Python Loops

12 Key excerpts on "Python Loops"

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  eBook - ePub

  The Statistics and Calculus with Python Workshop

  • Peter Farrell, Alvaro Fuentes, Ajinkya Sudhir Kolhe, Quan Nguyen, Alexander Joseph Sarver, Marios Tsatsos(Authors)
  • 2020(Publication Date)
  • Packt Publishing

    (Publisher)

  for loops. Let's understand each one in detail.

  The while Loop

  A while loop, just like an if statement, checks for a specified condition to determine whether the execution of a given program should keep on looping or not. For example, consider the following code:

  >>> x = 0 >>> while x < 3: ... print(x) ... x += 1 0 1 2

  In the preceding code, after x was initialized with the value 0 , a while loop was used to successively print out the value of the variable and increment the same variable at each iteration. As you can imagine, when this program executes, 0 , 1 , and 2 will be printed out and when x reaches 3 , the condition specified in the while loop is no longer met, and the loop therefore ends.

  Note that the x += 1 command corresponds to x = x + 1 , which increments the value of x during each iteration of the loop. If we remove this command, then we would get an infinite loop printing 0 each time.

  The for Loop

  A for loop, on the other hand, is typically used to iterate through a specific sequence of values. Using the range function in Python, the following code produces the exact same output that we had previously:

  >>> for x in range(3): ... print(x) 0 1 2

  The in keyword is the key to any for loop in Python: when it is used, the variable in front of it will be assigned values inside the iterator that we'd like to loop through sequentially. In the preceding case, the x variable is assigned the values inside the range(3) iterator—which are, in order, 0 , 1 , and 2 —at each iteration of the for loop.

  Instead of range() , other types of iterators can also be used in a Python for loop. The following table gives a brief summary of some of the most common iterators to be used in for

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  Job Ready Python

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

    (Publisher)

  P A R T I I Loops and Data Structures Lesson 7: Controlling Program Flow with Loops Lesson 8: Understanding Basic Data Structures: Lists Lesson 9: Understanding Basic Data Structures: Tuples Lesson 10: Diving Deeper into Data Structures: Dictionaries Lesson 11: Diving Deeper into Data Structures: Sets Lesson 12: Pulling It All Together: Prompting for an Address Lesson 13: Organizing with Functions Lesson 7 Controlling Program Flow with Loops P ython supports a variety of control structures that determine how content in a program will work and interact. This lesson continues the exploration including additional control structures that make creating algorithms possible. The lesson concludes with a discussion of how iteration aids in text processing. LEARNING OBJECTIVES By the end of this lesson, you will be able to: • Create a Python script that uses a for loop to repeat an activity until a specific crite -rion is met. • Create a Python script that uses a while loop to repeat an activity as long as a specific criterion holds true. • Using string operations to manipulate text. Job Ready Python 144 ITERATIONS OVERVIEW Lesson 5 introduced control structures. These structures are used to “control” the flow of data through an application. Python supports various structures through control state-ments of sequence, iteration, and selection. In Lesson 5 selection and progression were discussed. The sequence control structure (executed in a sequence statement) is com -pleted one statement after the other. The selection control structure (implemented in a selection statement) executes only after a particular condition is met. Although these types of code statements allow applications to run, in most cases, using only sequence and selection statements would make for very long program code. In this lesson, a third control structure is introduced: iteration.

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  eBook - ePub

  Python for Beginners

  • Kuldeep Singh Kaswan, Jagjit Singh Dhatterwal, B Balamurugan(Authors)
  • 2023(Publication Date)
  • Chapman and Hall/CRC

    (Publisher)

  5 Iterative Control Structure

  DOI: 10.1201/9781003202035-5

  Looping is a program or script procedure that performs repetitive (or iterative) tasks. The loop is a sequence of instructions (a block) that is executed repeatedly while or until some condition is met or satisfied. Each repetition is called an iteration. All programming and scripting languages provide loop structures, and, like other languages, Python has several looping constructs [28 ].

