Starting Concepts

• 1: Variables and Data Types
• 2: Input and output operations
• 3: Flow Control
• 4: Functions
• 5: Recursive Functions

1 - Variables and Data Types

Variables

A variable is a container to store data in the computer’s memory. We can think of it as a box with a label. The label is the variable name and inside the box its value is stored.

To declare a variable in Python we just write the name and assign a value:

age = 28
price = 19.95
student = True

Variable names must start with letters or underscore, and can only contain letters, numbers and underscores. It is recommended to use meaningful names that represent the purpose of the variable.

In Python variables do not need to be declared with a particular type. The type is inferred automatically when assigning the value:

age = 28 # age is integer type
price = 19.95 # price is float type
single = True # single is boolean type

Once assigned, a variable can change its value at any time:

age = 30 # We change age to 30

Scope and lifetime

The scope of a variable refers to the parts of the code where it is available. Variables declared outside functions are global and available throughout the file. Variables inside a function are local and only visible within it.

The lifetime is the period during which the variable exists in memory. Local variables exist while the function executes, then they are destroyed. Global variables exist while the program is running.

Assignment

Assignment with the = operator allows changing or initializing a variable’s value:

number = 10
number = 20 # Now number is 20

There are also compound assignment operators like += and -= that combine an operation with assignment:

number += 5 # Adds 5 to number (number = number + 5)
number -= 2 # Subtracts 2 from number

Data types

Data types define what kind of value a variable can store. Python has several built-in types, including:

Numerical: To store integer, float, and complex numeric values:

integer = 10
float = 10.5
complex = 3 + 4j

Strings: To store text:

text = "Hello World"

Boolean: For True or False logical values:

true_variable = True
false_variable = False

Collections: To store multiple values like lists, tuples and dictionaries:

• Lists: Mutable sequences of values:

  list = [1, 2, 3]
  

• Tuples: Immutable sequences of values:

  tuple = (1, 2, 3)
  

• Dictionaries: Key-value pair structures:

  dictionary = {"name":"John", "age": 20}
  

It is important to choose the data type that best represents the information we want to store.

Operators

Operators allow us to perform operations with values and variables in Python. Some common operators are:

• Arithmetic: +, -, *, /, %, //, **

• Comparison: ==, !=, >, <, >=, <=

• Logical: and, or, not

• Assignment: =, +=, -=, *=, /=

Let’s see concrete examples of expressions using these operators in Python:

# Arithmetic
5 + 4 # Addition, result 9
10 - 3 # Subtraction, result 7
4 * 5 # Multiplication, result 20

# Comparison
5 > 4 # Greater than, result True
7 < 10 # Less than, result True

# Logical
True and False # Result False
True or False # Result True
not True # Result False

# Assignment
number = 10
number += 5 # Adds 5 to number, equivalent to number = number + 5

Each type of operator works with specific data types. We must use them consistently according to our variable data types.

Type conversions

Sometimes we need to convert one data type to another to perform certain operations. In Python we can convert explicitly or implicitly:

Explicit: Using functions like int(), float(), str():

float = 13.5
integer = int(float) # converts 13.5 to 13

text = "100"
number = int(text) # converts "100" to 100

Implicit: Python automatically converts in some cases:

integer = 100
float = 3.5
result = integer + float # result is 103.5, integer converted to float

Some conversions can generate data loss or errors:

float = 13.5
integer = int(float)

print(integer) # 13, decimals are lost

To prevent this we must explicitly choose conversions that make sense for our data.

Conclusion

In this article we reviewed key concepts like variables, operators, data types and conversions in Python. Applying these concepts well will allow you to efficiently manipulate data in your programs. I recommend practising with your own examples to gain experience using these features. Good luck in your Python learning!

2 - Input and output operations

Screen output

Python also provides functions to send program output to “standard output”, usually the screen or terminal¹.

The print() function displays the value passed as a parameter:

name = "Eric"
print(name) # displays "Eric"

We can print multiple values separated by commas²:

print("Hello", name, "!") # displays "Hello Eric!"

