F1.2 · LESSON 2 OF 6

Strings and sequences

Write a small program that transforms data using conditions, loops and functions.

THE IDEA

A string is an ordered row of characters

A string such as "model" is a sequence: its characters have a fixed order and each sits at a numbered position called an index. Python starts counting at zero, so the first character is at index 0 and a five letter word ends at index 4. Negative indexes count from the end: -1 is the last character.

text start stop step

0t-11

1r-10

2a-9

3n-8

4s-7

5f-6

6o-5

7r-4

8m-3

9e-2

10r-1

Top number: index from the start. Bottom number: negative index from the end. Leave a box empty to mean "go to that end".

"transformer"[1:4] gives "ran"

The stop position is never included. Try 0 and 3, then 3 and 0 with step -1, then an empty start and stop with step -1.

Edit the text and the three boxes. Highlighted characters are what the slice picks.

Slicing: from start, up to but not including stop

text[a:b] takes the characters from index a up to, but not including, index b. That makes the length simply b - a, and it means text[:3] + text[3:] is always the whole text with nothing repeated or lost. A third number is the step, and a step of -1 walks backwards.

Predict first. What does "model"[1:4] give?

Strings cannot change. Lists can.

You cannot edit a string in place: word[0] = "T" raises TypeError: 'str' object does not support item assignment. To get a changed string you build a new one. A list is also a sequence, with the same indexing and slicing, but it can be changed in place with append or scores[0] = 75.

That power has a trap, which the name board makes visible. b = a never copies a list. It puts a second name on the same list.

Program (line 0 of 3)

a = [1, 2]b = ab.append(3)

Names

no names yet

Objects in memory

no objects yet

Nothing has happened yet. No names, no objects.

Switch between the two programs and compare the final step.

Predict first. After a = [1, 2], b = a and b.append(3), what is a?

Chapter context

A program is a precise sequence of state changes. Python names refer to objects; reading a program means tracking the values those names refer to, rather than guessing from how the code sounds.

TRY IT YOURSELF

Exercise 1: pick a string apart

Use only indexing and slicing on word. Counting starts at 0, a negative index counts from the end, and a slice s[a:b] stops just before b.
Your program is run again with a different word, so the code has to work for any word of at least four letters.

word = "transformer"

first = None       # TODO: the first character (index 0)
last = None        # TODO: the last character, using a negative index
middle = None      # TODO: everything except the first and last character (a slice)
start3 = None      # TODO: the first three characters
backwards = None   # TODO: the whole word reversed (a slice with step -1)

print(first, last, middle, start3, backwards)

Ctrl/⌘ + Enter runs it

Exercise 2: a copy is not a second name

backup is meant to remember the original scores, but backup = scores only puts a second label on the same list, so the changes below show up in both.
Fix the backup line so it is a real, separate copy. Then add last_two, the last two scores, with a slice.

scores = [70, 85, 90]
backup = scores      # TODO: this line does NOT make a copy. Change it.

scores.append(100)
scores[0] = 75

# When the program ends, backup must still be [70, 85, 90]
# and scores must be [75, 85, 90, 100].

last_two = None      # TODO: the last two scores, as a slice with a negative start

print(scores, backup, last_two)

Ctrl/⌘ + Enter runs it

Both exercises must be solved to finish this lesson.

Stuck, or something not working? You can come back to it any time.

CHECK YOUR UNDERSTANDING

Evaluate "model"[1:4].

AThe result is "odel", because the slice runs from index 1 to the end of the string.BThe result is "mode", because the slice includes both index 1 and index 4.CThe result is "ode", because index 1 is "o" and the slice stops before index 4.

Chapter reference: math, worked example & code

This section belongs to the whole chapter, so it is the same on every lesson in it and may cover a different lesson's topic. The code for the experiment you just ran is inside the experiment itself.

Math and notation

For an accumulator over numbers $x_1,\ldots,x_n$ :

s_0=0,\qquad s_i=s_{i-1}+x_i.

The subscript denotes a step, not a Python index. After the last step, $s_n$ is the sum. For a mean, divide by the number of items, and define what should happen for an empty list.

Worked example

Trace a robot's forward distances [2, 1, 3]. Position starts at zero and changes to 2, then 3, then 6. Replacing three copy-pasted moves with a loop preserves the order and final state. A mean distance is 6/3 = 2; the count is 3, not the last list index 2.

Reference code

The following example uses the Python standard library and does not call a remote service.

moves = [2, 1, 3]
position = 0
for distance in moves:
    position += distance
assert position == 6
assert sum(moves) / len(moves) == 2

Python examples are provided for study and local execution; this page does not execute Python.

Common mistakes

Assignment does not copy a mutable object. A loop stop value is exclusive. A function can return None even when its print output looks correct.