causes an error. Distinguish searching for a character's position from obtaining the character at a known position.

Substrings and concatenation

A substring extracts part of a string, while concatenation joins strings into a new string.

Python's slice text[start:stop] includes the start index and excludes the stop index. For PLANET, [1:4] selects indexes 1, 2, 3 to produce LAN. AQA supplied SUBSTRING(1,3,word) includes the final index and therefore selects the same characters. Translate the endpoint rather than copying it unchanged.

word = "PLANET"
label = "Route " + word[1:4]
print(label)

The result is Route LAN. + joins the characters of its two string operands in order; it does not add a space automatically. The space belongs in "Route ". Strings are immutable: a slice produces a new string rather than changing the original. Store a new value if it is needed later.