We can also use literal values without assigning to variables³:

print("2 + 3 =", 2 + 3) # displays "2 + 3 = 5"

Output formatting

Python provides various ways to format output⁴:

f-Strings: Allow inserting variables into a string:

name = "Eric"
print(f"Hello {name}") # displays "Hello Eric"

%s: Inserts string text into a format string:

name = "Eric"
print("Hello %s" % name) # displays "Hello Eric"

%d: Inserts integer numbers:

value = 15
print("The value is %d" % value) # displays "The value is 15"

.format(): Inserts values into a format string:

name = "Eric"
print("Hello {}. Welcome".format(name))
# displays "Hello Eric. Welcome"

These formatting options allow us to interpolate variables and values into text strings to generate custom outputs. We can combine multiple values and formats in a single output string².

Keyboard input

Python provides built-in functions to read data entered by the user at runtime. This is known as “standard input”⁴.

The input() function allows reading a value entered by the user and assigning it to a variable. For example:

name = input("Enter your name: ")

This displays the message “Enter your name: " and waits for the user to enter text and press Enter. That value is assigned to the name variable².

The input() function always returns a string. If we want to ask for a number or other data type, we must convert it using int(), float(), etc¹:

age = int(input("Enter your age: "))
pi = float(input("Enter the value of pi: "))

Reading multiple values

We can ask for and read multiple values on the same line separating them with commas³:

name, age = input("Enter name and age: ").split()

The split() method divides the input into parts and returns a list of strings. We then assign the list elements to separate variables.

We can also read multiple lines of input with a loop⁴:

names = [] # empty list

for x in range(3):
   name = input("Enter a name: ")
   names.append(name)

This code reads 3 names entered by the user and adds them to a list.

Output to a file

In addition to printing to the screen, we can write output to a file using the open() function¹:

file = open("data.txt", "w")

This opens data.txt for writing (“w”) and returns a file object.

Then we use file.write() to write to that file³:

file.write("Hello World!")
file.write("This text goes to the file")

We must close the file with file.close() when finished⁴:

file.close()

We can also use with to open and automatically close:

with open("data.txt", "w") as file:
   file.write("Hello World!")
   # no need to close, it's automatic

Reading files

To read a file we use open() in “r” mode and iterate over the file object¹:

with open("data.txt", "r") as file:
   for line in file:
      print(line) # prints each line of the file

This prints each line, including newlines.

We can read all lines to a list with readlines()³:

lines = file.readlines()
print(lines)

To read the full content to a string we use read()⁴:

text = file.read()
print(text)

We can also read a specific number of bytes or characters with read(n)².

File handling operations

There are several built-in functions to handle files in Python¹:

• open() - Opens a file and returns a file object
• close() - Closes the file
• write() - Writes data to the file
• read() - Reads data from the file
• readline() - Reads a line from the file
• truncate() - Empties the file
• seek() - Changes the reading/writing position
• rename() - Renames the file
• remove() - Deletes the file

These allow us to perform advanced operations to read, write and maintain files.

Conclusion

In this article we explained Python input and output operations in detail, including reading from standard input and writing to standard output or files. Properly handling input and output is essential for many Python applications. I recommend practising with your own examples to master these functions³.

References

3 - Flow Control

Conditions: making decisions in code

Life is full of decisions: “If it rains, I’ll take an umbrella. Otherwise, I’ll wear sunglasses.” These decisions are also present in the world of programming. Conditions are like questions the computer asks itself. They allow us to make decisions and execute specific code based on a condition¹. They can be as simple as “Is it raining?” or as complex as “Is it the weekend and do I have less than $100 in my bank account?”.

if

The if structure allows us to evaluate conditions and make decisions based on the result of that evaluation.

age = 15

if age >= 18:
    print("You are an adult")

The code above allows executing a portion of code if a person’s age is greater than or equal to 18 years.

if-else

When you want to execute alternative code if the condition is false, you use the if-else structure.

age = 21
if age >= 18:
    print("You are an adult")
else:
    print("You are a minor")

In this case, it determines if the person is an adult or a minor, and the message displayed is different.

if-elif-else

When conditions are multiple and two paths are not enough, the if-elif-else structure is used to evaluate them in a chained way.

age = 5
if age <= 13:
    print("You are a child")
elif age > 13 and age < 18:
    print("You are a teenager")
else:
    print("You are an adult")

In the code above, there are three clear paths: one for when age is less than or equal to 13, one for when age is between 13 and 18, and another for when age is greater than or equal to 18.

