Glossary

5.1 While loop

3 min read•june 24, 2024

loops are powerful tools for repetitive tasks in Python. They execute a block of code repeatedly as long as a specified condition remains true, offering flexibility in controlling program flow and .

Mastering while loops involves understanding loop conditions, variables, and control statements like and . These elements allow programmers to create dynamic, efficient loops that can handle complex repetitive tasks while maintaining precise control over execution.

Looping Constructs

While loops for repetitive tasks

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Top images from around the web for While loops for repetitive tasks

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  • Repeatedly execute a block of code as long as a specified condition remains true
    • Condition checked at the beginning of each
    • executed if condition evaluates to True
    • Loop terminates and program execution continues with next statement after loop if condition evaluates to False
  • Basic syntax of a while loop in Python: 
    while condition:     # Loop body     # Code to be executed repeatedly
  • Ensure condition eventually becomes False to prevent
    • Infinite loops occur when condition always remains True, causing loop to run indefinitely
    • Modify variables or state that affect condition in loop body to avoid infinite loops (counter variables)

Control of program flow

  • is an expression that evaluates to a Boolean value (True or False)
    • Can include variables, comparison operators (====, !=!=, >>, <<, >=>=, <=<=), logical operators (and, or, not), and function calls
    • The condition is also known as a
  • Carefully design loop condition to control number of iterations and ensure desired program flow
    • Modify loop condition within loop body for dynamic control over loop's execution
    • Update variables used in condition within loop to eventually make condition False and terminate loop (incrementing or decrementing counters)

Counting loops with variables

  • Use counter variables to keep track of number of iterations or control loop's behavior
    • Initialize counter variable (also called a ) before loop and increment or decrement within loop body
    • Often used as part of loop condition to determine when to exit loop (counter < 10)
  • Step sizes determine increment or decrement value applied to counter variable in each iteration
    • Can be positive or negative integer for increasing or decreasing sequences
    • Adjust to control loop's progression and values generated within loop
  • Example of counting loop with counter variable and step size: 
    counter = 0 while counter < 10:     print(counter)     counter += 2
    • Counter variable starts at 0 and increments by 2 in each iteration until it reaches or exceeds 10

Controlling Loop Execution

While loops for repetitive tasks

  • Use 
    [break](https://www.fiveableKeyTerm:Break)
    statement to prematurely exit loop based on certain condition
    • Loop immediately terminates when 
      break
      encountered within loop body
    • Program execution continues with next statement after loop
    • Often used with conditional statements (
      if
      ) to exit loop when specific condition met
  • Use 
    continue
    statement to skip rest of current iteration and move to next iteration
    • Remaining code in loop body skipped when 
      continue
      encountered
    • Loop proceeds with next iteration
    • Useful for skipping certain iterations based on condition without terminating entire loop
  • Example using 
    break
    and 
    continue
    : 
    counter = 0 while counter < 10:     counter += 1     if counter == 5:         continue     if counter == 8:         break     print(counter)
    • Loop skips printing number 5 due to 
      continue
      statement
    • Loop terminates when counter reaches 8 due to 
      break
      statement

Loop Control and Tracking

  • Iteration count: Keep track of the number of times the loop has executed
  • : The specific condition that causes the loop to terminate
  • Use these elements to monitor and control the loop's behavior and ensure it performs the desired number of iterations

Key Terms to Review (26)