  Algorithm design is frequently based on iteration and conditional execution.

  5.1 The While Loop

  The output of Program 5.1 (counting.py) counts up to five.

  print (5) print (4) print (3) print (2) print (1)

  Output 5 4 3 2 1

  How would you write the code to count to 10,000? Would you copy, paste, and modify 10,000 printing statements? You could, but that would be impractical! Counting is such a common activity, and computers routinely count up to very large values, that there must be a better way. What we would really like to do is print the value of a variable (call it count), then increment the variable (count += 1), repeating this process until the variable is large enough (count == 5 or maybe count == 10000). Python has two different keywords, while and for , that enable iteration [29 ].

  Program 5.2 (loopcounting.py) uses the keyword while to count up to five:

  count = 1 while count <= 5: print (count) count += 1

  Output 1 2 3 4 5

  The while keyword in Program 5.2 (loopcounting.py) causes the variable ‘count’ to be repeatedly printed then incremented until its value reaches five. The block of statements

  print(count) count += 1

  is executed five times. After each print of the variable count

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  Programming in Python

  Learn the Powerful Object-Oriented Programming

  • Dr. Pooja Sharma(Author)
  • 2018(Publication Date)
  • BPB Publications

    (Publisher)

  Table 4.2 .

  Loop	Description
  while	It iterates a statement or group of statements while a given condition evaluates to true. It evaluates the test expression before executing the loop body.
  for	It executes a sequence of statements multiple times until the test expression evaluates to true.
  Nested loop	A loop either while or for, inside another loop is called nesting of loop.

  Table 4.2. Python loop control structures

  4.2.2. Python while Loop

  The while loop in Python is used to iterate over a block of code as long as the test expression (condition) is true. It is also called a preset loop that is used when it is not predetermined how many times the loop will be iterated. The syntax of while loop is given as follows:

  while test_condition: body of loop

  In while loop, initially, the test expression is evaluated. If the test_expression evaluates to true only then the body of the loop is executed. After one iteration, the test expression is evaluated and tested again for true or false. This process continues until the test_expression evaluates to false. On the other hand, if the test_expression evaluates to false at the first time, the body of loop will not be executed at all and the first statement after the while loop will be executed.

  Fig. 4.4. Flow chart representing while loop

  Indentation determines the body of the while loop in Python. Body starts with indentation and the first unindented line marks the end of the loop. The flow chart of while loop is given in Fig. 4.4 .

  Code 4.7

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  eBook - PDF

  A Student's Guide to Python for Physical Modeling

  Updated Edition

  • Jesse Kinder, Philip Nelson(Authors)
  • 2018(Publication Date)
  • Princeton University Press

    (Publisher)

  C H A P T E R 3 Structure and Control When you come to a fork in the road, take it. — Yogi Berra The previous chapter introduced some useful structures for storing and organizing data. To utilize them effectively, we now need to automate repetitive operations on the data. This chapter describes how to group code into repeated blocks (looping) and contingent blocks (branching), and how to assemble code blocks into reusable computer programs called scripts. 3.1 LOOPS So far, we have described Python as a glorified calculator with some string processing capabilities. However, we can continue to build on what we have learned and do increasingly complex tasks. In this section, we will explore two control structures that will allow us to repeat a set of operations as many times as we need: for loops and while loops. 3.1.1 for loops Instead of solving a quadratic equation, let’s go a step further and create a table of solutions for various values of a , holding b and c fixed. To do this, we will use a loop . Try typing the following in the IPython console: # for_loop.py [get code] b, c = 2, -1 for a in np . arange (-1, 2, 0.3): x = (-b + np . sqrt (b ** 2 -4 * a * c)) / (2 * a) 5 print ( a= {:.4f}, x= {:.4f} . format (a, x)) (Hit on a blank line to terminate the loop and run it.) Note the colon after the for statement. This is essential. It tells Python that the next lines of indented code are to be associated with the for loop. The colon is also used in while loops and if , elif , and else statements, which will be discussed shortly. The keyword for instructs Python to perform a block of code repeatedly. It works as follows: 1. The function np . arange (-1,2,0.3) indicates a series of values with which to execute the indented block of code. Python starts with the first entry and cycles over each value in the array. 2. Python initially assigns a the value -1. Then, it evaluates a solution to the quadratic equation and prints the result.