Another way to solve this problem is through the switch-case structure, which, although Python does not natively incorporate, other languages like Java or C++ do, and it is an important tool to be familiar with. This structure allows programmers to handle multiple conditions in a more organized way than a series of if-elif-else.

In Java, for example:

int day = 3;
switch(day) {
  case 1:
    System.out.println("Monday");
    break;
  case 2:
    System.out.println("Tuesday");
    break;
  case 3:
    System.out.println("Wednesday");
    break;
  // ... and so on
  default:
    System.out.println("Invalid day");
}

In the previous example, depending on the value of day, the corresponding day will be printed².

Loops: repeating actions

Sometimes in programming we need to repeat an action several times. Instead of writing the same code many times, we can use loops. These allow repeating the execution of a block of code while a condition is met³.

while

The while loop is useful when we want to repeat an action based on a condition.

# Prints 1 to 5
i = 1
while i <= 5:
    print(i)
    i = i + 1

do-while

Similar to while but guarantees at least one execution since the code block is executed first and then the condition is evaluated. Python does not implement this structure, but other languages like Java and C++ do.

int i = 1;

do {
  System.out.println(i);
  i++;
} while(i <= 5);

int number = 0;
do {
  std::cout << "Hello, world!" << std::endl;
  number++;
} while (number < 5);

for

The for loop is useful when we know how many times we want to repeat an action.

for i in range(5):
    print("Hello, world!")

The code above will print “Hello, world!” five times.

We can also iterate over the elements of a list or iterable object:

names = ["Maria", "Florencia", "Julian"]
for name in names:
    print(f"Hello {name}")

# Prints
# Hello Maria
# Hello Florencia
# Hello Julian

The break and continue statements

We can use break to terminate the loop and continue to skip to the next iteration.

break is used to completely terminate the loop when a condition is met, in the following example, when i reaches 5.

# break example
i = 0
while i < 10:
  print(i)
  if i == 5:
    break
  i += 1

# Prints:
# 0
# 1
# 2
# 3
# 4
# 5

continue is used to skip an iteration of the loop and continue with the next one when a condition is met. Here we use it to skip even numbers.

# continue example
i = 0
while i < 10:
  i += 1
  if i % 2 == 0:
    continue
  print(i)

# Prints:
# 1
# 3
# 5
# 7
# 9

Nesting: combining structures

Control flow structures can be nested within each other. For example, we can have loops within loops or conditions within loops.

for i in range(5):
  for j in range(10):
    if (i % 2 == 0 and j % 3 == 0):
      print(f"i = {i}, j = {j}")

This code will print combinations of i and j only when i is divisible by 2 and j is divisible by 3, demonstrating how loops are nested and executed³.

Common usage patterns

There are specific patterns to solve common needs with control flow.

Search

Search for a value in a collection:

fruits = ["apple", "orange"]

searching = "orange"
found = False

for fruit in fruits:
    if fruit == searching:
        found = True
        break

if found:
    print("Fruit found!")

Accumulation

Accumulate incremental values in a loop:

total = 0

for i in range(10):
    total += i

print(total) # Sum from 0..9 = 45

Flowcharts: the visual route to understanding code

Programmers, whether beginners or experts, often find themselves facing challenges that require detailed planning before diving into code. This is where flowcharts come into play as an essential tool. These charts are graphical representations of the processes and logic behind a program or system. In this article, we will unravel the world of flowcharts, from basic concepts to advanced techniques, and how they can benefit programmers of all levels.

A flowchart is a graphical representation of a process. It uses specific symbols to represent different types of instructions or actions. Its main purpose is to simplify understanding of a process by showing step by step how information or decisions flow. These charts:

• Facilitate understanding of complex processes.
• Aid in the design and planning phase of a program.
• Serve as documentation and reference for future developments.

Flowcharts are a powerful tool that not only benefits beginners but also experienced programmers. They provide a clear and structured view of a process or program, facilitating planning, design, and communication between team members.