Accumulator: An accumulator is a variable that is used to collect or accumulate values over time, typically in the context of loops. It allows for the aggregation of data as a program executes, enabling calculations like sums, counts, or other cumulative metrics. This concept is especially useful in iterative processes, where repetitive actions can build upon each other to produce a final result.
Boolean Expression: A Boolean expression is a logical statement that evaluates to either true or false. It is a fundamental concept in programming that is used to control the flow of execution in a program based on conditions.
Break: A break statement in Python is used to exit a loop prematurely when a certain condition is met. It immediately stops the execution of the innermost loop and transfers control to the statement following that loop.
Break: The 'break' statement is a control flow statement in programming that allows a loop to be terminated prematurely, exiting the loop immediately. It is a powerful tool that provides flexibility and control over the execution of loops in various programming constructs.
Continue: The 'continue' statement is a control flow statement in programming that allows a loop to skip the current iteration and move on to the next one, effectively continuing the loop's execution from the next iteration. It is commonly used in the context of various loop structures, including while loops, for loops, and nested loops, to selectively bypass certain iterations based on specific conditions.
Counter: A counter is a variable that keeps track of the number of times a specific event or action has occurred within a program. It is a fundamental concept in programming, particularly in the context of control flow structures like loops, where it is used to manage and monitor the execution of repetitive operations.
Dictionary: A dictionary in Python is a collection of key-value pairs where each key is unique. It allows for efficient data retrieval based on the keys.
Dictionary: A dictionary in Python is an unordered collection of key-value pairs, where each key is unique and is associated with a corresponding value. Dictionaries provide a flexible and efficient way to store and retrieve data, making them a fundamental data structure in the language.
Exit Condition: An exit condition is a logical expression or test that determines when a loop should terminate or stop executing. It is a crucial component in controlling the flow of a program, ensuring that a loop runs only for the desired number of iterations or until a specific condition is met.
Flag Variable: A flag variable is a boolean (true/false) variable used to track the state or condition of a program or algorithm. It serves as a marker or signal to indicate whether a particular event or condition has occurred.
Infinite loop: An infinite loop is a sequence of instructions in programming that loops endlessly without a terminating condition. This typically occurs due to logic errors or incorrect conditions in the loop.
Infinite Loop: An infinite loop is a programming construct where a sequence of instructions or a block of code is repeated indefinitely, without any condition to terminate the loop. This can occur in various programming constructs, such as while loops, for loops, and do-while loops.
Input: Input in Python is data provided by the user to the program, typically through the keyboard. It can be captured using the input() function and stored in variables for further processing.
Input(): The input() function in Python is used to receive data from the user during the execution of a program. It allows the program to pause and wait for the user to enter some information, which is then stored in a variable for further processing.
Iteration: Iteration is the process of repeating a set of instructions or operations multiple times in a computer program or algorithm. It is a fundamental concept in programming that allows for the execution of a block of code repeatedly until a specific condition is met.
Iteration Count: Iteration count refers to the number of times a loop, such as a while loop, executes before terminating. It is a crucial concept in programming that helps control the flow of execution and ensures the loop performs the desired number of iterations.
Len(): The len() function is a built-in function in Python that returns the length or count of elements in a given object, such as a string, list, tuple, or dictionary. It is a fundamental operation that is widely used across various programming topics in Python.
List: A list in Python is an ordered collection of items, where each item can be of a different data type. Lists are one of the most fundamental and versatile data structures in the Python programming language, allowing you to store and manipulate multiple values in a single variable.
Loop Body: The loop body refers to the set of instructions or statements that are executed repeatedly within a loop construct, such as a while loop or a for loop, until the loop's termination condition is met. It is the core of the looping mechanism, where the actual operations or computations take place during each iteration of the loop.
Loop Condition: The loop condition is a logical expression that controls the execution of a loop in a programming language. It determines whether the loop should continue to execute or terminate based on the evaluation of the condition.
Loop Variable: A loop variable is a variable that is used to control the number of iterations or repetitions in a loop. It is a key component in both while loops and for loops, as it determines when the loop should stop executing.
Nested while loop: A nested while loop is a loop inside another while loop, allowing for more complex iterations. This structure enables you to perform repeated actions within repeated actions, which is useful when working with multi-dimensional data or when you need to repeat an operation multiple times based on certain conditions. By leveraging nested loops, you can create intricate control flows that enhance the functionality of your programs.
Sentinel Value: A sentinel value is a special value used to signify the end of a data input sequence or loop, often acting as a stopping point for iteration. In the context of a while loop, sentinel values are important because they allow programmers to control when to terminate the loop based on user input or specific conditions. This helps prevent infinite loops and ensures that programs can handle user inputs efficiently and gracefully.
Step size: Step size is the increment or decrement by which a loop variable is updated each iteration. It determines how quickly the loop progresses towards its termination condition.
While: The 'while' statement in programming is a control flow statement that repeatedly executes a block of code as long as a specified condition remains true. It allows for the creation of loops, where a set of instructions can be repeated until a particular condition is met.
While-else: The while-else statement is a control flow structure in programming that allows for the execution of a block of code as long as a certain condition is true, and provides an optional 'else' block to be executed if the condition becomes false. It is used to create loops and handle different scenarios based on the evaluation of the condition.
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