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  The Python Audio Cookbook

  Recipes for Audio Scripting with Python

  • Alexandros Drymonitis(Author)
  • 2023(Publication Date)
  • Focal Press

    (Publisher)

  2.6 Loops in Python Python has structures that make parts of our code run in a loop, for a specified number of times. In this section we will look at two different ways to run loops in Python. Ingredients: • Python3 • An interactive Python shell Process: There are two different ways to run code in a loop, using the for or the while reserved keywords. Let us include the if test we already wrote, in a for loop. The code is shown in Shell 2.25. Shell 2.25 A for loop. >>> for i in range(10): ... if i == 5: ... continue ... print(i) ... >>> In this script, we have used the for keyword, combined with range(). range(), which is a class rather than a function (more on classes in Chapter 8), takes from one to three arguments. Here we have passed one argument which is the “stop” argument. This means that range() will iterate over a range from value 0, until it reaches [stop – 1], and it will increment by 1 every time. Essentially, when using range() with one argument only, it will iterate as many times as the argument we pass to it, in our case, it will iterate ten times. 26 Writing Your First Python Programs range() returns a list with the values it iterates over. In this code it will return the follow- ing list: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] The for loop will iterate over this list and will assign each of its values to the variable i, every time assigning the next item of the list. Inside the body of our loop we can use this variable for any task we want. This example is very simple and all it does is print on the console the value of i except from the iteration where i is equal to 5. This is achieved by using the reversed keyword continue. This word skips any code of the structure that includes it, that is written below it. So, when our if test is true, the code written below this test will be skipped, and the for loop will continue to the next iteration. Running this script will print the following on the console.

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  eBook - PDF

  Python for Everyone

  • Cay S. Horstmann, Rance D. Necaise(Authors)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  125 C H A P T E R 4 LOOPS C H A P T E R G O A L S To implement while and for loops To hand-trace the execution of a program To become familiar with common loop algorithms To understand nested loops To process strings To use a computer for simulations C H A P T E R C O N T E N T S © photo75/iStockphoto. 4.1 THE WHILE LOOP 126 SYN while Statement 127 CE 1 Don’t Think “Are We There Yet?” 130 CE 2 Infinite Loops 130 CE 3 Off-by-One Errors 131 ST 1 Special Form of the print Function 132 C&S The First Bug 132 4.2 PROBLEM SOLVING: HAND-TRACING 133 4.3 APPLICATION: PROCESSING SENTINEL VALUES 135 ST 2 Processing Sentinel Values with a Boolean Variable 138 ST 3 Redirection of Input and Output 138 4.4 PROBLEM SOLVING: STORYBOARDS 139 4.5 COMMON LOOP ALGORITHMS 141 4.6 THE FOR LOOP 145 SYN for Statement 145 SYN for Statement with range Function 146 PT 1 Count Iterations 148 HT 1 Writing a Loop 149 4.7 NESTED LOOPS 152 WE1 Average Exam Grades 155 WE2 A Grade Distribution Histogram 157 4.8 PROCESSING STRINGS 159 4.9 APPLICATION: RANDOM NUMBERS AND SIMULATIONS 164 WE 3 Graphics: Bull’s Eye 167 4.10 GRAPHICS: DIGITAL IMAGE PROCESSING 169 4.11 PROBLEM SOLVING: SOLVE A SIMPLER PROBLEM FIRST 174 C&S Digital Piracy 180 126 In a loop, a part of a program is repeated over and over, until a specific goal is reached. Loops are important for calculations that require repeated steps and for processing input consisting of many data items. In this chapter, you will learn about loop statements in Python, as well as techniques for writing programs that process input and simulate activities in the real world. 4.1 The while Loop In this section, you will learn about loop statements that repeatedly execute instructions until a goal has been reached. Recall the investment problem from Chapter 1. You put $10,000 into a bank account that earns 5 percent inter- est per year.

