Python from zero: part 2: decisions and repeats

1 · Observe

Real work is deciding and repeating

Words used in this lesson
Word	What it means
Condition	A question whose answer is True or False, such as price <= 70.
if / elif / else	If the condition is true do this, else if another is true do that, otherwise do the last thing.
Indentation	The spaces at the start of a line. In Python they are part of the meaning.
List	An ordered collection of values in square brackets: [62.5, 48.0, 71.2].
Index	A value's position in a list. Counting starts at 0.
Loop	Repeating the same steps for each value.
Accumulator	A name you update inside a loop to build a total or a count.

You are shortlisting flats. For each one you ask: is it within budget? If yes, keep it. If not, skip it. That is a decision (if) repeated over a collection (for). Nearly every data task is built from these two moves.

Everyday words and Python words
You say	Python writes	Meaning
If the price is within budget	if price <= budget:	Do the next, indented, lines only when True
Otherwise	else:	Do these lines when the condition was False
For each flat in my list	for price in prices:	Run the indented lines once per value
Keep a running total	total = total + price	Store the old total plus this price

2 · Express

The shape of if and for

Both start with a line ending in a colon. The lines that belong to it are indented by the same number of spaces (four is standard). The block ends when the indentation stops.

if condition:

lines that run only when it is True

else:

lines that run when it is False

The colon opens the block. The indentation shows what is inside it.

for name in collection:

lines that run once for each value, with name holding that value

Joining questions
Word	Means	Example	Answer when price = 62.5
and	both must be True	price > 50 and price < 70	True
or	at least one is True	price < 50 or price > 100	False
not	flips the answer	not price > 100	True

A list stores values in order. The first value is at position 0, not 1, because the position is how far from the start you are. A list of four values has positions 0, 1, 2 and 3. len(list) tells you how many values there are.

Check yourself

A list has five values. What is the position of the last one?

5 4 6

3 · Visualize

Trace a loop, one pass at a time

A loop runs its block once per value. Trace it just as you traced the lines in part 1. Here the budget is 60 and the list is [62.5, 48.0, 71.2].

Tracing the shortlist loop
Pass	price	price <= 60?	shortlist afterwards
Start			[]
1	62.5	False, so skip	[]
2	48.0	True, so keep	[48.0]
3	71.2	False, so skip	[48.0]
End			[48.0]

4 · Discover

Try each piece

First a decision. Change price_lakh to 40, then to 62.5, then to 90, and predict which message appears before you run it.

if, elif and else: pick one path

price_lakh = 62.5
budget_lakh = 70

if price_lakh <= 50:
    print("cheap: shortlist it")
elif price_lakh <= budget_lakh:
    print("within budget: visit it")
else:
    print("too expensive: skip it")

within budget: visit it

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if, elif and else: pick one path

Runs in your browser on your device. Nothing you type is sent anywhere, and nothing here changes your saved progress.

Python code (editable)

Press Ctrl+Enter (⌘+Enter on Mac) to run. Tab moves focus to the next control, as usual; use spaces to indent.

Only one branch runs. Python checks the conditions from the top and takes the first that is True. Now a list, and counting from zero.

A list holds many values, counted from zero

prices = [62.5, 48.0, 71.2, 55.5]
print(len(prices))        # how many values
print(prices[0])          # the first one: counting starts at 0
print(prices[3])          # the last one
print(prices[-1])         # negative counts from the end
prices.append(80.0)       # add one at the end
print(prices)
print(48.0 in prices)     # is it in the list?

4
62.5
55.5
55.5
[62.5, 48.0, 71.2, 55.5, 80.0]
True

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A list holds many values, counted from zero

Runs in your browser on your device. Nothing you type is sent anywhere, and nothing here changes your saved progress.

Python code (editable)

Press Ctrl+Enter (⌘+Enter on Mac) to run. Tab moves focus to the next control, as usual; use spaces to indent.

Now a loop with a decision inside it. This is the shortlist trace from the table, made real.

for: do the same thing to every value

prices = [62.5, 48.0, 71.2, 55.5]
budget = 60
shortlist = []
for price in prices:          # price takes each value in turn
    if price <= budget:
        shortlist.append(price)
print("shortlist:", shortlist)
print("how many:", len(shortlist))

shortlist: [48.0, 55.5]
how many: 2

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for: do the same thing to every value

Runs in your browser on your device. Nothing you type is sent anywhere, and nothing here changes your saved progress.

Python code (editable)

Press Ctrl+Enter (⌘+Enter on Mac) to run. Tab moves focus to the next control, as usual; use spaces to indent.