Basic elements

Flowcharts consist of several symbols, each with a specific meaning:

• Oval: Represents the start or end of a process.
• Rectangle: Denotes an operation or instruction.
• Diamond: Indicates a decision based on a condition.
• Arrows: Show the direction of flow.

graph TD;
    start((Start))
    process[Process]
    decision{Decision?}
    final((End))

    start --> process;
    process --> decision;
    decision --> |Yes| process
    decision --> |No| final

Examples

Let’s design a flowchart for a program that asks for a number and tells us if it’s even or odd.

graph TB
    start((Start))
    input[Input number]
    decision{Even?}
    isEven[Is even]
    isOdd[Is odd]
    final((End))

    start --> input
    input --> decision
    decision --> |Yes| isEven
    decision --> |No| isOdd
    isEven --> final
    isOdd --> final

As programs become more complex, you may need to incorporate loops, multiple conditions, and other advanced elements into your flowchart. For example, here we diagram a program that sums numbers from 1 to a number entered by the user.

graph TD
    start((Start))
    input[Input number]
    setVariables[Set sum=0 and counter=1]
    loop_condition{counter <= N?}
    loop_code[Add value and increment counter]
    result[Show sum]
    final((End))

    start --> input
    input --> setVariables
    setVariables --> loop_condition
    loop_condition --> |Yes| loop_code
    loop_code --> loop_condition
    loop_condition --> |No| result
    result --> final

Conclusion

Control flow is the heart of programming. Without it, programs would be linear sequences of actions without the ability to make decisions or repeat tasks. By mastering these structures not only do you improve your ability to write code, but also your ability to think logically and solve complex problems.

References

4 - Functions

What are functions?

A function, in simple terms, is a block of code that executes only when called. You can think of it as a small program within your main program, designed to perform a specific task¹. A function can also be seen as a black box: we pass an input (parameters), some internal processing occurs, and it produces an output (return value).

Functions allow us to segment our code into logical parts where each part performs a single action. This provides several benefits²:

• Reusability: Once defined, we can execute (call) that code from anywhere in our program as many times as needed.
• Organization: It allows dividing a large program into smaller, more manageable parts.
• Encapsulation: Functions reduce complexity by hiding internal implementation details.
• Maintainability: If we need to make changes, we only have to modify the code in one place (the function) instead of tracking down all instances of that code.

Procedures vs. Functions

It is vital to distinguish between these two concepts. While a function always returns a value, a procedure performs a task but does not return anything. In some languages, this difference is clearer than in others. Python, for example, has functions that can optionally return values.

Anatomy of a function

In Python, a function is declared using the def keyword, followed by the function name and parentheses. The code inside the function is called the body of the function³ and contains the set of instructions to execute to perform its task.

def my_function():
    print("Hello from my function!")

To call or invoke a function, we simply use its name followed by parentheses:

my_function() # Output: Hello from my function!

Parameters and arguments

Functions become even more powerful when we pass information to them, known as parameters. These act as “variables” inside the function, allowing the function to work with different data each time it is called.

While parameters are variables defined in the function definition, arguments are the actual values passed when calling the function.

def greet(name):
    print(f"Hello {name}!")

greet("Maria")
# Output:
#   Hello Maria!

Python allows default parameters, which have a predetermined value, making passing those arguments optional when calling the function. It also allows named parameters which enable passing arguments in any order by specifying their name.

def greet(name="Maria", repetitions=3):
    repetition = 1
    while repetition <= repetitions:
        print(f"Hello {name}!")
        repetition += 1

greet()
# Output:
#   Hello Maria!
#   Hello Maria!
#   Hello Maria!

greet("Florencia", 4)
# Output:
#   Hello Florencia!
#   Hello Florencia!
#   Hello Florencia!
#   Hello Florencia!

greet(repetitions=2, name="Julian")
# Output
#   Hello Julian!
#   Hello Julian!

Returning values

Functions can return a result or return value using the return keyword.

def circle_area(radius):
    return 3.14 * (radius ** 2)

result = circle_area(10)
print(result) # Output: 314

The return value is passed back to where the function was called and can be assigned to a variable for later use.

Functions can also perform some task without explicitly returning anything. In Python this is known as returning None.