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  eBook - PDF

  Introduction to Computing Using Python

  An Application Development Focus

  • Ljubomir Perkovic(Author)
  • 2015(Publication Date)
  • Wiley

    (Publisher)

  When Python runs the for loop, it assigns successive values in to and executes the for every value of . After the has been executed for the last value in , execution resumes with statement that is below the indented block and has the same indentation as the first line of the for loop statement. The for loop, and loops in general, have many uses in programming, and there are different ways to use loops. In this section, we describe several basic loop usage patterns. Loop Pattern: Iteration Loop So far in this book, we have used the for loop to iterate over the items of a list: >>> l = ['cat', 'dog', 'chicken'] >>> for animal in l: print(animal) cat dog chicken We have used it to iterate over the characters of a string: >>> s = 'cupcake' >>> for c in s: if c in 'aeiou': print(c) u a e Iterating through an explicit sequence of values and performing some action on each value represents the simplest usage pattern for a for loop. We call this usage pattern the iteration loop pattern. This is the loop pattern we have used most so far in this book. We include, as our final example of an iteration loop pattern, the code from Chapter 4 that reads 132 Chapter 5 Execution Control Structures a file line by line and prints each line in the interactive shell: >>> infile = open('test.txt', 'r') >>> for line in infile: print(line, end='') In this example, the iteration is not over characters of a string or items of a list but over the lines of the file-like object infile. Even though the container is different, the basic iteration pattern is the same.

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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)

  break statement, then the associated else block associated with it is not executed.

  for element in range(10): if(element*2 == 8): break print(element) print('For loop over')

  When element ==4, element*2 ==8, the break statement breaks the for loop, the control goes to the next statement after the for loop that is, the print statement and it is executed.

  Output: 0 1 2 3 For loop over >>> Note: Continue, break and pass statements inside for and while loops are specific to Python.

  8.6 Recap on control structures

  In Python, control structures are of two types:

  • Branching programs
  • Iterations

  The branching programs can be divided into three parts:

  • A test condition which is an expression that has a true or false output.
  • A block of code that must be executed if a condition evaluates to true .
  • Another block of code that can be executed if the condition evaluates to false . This is optional.

  Once the conditional block has been executed, the control passes on to the statements after the block. The condition statement is of the form: if Boolean_expression: ------------------------ ------------------------ else: ------------------------ ------------------------

  The other form of control structure is known as iteration , which begins with a conditional statement. If the condition evaluates to true, the loop will be executed once and the control goes back to the testing condition to check if it still evaluates to true . If yes, then the loop is executed one more time. This process continues till the condition evaluates to false . When the condition evaluates to false , the control passes on to the next line of code. The for and while