The loop from the table, run for real

prices = [62.5, 48.0, 71.2]
shortlist = []
for price in prices:
    keep = price <= 60
    print(price, "<= 60?", keep)
    if keep:
        shortlist.append(price)
print(shortlist)

62.5 <= 60? False
48.0 <= 60? True
71.2 <= 60? False
[48.0]

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The loop from the table, run for real

Runs in your browser on your device. Nothing you type is sent anywhere, and nothing here changes your saved progress.

Python code (editable)

Press Ctrl+Enter (⌘+Enter on Mac) to run. Tab moves focus to the next control, as usual; use spaces to indent.

5 · Apply

Two worked examples

Example 1, a running total. Start total at 0. On every pass, store the old total plus the current price. After the loop, divide by how many values there were to get the average.

A running total (the accumulator pattern)

prices = [62.5, 48.0, 71.2, 55.5]
total = 0
for price in prices:
    total = total + price     # store the old total plus this price
print("total:", total)
print("average:", round(total / len(prices), 2))
print("same answer with sum():", sum(prices) / len(prices))

total: 237.2
average: 59.3
same answer with sum(): 59.3

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A running total (the accumulator pattern)

Runs in your browser on your device. Nothing you type is sent anywhere, and nothing here changes your saved progress.

Python code (editable)

Press Ctrl+Enter (⌘+Enter on Mac) to run. Tab moves focus to the next control, as usual; use spaces to indent.

The four prices add to 237.2, so the average is 59.3. Python also has sum(), which does the same job in one step. Writing the loop once teaches you what sum() does.

Example 2 is counting: start a count at 0 and add 1 each time a condition is True. This is how you would count missing values in a column.

Counting with a condition

areas = [1075, None, 900, None, 1200]    # None means the value is missing
missing = 0
for area in areas:
    if area is None:
        missing = missing + 1
print("missing values:", missing, "out of", len(areas))

missing values: 2 out of 5

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Counting with a condition

Runs in your browser on your device. Nothing you type is sent anywhere, and nothing here changes your saved progress.

Python code (editable)

Press Ctrl+Enter (⌘+Enter on Mac) to run. Tab moves focus to the next control, as usual; use spaces to indent.

Check yourself

In for price in prices: what does price hold on the second pass?

The whole list The second value in the list Always the first value

6 · Where it goes wrong

Four classic slips

What Python says when these go wrong

try:
    compile("if 5 > 3\n    pass", "<cell>", "exec")        # forgot the colon
except SyntaxError as e:
    print("SyntaxError:", e.msg)
try:
    compile("for n in [1, 2]:\nprint(n)", "<cell>", "exec")  # forgot to indent
except SyntaxError as e:
    print(type(e).__name__ + ":", e.msg)
prices = [62.5, 48.0, 71.2]
try:
    print(prices[3])           # there are only three values: positions 0, 1, 2
except IndexError as e:
    print("IndexError:", e)

SyntaxError: expected ':'
IndentationError: expected an indented block after 'for' statement on line 1
IndexError: list index out of range

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What Python says when these go wrong

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Python code (editable)

Press Ctrl+Enter (⌘+Enter on Mac) to run. Tab moves focus to the next control, as usual; use spaces to indent.

Indentation decides what repeats

for n in [1, 2, 3]:
    print("inside the loop:", n)
print("after the loop: done")

inside the loop: 1
inside the loop: 2
inside the loop: 3
after the loop: done

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Indentation decides what repeats

Runs in your browser on your device. Nothing you type is sent anywhere, and nothing here changes your saved progress.

Python code (editable)

Press Ctrl+Enter (⌘+Enter on Mac) to run. Tab moves focus to the next control, as usual; use spaces to indent.

Compare that output with the code: the line that is indented runs three times, and the line that is not runs once, after the loop. Indentation is the only difference.

7 · In Python

A small program with a decision inside a loop

Here is everything from parts 1 and 2 together. Read it before you run it, and predict what it will print.

Decide and repeat in one program

prices = [62.5, 48.0, 71.2, 55.5, 90.0]
budget = 65

within = []
too_high = 0
for price in prices:
    if price <= budget:
        within.append(price)
    else:
        too_high = too_high + 1

print("within budget:", within)
print("too expensive:", too_high)
print("cheapest:", min(within), "| dearest:", max(within))
print("average of the affordable ones:", round(sum(within) / len(within), 2))

within budget: [62.5, 48.0, 55.5]
too expensive: 2
cheapest: 48.0 | dearest: 62.5
average of the affordable ones: 55.33

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Decide and repeat in one program

Runs in your browser on your device. Nothing you type is sent anywhere, and nothing here changes your saved progress.

Python code (editable)

Press Ctrl+Enter (⌘+Enter on Mac) to run. Tab moves focus to the next control, as usual; use spaces to indent.

Change budget to 50. How many are too expensive now?
Add a new price to the list, such as 45.5, and check the totals update.
Change <= to < in the condition. Does any answer change? Why?