Module 2 review: sections 1.6 to 2.2

1.6 Errors

Every programmer makes mistakes. The book sorts them into three kinds, and the quiz expects you to tell them apart.

• A syntax error breaks a rule of the language, such as a missing quotation mark or wrong indentation. Python catches it before the program runs and refuses to start.
• A runtime error happens after the program starts. The code is legal, but it asks for something impossible, like dividing by zero or using a variable that was never set. Python stops at that point and prints a message. These are also called exceptions.
• A logic error is the quiet one. The program runs from start to finish with no complaint, but the answer is wrong. The computer did what you wrote, not what you meant, and nothing points at the mistake except the bad result.

The same three errors, side by side.

Kind	When Python notices	Does it run?	Example
Syntax	Before it runs	No, never starts	print("hi)
Runtime	While it runs	Starts, then stops	total / 0
Logic	Never	Yes, to the end	area = w + h

1.7 Problem solving: algorithm design

Work out the solution before you write any Python.

An algorithm is a step by step description of how to solve a problem. To count as one, it has to be all three of these:

• unambiguous: every step has exactly one meaning
• executable: every step is something that can actually be carried out
• terminating: it stops after a limited number of steps

You usually write the solution first as pseudocode, which is just the steps in plain, orderly English. Pseudocode has no strict rules. The point is to settle the logic before you fight with syntax.

Here is a typical book example. You have two packs of soda at different prices and sizes, and you want the better deal.

1. Read price1 and count1.  unit1 = price1 / count1
2. Read price2 and count2.  unit2 = price2 / count2
3. If unit1 < unit2, report pack 1.  Otherwise report pack 2.

2.1 Variables

A variable is a named box in memory that holds one value.

You make a variable with an assignment:

cans_per_pack = 6

The single = is the assignment operator. It is not the equals sign from math. It means take the value on the right and store it in the name on the left. Read it as "cans_per_pack gets 6". You can put a new value in the same box later, and the old one is gone.

cans_per_pack = 6
cans_per_pack = 12   # the box now holds 12

A very common pattern updates a variable using its own current value.

total = 0
total = total + 2

Python always finishes the right side first. It looks up the current total (0), adds 2, gets 2, and only then stores 2 back into total.

Number types

An int is a whole number with no fractional part: 6, 0, -25. A float has a decimal point: 6.0, 3.14, -0.5. Division with / always produces a float, so even 4 / 2 gives 2.0.

Variable names

• Start with a letter or an underscore, then any mix of letters, digits, and underscores.
• No spaces, and no punctuation.
• Cannot be a reserved word such as if, for, def, or class.
• Names are case sensitive, so count and Count are two different variables.

The convention in this course: lowercase words joined by underscores, and names that say what the value is. total_cost beats tc.

Constants

A constant is a value that should not change while the program runs. Python does not enforce this, so by agreement we write constant names in capitals.

BOTTLE_VOLUME = 2.0
CANS_PER_PACK = 6

Comments

Anything after a # on a line is a note for humans and is ignored by Python. Use comments to explain why, not to repeat what the code already says plainly.

# a six pack, used to size the order
CANS_PER_PACK = 6

2.2 Arithmetic

This is the part the quiz drills. Learn what each operator returns.

The basic operators are + - * /. You must write * for multiplication. 2 * n is fine; 2n is a syntax error.

• ** raises to a power. 2 ** 10 is 1024.
• / is true division and always gives a float. 7 / 2 is 3.5.
• // is floor division. It divides and drops the fraction. 7 // 2 is 3.
• % is the remainder, also called modulus. 7 % 2 is 1. It is handy for testing even or odd with n % 2 == 0, and for splitting a total, such as turning 234 cents into 2 dollars (234 // 100) and 34 cents (234 % 100).

What each operator returns.

Expression	Result	Note
7 / 2	3.5	true division, always a float
7 // 2	3	floor division, fraction dropped
7 % 2	1	remainder
2 ** 5	32	power
5 + 2.0	7.0	an int plus a float is a float

Order of operations

Python follows the usual math rules:

1. ** first
2. then * / // %
3. then + -

Operators at the same level run left to right. Put parentheses around the top or the bottom of any fraction you copy from math: write (a + b) / 2, not a + b / 2.

3 + 4 * 5 ** 2
3 + 4 * 25      # ** first
3 + 100         # * next
103             # + last

Calling functions

round(x) rounds to the nearest whole number, and round(x, 2) keeps two decimal places. abs(x) gives the size of a number without its sign. Both are built in and always available.

The math module has the rest. Import it once, then reach into it with a dot.

import math

r = 2.5
area = math.pi * r ** 2
side = math.sqrt(area)

Quick check