Local and global variables

Local variables are defined inside a function and only exist in that scope, while global variables are defined outside and can be accessed from anywhere in the code. It is crucial to understand their scope (where a variable is accessible) and lifetime (how long a variable lives).

x = 10 # x is global

def add():
    y = 5 # y is local
    return x + y

add() # Output: 15
print(y) # Error, y does not exist outside the function

We can read global variables from a function, but if we need to modify it we must declare it global.

x = 10

def add():
    global x
    x = x + 5

add()
print(x) # 15

Best Practices

When creating functions we should follow certain principles and patterns⁴:

• The name of a function should clearly indicate its purpose.
• Make functions small, simple, and focused on one task. A function should do one thing and do it well.
• Use descriptive names for functions and parameters.
• Avoid side effects and modifying global variables.
• Properly document the purpose and usage of each function.
• Limit the number of parameters, ideally 0 to 3 parameters.

Following these best practices will help us create reusable, encapsulated, and maintainable functions.

Conclusion

Functions are core components in programming, allowing us to organize, reuse, and encapsulate code. By defining functions that perform a single task we keep our programs simplified, easy to understand, and modify. By understanding and mastering this concept, you not only improve the quality of your code but also your efficiency as a developer.

References

5 - Recursive Functions

Recursion: the art of calling yourself

Imagine a box of mirrors where each mirror reflects what it sees in the next, creating an infinite series of reflections. Recursion in programming is something similar. It is a technique where a function calls itself directly or indirectly¹. This creates a cycle where the function solves a problem by dividing it into smaller instances of the same problem, until reaching a simple base case that can be solved directly.

For example, let’s imagine a function that prints a countdown:

def countdown(number):

    if number > 0:
        print(number)
        countdown(number - 1)
    else:
        print("Blastoff!")

countdown(5)

This function calls itself recursively decrementing the number each time until reaching 0, and then prints the blastoff message.

Recursion is a declarative approach that focuses on dividing a problem into recursive cases without needing to explicitly control the loop using iterators or counters like in imperative programming.

The structure of a recursive function

The power of recursion lies in its simplicity. However, it is essential to understand its structure to avoid common pitfalls. A typical recursive function has two main parts²:

1. Base case: The simplest case with a known solution that doesn’t require recursion. It is the stopping condition that halts the recursion. Without a base case, we would fall into infinite recursion which eventually overflows the call stack.
2. Recursive case: This is where the magical recursive call occurs. At this point, the function calls itself with a modified argument, usually a reduced version of the original problem.

Classic recursion examples

Factorial

The factorial of a positive integer \(n\) is the product of all positive integers less than or equal to \(n\). For example:

• \(5! = 5 * 4 * 3 * 2 * 1 = 120\)
• \(4! = 4 * 3 * 2 * 1 = 24\)
• \(3! = 3 * 2 * 1 = 6\)

Here is the Python code for calculating factorial using recursion:

def factorial(n):
    if n == 1:
        return 1       # Base case
    return n * factorial(n-1) # Recursive case

• Base case: The base case is the simplest, smallest instance of the problem that can be answered directly. For factorial, when \(n = 1\), the result is \(1\).
• Recursive case: If \(n\) is greater than \(1\), the function calls itself with \(n-1\), and multiplies the result by \(n\).

Let’s say you want to calculate the factorial of \(5\), so you call factorial(5).

Here is what happens:

1. Step 1: Since \(n = 5\) is not \(1\), the function calls factorial(4), then multiplies the result by \(5\).
2. Step 2: Now, inside factorial(4), \(n = 4\), so the function calls factorial(3), then multiplies the result by \(4\).
3. Step 3: Inside factorial(3), \(n = 3\), so it calls factorial(2), then multiplies the result by \(3\).
4. Step 4: Inside factorial(2), \(n = 2\), so it calls factorial(1), then multiplies the result by \(2\).
5. Step 5: Finally, factorial(1) reaches the base case, where \(n = 1\), so it returns \(1\).

Now the results unwind:

• factorial(2) returns \(2 * 1 = 2\)
• factorial(3) returns \(3 * 2 = 6\)
• factorial(4) returns \(4 * 6 = 24\)
• factorial(5) returns \(5 * 24 = 120\)

The final result is \(120\), which is the value of \(5!\).

