CSC108H1 Chap.7 Strings: Indexing, Slicing and Methods
Strings: Indexing, Slicing and Methods
Text as a sequence
A string is a sequence of characters. Each character has an index counted from 0 on the left and from -1 on the right, and a slice collects the characters from one index onward, stopping just before a second index. A third number in a slice sets the stride.
Methods and the help sheet
Methods are functions that belong to a type and are called with dot notation.
String methods always return a new value and never change the original. Quizzes in the course supply help entries for the functions and methods you need, so reading those entries accurately is part of the skill.
How strings are assessed
Worksheet 12 asks you to build a target string from indices, to fill a table of slice values and to pick out expressions using in that produce False.
Worksheet 13 asks which expressions produce a given result using a printed help sheet for str, and notes that the sheet is similar to the one given on the midterm test. Both reward reading carefully rather than memorising.
What this chapter covers
- 01
Quotes, the empty string and len
- 02
Concatenation, repetition and conversion
- 03
Positive and negative indices
- 04
Slices, omitted ends and strides
- 05
String methods return new strings
- 06
Reading help entries for find, count and strip
Worked example · free
Slice a string using both index directions
- 2s[1:4] takes indices 1, 2 and 3, giving 'apl'; its length is 4 - 1 = 3.
- 1s[-1] is the last character, 'e'.
- 1s[::2] takes indices 0, 2 and 4, giving 'mpe'.
Key terms
- Index
- Where a character sits in a string, numbered from 0 at the left or from -1 at the right.
- Slice
- A substring that begins at one index and stops just before another.
- Stride
- The optional third slice number that sets the step between characters taken.
- Method
- A function attached to a type, called on a value using dot notation.
- Concatenation
- Joining two strings end to end with the + operator.
- Empty String
- The string with no characters at all, written as two quotes with nothing between them.
Strings: Indexing, Slicing and Methods FAQ
Why does s[len(s)] raise an error?
Indices start at 0, so the last character of s is at len(s) - 1. Asking for position len(s) points one past the end, where no character exists, and Python raises an IndexError. Slices are more forgiving, but single indices are not.
Does calling upper change the original string?
No. String methods return a new string and leave the original untouched. To keep the result, assign it to a variable, and to replace the old text, reassign the same name to the value the method returned.
How do I find the second occurrence of a character?
Call find a second time with a start position one past the first result, as in s.find(c, s.find(c) + 1). Starting exactly at the first index returns that same index again, which is a common wrong answer.
What does a negative stride do in a slice?
It walks through the string from right to left. With the start and end left out, a stride of -1 visits every character backwards, so the slice reverses the string. Larger negative strides skip characters while moving backwards.
Why does 'Room ' + 101 raise an error?
Because + joins two strings or adds two numbers, but cannot combine a string with an int. Convert the number first with str, as in 'Room ' + str(101), which gives the string 'Room 101'. The reverse conversion uses int on a string of digits.
Exam move
Pick three strings of different lengths, including one with spaces, and write their index tables with positive and negative indices. Set yourself ten slices and five method calls on them, answer on paper, then check in the shell. Practise with only the help sheet beside you, reading each entry's return type before answering. Time yourself on one full worksheet's worth of questions to build speed for the quizzes.
Include the empty string in at least two of your questions, since the is-methods treat it specially, and include one negative stride.
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