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  eBook - PDF

  Microsoft Visual C#: An Introduction to Object-Oriented Programming

  • Joyce Farrell(Author)
  • 2017(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  C H A P T E R 5 Looping Upon completion of this chapter, you will be able to: • Create loops using the while statement • Create loops using the for statement • Create loops using the do statement • Use nested loops • Accumulate totals • Understand how to improve loop performance • Appreciate looping issues in GUI programs Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 186 C H A P T E R 5 Looping In the previous chapter, you learned how computers make decisions by evaluating Boolean expressions. Looping allows a program to repeat tasks based on the value of a Boolean expression. For example, programs that produce thousands of paychecks or invoices rely on the ability to loop to repeat instructions. Likewise, programs that repeatedly prompt users for a valid credit card number or for the correct answer to a tutorial question run more efficiently with loop structures. In this chapter, you will learn to create loops in C# programs. Computer programs seem smart because of their ability to make decisions; looping makes programs seem powerful. Creating Loops with the while Statement A loop is a structure that allows repeated execution of a block of statements. Within a looping structure, a Boolean expression is evaluated. If it is true , a block of statements called the loop body executes and the Boolean expression is evaluated again. As long as the expression is true , the statements in the loop body continue to execute and the loop-controlling Boolean expression continues to be reevaluated. When the Boolean evaluation is false , the loop ends.

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  Programming Logic and Design, Introductory

  • Joyce Farrell(Author)
  • 2017(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  If you omit any of these actions—initialize, test, alter—or perform them incorrectly, you run the risk of creating an infinite loop. Once your logic enters the body of a structured loop, the entire loop body must execute. Program logic can leave a structured loop only at the evaluation of the loop control variable. Commonly, you can control a loop’s repetitions in one of two ways: • Use a counter to create a definite, counter-controlled loop. • Use a sentinel value to create an indefinite loop. Using a Definite Loop with a Counter Figure 5-3 shows a loop that displays Hello four times. The variable count is the loop control variable. This loop is a definite loop because it executes a definite, predetermined number of times—in this case, four. The loop is a counted loop, or counter-controlled loop, because the program keeps track of the number of loop repetitions by counting them. The false statement is #3. A loop is a structure that repeats actions while some condition continues. TWO TRUTHS & A LIE Appreciating the Advantages of Looping 1. When you use a loop, you can write one set of instructions that operates on multiple, separate sets of data. 2. A major advantage of having a computer perform complicated tasks is the ability to repeat them. 3. A loop is a structure that branches in two logical paths before continuing. 179 Using a Loop Control Variable Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. The loop in Figure 5-3 executes as follows: • The loop control variable, count, is initialized to 0. • The while expression compares count to 4. • The value of count is less than 4, and so the loop body executes. The loop body shown in Figure 5-3 consists of two statements that, in sequence, display Hello and add 1 to count. • The next time the condition count < 4 is evaluated, the value of count is 1, which is still less than 4, so the loop body executes again.

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  eBook - PDF

  Programming Logic & Design, Comprehensive

  • Joyce Farrell(Author)
  • 2017(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  If you omit any of these actions—initialize, test, alter—or perform them incorrectly, you run the risk of creating an infinite loop. Once your logic enters the body of a structured loop, the entire loop body must execute. Program logic can leave a structured loop only at the evaluation of the loop control variable. Commonly, you can control a loop’s repetitions in one of two ways: • Use a counter to create a definite, counter-controlled loop. • Use a sentinel value to create an indefinite loop. Using a Definite Loop with a Counter Figure 5-3 shows a loop that displays Hello four times. The variable count is the loop control variable. This loop is a definite loop because it executes a definite, predetermined number of times—in this case, four. The loop is a counted loop , or counter-controlled loop , because the program keeps track of the number of loop repetitions by counting them. The false statement is #3. A loop is a structure that repeats actions while some condition continues. TWO TRUTHS & A LIE Appreciating the Advantages of Looping 1. When you use a loop, you can write one set of instructions that operates on multiple, separate sets of data. 2. A major advantage of having a computer perform complicated tasks is the ability to repeat them. 3. A loop is a structure that branches in two logical paths before continuing. 179 Using a Loop Control Variable Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-300 The loop in Figure 5-3 executes as follows: • The loop control variable, count , is initialized to 0. • The while expression compares count to 4. • The value of count is less than 4, and so the loop body executes. The loop body shown in Figure 5-3 consists of two statements that, in sequence, display Hello and add 1 to count . • The next time the condition count < 4 is evaluated, the value of count is 1, which is still less than 4, so the loop body executes again.

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