Here is a visual representation of the call stack:

factorial(5)
  -> factorial(4)
    -> factorial(3)
      -> factorial(2)
        -> factorial(1)
          return 1
        return 2
      return 6
    return 24
  return 120

Fibonacci sequence

The Fibonacci sequence is a series of numbers where each number is the sum of the previous two. It starts with \(0\) and \(1\), and each subsequent number is the sum of the two numbers before it. The beginning of the sequence is: \(0, 1, 1, 2, 3, 5, 8, 13, 21, 34, …\)

Here is the Python code for calculating the \(n^th\) Fibonacci number using tail recursion:

def fibonacci(n, a=0, b=1):
    if n == 0:
        return a
    return fibonacci(n-1, b, a+b)

The function takes three parameters:

• \(n\): The position of the desired number in the sequence.
• \(a\) and \(b\): Two numbers that aid in the sequence calculation.

Here is a breakdown of how the function works:

1. Base case: If \(n\) is \(0\), the function returns \(a\). This is the value of the \(n^th\) number in the sequence.
2. Recursive case: If \(n\) is not \(0\), the function calls itself with \(n-1\), \(b\), and \(a+b\). These parameters change the position in the sequence and prepare the next numbers for summation.

Suppose we want to find the \(5^th\) number in the Fibonacci sequence by calling fibonacci(5).

Here is what happens:

1. Step 1: Since \(n = 5\), it calls fibonacci(4, 1, 1) (because \(a = 0\), \(b = 1\), \(a + b = 1\)).
2. Step 2: Since \(n = 4\), it calls fibonacci(3, 1, 2) (because \(a = 1\), \(b = 1\), \(a + b = 2\)).
3. Step 3: Since \(n = 3\), it calls fibonacci(2, 2, 3) (because \(a = 1\), \(b = 2\), \(a + b = 3\)).
4. Step 4: Since \(n = 2\), it calls fibonacci(1, 3, 5) (because \(a = 2\), \(b = 3\), \(a + b = 5\)).
5. Step 5: Since \(n = 1\), it calls fibonacci(0, 5, 8) (because \(a = 3\), \(b = 5\), \(a + b = 8\)).
6. Step 6: Since \(n = 0\), it returns \(a\), which is \(5\).

The result is \(5\), which is the \(5^th\) number in the Fibonacci sequence.

Here is a visual representation of the call stack:

fibonacci(5, 0, 1)
  -> fibonacci(4, 1, 1)
    -> fibonacci(3, 1, 2)
      -> fibonacci(2, 2, 3)
        -> fibonacci(1, 3, 5)
          -> fibonacci(0, 5, 8)
            return 5

Advantages and Disadvantages

Recursion has certain advantages³:

• It can result in simple, elegant solutions for problems that easily break down into subproblems.
• It eliminates the need for explicit loop control.
• It mirrors the mathematical structure of a recursive definition.

The disadvantages include:

• It can be less efficient (high memory consumption) than iteration due to repeated function calls and stack frame creation.
• Too much recursion can overflow the call stack and cause crashes.
• It can be harder to debug and analyze than iteration.

Therefore, recursion is a powerful tool that should be used judiciously in appropriate cases.

Recursion vs Iteration

Recursion and iteration (using loops) are parallel tools and we can use either one to solve many problems. Both techniques have the potential to solve the same problems, but their implementation and efficiency may vary. Let’s take the factorial example:

Iterative

def factorial_iterative(n):
    result = 1
    for i in range(1, n+1):
        result *= i
    return result

Recursive

def factorial_recursive(n):
    if n == 1:
        return 1
    return n * factorial(n-1)

The iterative version is more efficient in terms of space, but the recursive is cleaner and easier to understand. The choice between recursion and iteration often depends on the specific problem, memory constraints, and programmer preferences.

Conclusion

Recursion is a key technique that allows writing elegant, natural, and efficient algorithms when properly leveraged. Understanding how to break down a problem into recursive cases is essential to master this skill. Recursion provides a unique declarative alternative to solve complex problems without managing explicit loops. However, it is crucial to always remember to define an adequate base case and be aware of recursion limitations in terms of efficiency and memory usage. Knowing how to combine recursion and iteration gives flexibility when creating optimal solutions.

As always, the key lies in finding the right balance and using the right tool for the right job.

References