Chapter 1 learning objectives

By the end of this chapter, you will be able to:

1. Explain what programming is at a high level
2. Describe how Python fits into the broader computing landscape
3. Distinguish programming ideas from Python-specific syntax
4. Find environment and Jupyter setup guidance
5. Use basic built-in functions in simple Python code

Today's goals

• Define an algorithm and turn one into a program
• Contrast natural and formal languages
• Place Python among levels of abstraction
• Tell interpreted from compiled execution
• Name the core programming constructs
• Separate expressions from statements
• Read binary, hexadecimal, and character codes

How you learn to program

Learning programming is largely about connecting the dots.

1. Repeat: keep collecting and revisiting the core ideas
2. Associate: keep linking new ideas to ideas you already have

You need enough concepts, examples, and practice to work with, and then you need to connect them.

An algorithm is a plan

An algorithm is a finite, ordered set of unambiguous steps that turns input into output.

• Each step is clear enough that anyone, or any machine, gets the same result
• The steps eventually come to an end
• It describes the logic, not the syntax of a language

A recipe is an algorithm: ingredients in, fixed steps, a finished dish out.

Example: find the largest number

1. Look at the first number and remember it as the largest so far
2. Look at the next number; if it is greater, remember it instead
3. Repeat step 2 until no numbers are left
4. Report the number you remembered

Input [42, 17, 88, 63]: remember 42, keep 42, switch to 88, keep 88.

Output: 88

From algorithm to program

numbers = [42, 17, 88, 63]

largest = numbers[0]          # step 1
for num in numbers[1:]:       # step 3
    if num > largest:         # step 2
        largest = num
print(largest)                # step 4
print(max(numbers))           # built-in version

88
88

Natural vs formal languages

Formal languages are denser, their structure matters, and the details matter.

Levels of abstraction

Python, Java, JavaScript, C, and C++ are high-level languages.

Interpreted, compiled, and hybrid

Programming constructs

Expressions produce values

An expression combines values, variables, operators, and function calls. Python evaluates it to produce a single value.

Statements perform actions

Computing an expression is evaluation; running a statement is execution.

Expressions and statements in code

x = 5            # statement: no value is shown
print(x + 5)     # x + 5 is an expression: 10
if x > 0:        # statement: controls the flow
    print("x is positive")

10
x is positive

Number systems

Prefixes: 0b binary, 0o octal, 0x hexadecimal.

Positional notation

Each digit's value = digit × base^position

Binary place values: 8, 4, 2, 1. So 1011₂ = 8 + 2 + 1 = 11.

Reading 100 in binary

0b1100100
  ││││││└ 0 × 2^0 = 0
  │││││└─ 0 × 2^1 = 0
  ││││└── 1 × 2^2 = 4
  │││└─── 0 × 2^3 = 0
  ││└──── 0 × 2^4 = 0
  │└───── 1 × 2^5 = 32
  └────── 1 × 2^6 = 64
                   ───
                   100

Converting in Python

print(bin(100), oct(100), hex(100))
print(hex(0b1100100))
print(int("0x64", 16), int("0b1100100", 2))
print(0b1011, 0x64)

0b1100100 0o144 0x64
0x64
100 100
11 100

Character encoding

Character codes with ord() and chr()

code = ord('A')
print(code, bin(code), hex(code))
print(chr(code))
print(ord('B') - ord('A'), 'B' > 'A')
print(ord('é'), chr(960))

65 0b1000001 0x41
A
1 True
233 π

Today's goals

• Use indentation to mark blocks
• Split long statements with parentheses
• Display output with print() and f-strings
• Read input and convert it to numbers
• Write comments and docstrings
• Inspect objects and recognize keywords
• Call built-in functions and import modules

Indentation defines blocks

for i in range(5):
print(i)

IndentationError: expected an indented block after 'for' statement on line 1

for i in range(3):
    print(i)

0
1
2

One statement per line

total = (
    100
    + 200
    + 300
)
print(total)
total = 100 + 200 + \
        300

600

• Preferred: one statement per line, parentheses for long expressions
• Works but fragile: a backslash (a trailing space breaks it)

print() with commas or +

print("Name:", "Alice", "Age:", 25)
print("Name:" + " Alice" + " Age:", 25)
print("I am " + 25 + " years old")

Name: Alice Age: 25
Name: Alice Age: 25
TypeError: can only concatenate str (not "int") to str

F-strings

name = "Alice"
age = 25
print(f"My name is {name} and I am {age} years old.")

x = 10
y = 20
print(f"The sum of {x} and {y} is {x + y}")

My name is Alice and I am 25 years old.
The sum of 10 and 20 is 30

Format specifications

A colon inside the braces controls how the value looks.

Combine them: :,.2f gives 1,234,567.89.

Format specifications in code

price = 4.5
revenue = 1234567.891
growth = 0.083
item = "apple"

print(f"Price: ${price:.2f}")
print(f"Revenue: {revenue:,.2f}")
print(f"Growth: {growth:.1%}")
print(f"|{item:>8}|{item:<8}|")

Price: $4.50
Revenue: 1,234,567.89
Growth: 8.3%
|   apple|apple   |

Format spec vs round()

print(round(42.5, 2))
print(f"{42.5:.2f}")

42.5
42.50

• round() returns a number; Python drops trailing zeros
• A format spec returns text with exactly the places you ask for

print() parameters: sep and end

print("A", "B", "C")
print("A", "B", "C", sep="-")
for num in range(5):
    print(num, end=" ")

A B C
A-B-C
0 1 2 3 4

• sep: placed between values (default a space)
• end: added at the end (default a newline)

input() returns a string

age = input("Enter your age: ")
print(type(age))
age = int(age)
print(type(age))
print(f"You are {age} years old.")

Enter your age: 25
<class 'str'>
<class 'int'>
You are 25 years old.

Type conversion: int(), float(), str()

price_text = "19.99"
price = float(price_text)
print(price * 2)

age = 25
message = "I am " + str(age) + " years old"
print(message)

39.98
I am 25 years old

Update a variable from its old value

total = 10
total = total + 5
print(total)

total_minutes = int("135")
print(total_minutes // 60, total_minutes % 60)

15
2 15

Comments and docstrings

def area_circle(radius):
    """Return the area of a circle given its radius."""
    import math
    return math.pi * radius**2   # pi times r squared

print(area_circle(3))
print(area_circle.__doc__)

28.274333882308138
Return the area of a circle given its radius.

Every value is an object

score = 95
print(type(score))
print(score)
print(id(score))

<class 'int'>
95
4311411656

An object has a type, a value, and an identity.

Keywords are reserved

class = "my first class"

SyntaxError: invalid syntax

Built-in functions

scores = [88, 92, 75, 100, 83]
print(len(scores))
print(min(scores), max(scores))
print(sum(scores) / len(scores))
print(sorted(scores))

5
75 100
87.6
[75, 83, 88, 92, 100]

Arguments and return values

print(int("101"), int("101", 2), round(1.2345, 2))

return_value = print("display only")
print(return_value)

101 5 1.23
display only
None

• Arguments go inside the parentheses
• Most functions return a value; print() returns None

Modules and imports

import math
print(math.sqrt(25), math.pi)

5.0 3.141592653589793