Python while else

4 Key excerpts on "Python while else"

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  Python Fundamentals

  A practical guide for learning Python, complete with real-world projects for you to explore

  • Ryan Marvin, Mark Ng'ang'a, Amos Omondi(Authors)
  • 2018(Publication Date)
  • Packt Publishing

    (Publisher)

  if_python.py and add your code to it. Then, run it in a terminal. The output should be as follows:

  >python if_python.py Return TRUE or FALSE: Python was released in 1991: TRUE Correct Bye! >python if_python.py Return TRUE or FALSE: Python was released in 1991: FALSE Wrong Bye!

  Note

  Solution for this activity can be found at page 283.

  The while Statement

  A while statement allows you to execute a block of code repeatedly, as long as a condition remains true. That is to say, as long as condition X remains true, execute this code .

  A while statement can also have an else clause that will be executed exactly once when the condition, X , that's mentioned is no longer true. This can be read as follows: As long as X remains true, execute this block of code, else, execute this other block of code immediately so that the condition is no longer true .

  For instance, consider the traffic police officer we mentioned earlier, who could be letting traffic through while the exit is clear, and as soon as it is not clear, they stop the drivers from exiting.

  Here is the basic syntax of a while statement:

  while condition: # Run this code while condition is true # Replace the "condition" above with an actual condition # This code keeps running as long as the condition evaluates to True else: # Run the code in here once the condition is no longer true # This code only runs one time unlike the code in the while block

  As mentioned earlier, this can be read as: As long as the condition is true, execute the first block of code, and if the condition is not true, execute the second block of code .

  Exercise 15: Using the while Statement

  Now we will look at a practical use of the while statement:

  1. First, declare a variable called current and set its value to 1 :current = 1
  2. Declare the end variable and set its value to 10 :end = 10
  3. Then, write a while statement.
     The condition of this while statement is that the current

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  Job Ready Python

  • Haythem Balti, Kimberly A. Weiss(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  If you changed the range to a different number, then the for statement would iterate that many times. NOTE If you’ve used other programming languages such as C, C++, or Java, then you might notice that the for loop in Python doesn’t use braces ({}) to mark the beginning and end of the code block that will be iterated with the loop. Rather, Python uses indenting to determine what code to run. THE WHILE LOOP An if statement evaluates a condition, performs specific activities based on that condition, and then moves on to the next set of instructions in the code. Although similar in execution, a while loop repeats a specific set of instructions as many times as needed as long as a con -dition is True. We use the term iteration to refer to each time a set of instructions runs. The basic syntax for a while loop is: while ( condition == True): < statement(s) > In order to iterate through a loop a given number of times, we use a counter variable to increment the variable by a constant measure. For example, we might increase the counter variable by 1, or we may double it. We often refer to that counter variable as an “incrementer.” 147 Lesson 7: Controlling Program Flow with Loops This changing of the counter variable used in the condition is extremely important as it ensures that the condition will eventually stop being True . Without such a condition, we would end up with an endless loop that would run forever, commonly referred to as an infinite loop. In the example of using a while statement presented in Listing 7.2, we simply display the integers 0 through 9. LISTING 7.2 Using a while statement i = 0 while ((i < 10) == True): print(i) i = i + 1 This listing goes through the following steps: 1. It starts by setting a variable i to 0. 2. It then uses a while loop that will run as long as i is less than 10. We can see that the while statement checks to see if i is less than 10.

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  Python Programming for Biology

  Bioinformatics and Beyond

  • Tim J. Stevens, Wayne Boucher(Authors)
  • 2015(Publication Date)
  • Cambridge University Press

    (Publisher)

  Usually such loops are either done a specific number of times or until something in particular happens. When you have a collection of items, like a list, a loop can be used to consider all of the items in turn; the loop iterates over the items of the collection. A significant number of people who are new to programming find this the hardest idea to get to grips with, although Python’ s syntax makes it about as easy as it can be. Loops, conditional statements and functions are the three ways of controlling a program’ s flow that occur under ordinary circumstances. There is actually a fourth way that can cause a jump in program execution and that is if an error or exception occurs, i.e. something illegal has happened. In Python, like with Java and many other languages, when an error occurs inside a function, the exception propagates back up the stack of any function calls, until it finds the first of those functions. If the initial function does not specifically handle the error, then the program (or more precisely that specific thread) stops. Code blocks With all of the means by which Python code execution can jump about we naturally need to be aware of the boundaries of the block of code we jump into, so that it is clear at what point the job is done, and program execution can jump back again. In essence it is required that the end Figure 4.1. The changes to the flow of a program’s line-by-line execution using conditionals and loops. The normal flow of a program’ s execution proceeds from one line to the next. The presence of a conditional test, using an ‘if’ statement, means a block of code is only executed if the test evaluates to be logically true, and otherwise the block is skipped. The presence of a loop, starting with a ‘for’ or ‘while’ statement, causes the execution of a block of code to be repeated a number of times from the start of the block and after the last repeat the program execution resumes after the block. 44 Program control and logic

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  eBook - ePub

  Basic Core Python Programming

  A Complete Reference Book to Master Python with Practical Applications (English Edition)

  • Meenu Kohli(Author)
  • 2021(Publication Date)
  • BPB Publications

    (Publisher)

  HAPTER 8

  Program Flow Control in Python

  Introduction

  So far, you have seen simple statements in coding. The examples of code that we have tried so far in this book have executed line-by-line very smoothly without any issues. But in the real world of software development, you will be working on complex logic. In this chapter, you will learn about the statements used for decision making and controlling the program flow. These statements control the execution of code. So, get ready to work with control structures.

  Structure

  • Working with ‘if’
    • if .. elif..else
    • if statement
    • if..else statement
    • Nested if statement
  • For loops
    • Iterating over a range of numbers
      • Convert a range into a list
      • Negative steps in range
  • while loops
  • else clause with loops
  • continue, break and pass statements
  • Recap on control structures
  • Python iterators
  • Python generators

  Objective

  After reading this chapter, you will be able to:

  • Work with ‘if’ statements
  • Work with for loops
  • Work with while loops
  • Use else clause with loops
  • Work with continue, break, and pass statements
  • Work with Python iterators
  • Work with Python generators

  There is a block of code associated with every control flow statement. This block of code is defined by a colon (‘:’) and indentation. You are aware that Python uses four spaces (PEP-8 standard) of indentation to mark the beginning of a block of code. Usage of a tab instead of four spaces is not recommended as per the best practices defined in PEP-8. In all languages, indentation is required only to improve the readability and the block of code is defined in a curly brace ({}). However, in Python programming indentation is crucial.

  8.1 Working with ‘if’

  While programming you will often come to a point where you will have to decide how your program will work under different scenarios. For example, you build campus management software for your school but you cannot give the same privileges to all the users. You will have different privileges for students and teachers. So, if a student logs in then there are one set of privileges available and if the teacher logs in then another set of privileges are available.

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