Scripting Intro S1 — Cheatsheet

Python, algorithmic thinking and PowerShell — 2024–2025

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Table of Contents

• Algorithms and Basics
• Variables, Types and Operators
• Selection
• Loops
• Strings and Collections
• Functions
• Files
• Debugging and Error Handling
• PowerShell Basics

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Algorithms and Basics

An algorithm is a finite, unambiguous sequence of steps that turns input into output. Build the logic first with pseudocode or a flowchart, then translate it into code.

start
  read input
  validate input
  process data
  show result
stop

Flowchart shape	Meaning
Oval	start/end
Rectangle	processing/assignment
Diamond	decision
Parallelogram	input/output

Small steps

Get one case working correctly first. Then add validation, repetition and exception handling.

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Variables, Types and Operators

Python

name = input("Name: ")
age = int(input("Age: "))
height = float(input("Height in meters: "))
active = True

print(f"{name} is {age} years old")

Type	Example
int	42
float	3.14
str	"text"
bool	True, False
list	[1, 2, 3]
dict	{"name": "Ada"}

input() always returns a string. Convert explicitly when you want to calculate.

Operators

Kind	Python	PowerShell
add/subtract	+, -	+, -
multiply/divide	*, /	*, /
integer division/remainder	//, %	[math]::Floor($a/$b), %
equal/not equal	==, !=	-eq, -ne
comparison	<, <=, >, >=	-lt, -le, -gt, -ge
logical	and, or, not	-and, -or, -not

Order: parentheses → exponents → multiply/divide → add/subtract → comparisons → logical operators.

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Selection

Python

score = float(input("Score: "))

if score < 0 or score > 10:
    print("Invalid score")
elif score >= 8.5:
    print("A")
elif score >= 7.5:
    print("B")
else:
    print("C or lower")

Python uses indentation to define blocks. The order of conditions matters: test specific/high boundaries before broader/low ones.

PowerShell

$weight = [double](Read-Host 'Weight')

if ($weight -gt 20) {
    'Surcharge of 25 euros'
} elseif ($weight -eq 20) {
    'Exactly right'
} else {
    'Have a good trip!'
}

switch -Wildcard ($department) {
    'R&D'   { 'Research and development' }
    'Sales*' { 'Sales' }
    default { $department }
}

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Loops

Construct	Use
while	repeat while the condition is true; possibly zero times
do...while	at least once, repeat while true
do...until	at least once, stop once true
for	known number of iterations/counter
foreach	each element of a collection

Python

for number in range(1, 11):
    print(number)

attempts = 0
while attempts < 3:
    attempts += 1

PowerShell

for ($i = 1; $i -le 10; $i++) {
    $i
}

foreach ($item in $items) {
    $item
}

do {
    $number = [int](Read-Host 'Positive number')
} until ($number -gt 0)

• break: exits the nearest loop.
• continue: skips the rest of the current iteration.
• exit: ends the entire script/PowerShell session.

foreach versus ForEach-Object

foreach has the collection available upfront and supports break. ForEach-Object processes pipeline objects as they arrive and is suited for streaming.

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Strings and Collections

text = "The Holy Hand Grenade"
print(len(text))
print(text.lower())
print(text[0])
print(text[3:10])

vowels = set()
for char in text.lower():
    if char in "aeiou":
        vowels.add(char)
print(len(vowels))

numbers = [3, 1, 4]
numbers.append(2)
numbers.sort()
for number in numbers:
    print(number)

A list is ordered and mutable; a tuple is ordered and immutable; a set holds unique values; a dictionary maps keys to values.

$names = @('Ada', 'Linus', 'Grace')
$names.Count
$names[0]
$names += 'Guido'

$person = @{ Name = 'Ada'; Age = 36 }
$person.Name

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Functions

Functions split a problem into testable, reusable parts.

def calculate_bmi(weight, height):
    """Return BMI rounded to two decimals."""
    if height <= 0:
        raise ValueError("height must be positive")
    return round(weight / height ** 2, 2)

bmi = calculate_bmi(75, 1.80)

Concept	Meaning
Parameter	name in the function definition
Argument	concrete value at call time
Return value	result for the caller
Local variable	exists within the function

function Get-Bmi {
    param(
        [Parameter(Mandatory)] [double]$Weight,
        [Parameter(Mandatory)] [double]$Height
    )
    [math]::Round($Weight / ($Height * $Height), 2)
}

Design example: sliding puzzle

Split a larger program into responsibilities:

make_grid()       → build the grid and empty position
print_grid()      → show the current state
check_move()      → validate input
apply_move()      → change the state
check_grid()      → test the win condition

This avoids one long loop where input, validation, data changes and output are all tangled together.

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Files

with open("data.txt", "r", encoding="utf-8") as file:
    for line in file:
        print(line.rstrip())

with open("result.txt", "w", encoding="utf-8") as file:
    file.write("result\n")

Mode	Effect
r	read
w	write and overwrite
a	append at the end
x	create a new file; fails if it already exists

with also closes the file when an error occurs.

For /etc/passwd-style data:

with open("passwd.txt", encoding="utf-8") as file:
    for line in file:
        fields = line.rstrip().split(":")
        username, uid, home, shell = fields[0], fields[2], fields[5], fields[6]

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Debugging and Error Handling

Common errors:

Error	Cause
SyntaxError	code is grammatically invalid
IndentationError	incorrect indentation
NameError	unknown variable name
TypeError	operation doesn't fit the type
ValueError	invalid conversion/value
ZeroDivisionError	division by zero
IndexError	index out of range for the collection

while True:
    try:
        divisor = float(input("Divisor: "))
        result = 10 / divisor
    except ValueError:
        print("Enter a number")
    except ZeroDivisionError:
        print("Cannot divide by zero")
    else:
        print(result)
        break

Debug systematically: reproduce the error, read the traceback bottom to top, check types and intermediate values, shrink the problem, and test edge cases (0, empty, negative, first/last index).

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PowerShell Basics

PowerShell works with objects, not just text. Cmdlets mostly follow Verb-Noun.

Get-Command *Process*
Get-Help Get-Process -Examples
Get-Process | Get-Member
Get-Process |
    Where-Object CPU -gt 10 |
    Sort-Object CPU -Descending |
    Select-Object -First 5 Name, CPU

Variables and interpolation

$name = 'Ada'
$count = 3
"Hello $name, count: $count"        # interpolation
'Hello $name'                       # literal
"Result: $($count * 2)"             # expression in a string

Safely converting input

[int]$number = Read-Host 'Enter a whole number'
[double]$price = Read-Host 'Price'

Core rule for solutions

input → type conversion → validation → processing → clear output

Test deliberately

A program that handles one normal example is not done. Also test wrong types, empty input, boundary values and repeated execution.