Python Program Structures and the for Loop

Structure of Programs

Overview of Program Structures

Python programs are typically organized using three fundamental control structures: sequence, decision, and repetition structures. Understanding these is essential for writing clear and effective code.

• Sequence Structure: Executes statements in a linear, step-by-step order.

• Decision Structure: Uses conditions to determine which statements to execute (e.g., if statements).

• Repetition Structure: Repeats a block of code multiple times, typically using loops (while or for).

Repetition Structures: Loops in Python

The while Loop

The while loop repeatedly executes a block of statements as long as a specified condition is True.

• Syntax:

while condition: indented block of statements

• Sentinel Values: Special values used to terminate a loop.

• break and continue Statements: Used to alter the flow of loops.

The break and continue Statements

• break: Immediately exits the nearest enclosing loop, skipping any remaining statements in the loop body.

• continue: Skips the rest of the code inside the loop for the current iteration and jumps to the next iteration of the loop.

# Example of break while condition: statement if condition: break statement # Example of continue while condition: statement if condition: continue statement

The for Loop

Introduction to the for Loop

The for loop is used to iterate through a sequence of values, such as lists, strings, or ranges of numbers. It is especially useful when the number of iterations is known in advance.

• General form:

for variable in sequence: statement statement ...

• Sequence types: List, String, Arithmetic progression (using range()), File object

How the for Loop Works

• The variable is successively assigned each value in the sequence.

• The indented block of statements is executed after each assignment.

• Indentation is crucial in Python to define the scope of the loop body.

Examples of for Loops

• Iterating through a list:

list1 = ["one", 23, 17.5, "two", 33, 22.1, 242, "three"] for var in list1: print(var)

Output: Each element of the list is printed on a new line.

• Finding the first integer divisible by 11:

list1 = ["one", 23, 17.5, "two", 33, 22.1, 242, "three"] found = False for i in range(len(list1)): if type(list1[i]) == int and list1[i] % 11 == 0: found = True print(list1[i]) break

• Iterating through a string:

stateName = "Virginia" for letter in stateName: print(letter)

Comparison: while vs for Loops

• while loop: Uses an index variable (e.g., i) to access elements by position.

• for loop: Directly assigns each element in the sequence to the loop variable.

# while version stateName = "Virginia" i = 0 while i < len(stateName): letter = stateName[i] print(letter) i = i + 1 # for version stateName = "Virginia" for letter in stateName: print(letter)

Key difference: The while loop requires manual index management, while the for loop automatically iterates over elements.

Using range() in for Loops

Generating Arithmetic Progressions

The range() function generates a sequence of integers, commonly used for count-controlled loops.

• Syntax: range([start], stop[, step])

• If only stop is provided, the sequence starts at 0 and ends at stop - 1.

• If step is provided, it determines the increment (or decrement if negative).

Expression	Generated Sequence	Comment
range(5)	0, 1, 2, 3, 4	Executes 5 times, from 0 to 4
range(10, 15)	10, 11, 12, 13, 14	Ending value is not included
range(0, 9, 2)	0, 2, 4, 6, 8	Step value is 2
range(5, 0, -1)	5, 4, 3, 2, 1	Negative step for counting down

Example: Displaying Integers and Their Squares

for i in range(2, 6): print(i, i**2)

Output: Displays 2 4, 3 9, 4 16, 5 25

Example: Population Growth Table

pop = 300000 print("{0:10} {1}".format("Year", "Population")) for year in range(2014, 2019): print("{0:10d} {1:d}".format(year, round(pop))) pop += 0.03 * pop

Output: Displays a table of population growth from 2014 to 2018, increasing by 3% each year.

Counting Matches in a String

Counting Uppercase Letters

To count the number of uppercase letters in a string, iterate through each character and use the isupper() method.

string = "ISE 135" uppercase = 0 for char in string: if char.isupper(): uppercase += 1 print(uppercase)

Boolean String Methods

• isupper(): Returns True if all cased characters are uppercase.

• isdigit(): Returns True if all characters are digits.

• isalpha(): Returns True if all characters are alphabetic.

Looping Through Lists and Tuples

Example: Filtering List Items

To display months containing the letter 'r':

months = ("January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December") for month in months: if 'r' in month.lower(): print(month)

Nested for Loops

Definition and Use

A nested for loop is a loop inside another loop. The inner loop must use a different loop variable and is fully contained within the outer loop's body.

Example: Multiplication Table

for m in range(1, 6): for n in range(1, 6): print(f"{m}x{n}={m*n}", end="\t") print()

Output: Prints a 5x5 multiplication table.

Practice Problems

• Display a triangle of asterisks: Use nested for loops to print a triangle pattern based on user input.

• Average Exam Grades: Use nested loops to compute average grades for multiple students, prompting for each student's grades and whether to continue.

Comments and Additional Notes

• The range() function can take one, two, or three arguments: range([start], stop[, step]).

• If the step is positive and start ≥ stop, or if the step is negative and start ≤ stop, range() produces an empty sequence.

• Use list(range(...)) to display the generated sequence.

• Loops can be nested: for loops inside while loops and vice versa.

• break and continue are often used inside if statements to control loop execution efficiently.