Python Loops

9 Key excerpts on "Python Loops"

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  eBook - ePub

  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)

  while statement can be considered a loop. We'll look at looping structures next.

  Loops

  In Python, looks (just as in any other language) are a way to execute a specific block of code several times. In particular, loops are used to iterate or loop over what we call iterables. For the purposes of this chapter, we can define an iterable as follows:

  • Anything that can be looped over (that is, you can loop over a string or a file)
  • Anything that can appear on the right-hand side of a for loop, for example, for x in iterable

  A few examples of common iterables include the following:

  • Strings
  • Lists
  • Dictionaries
  • Files

  You can think of an iterable as a collection of homogeneous things that have been grouped together to form a large collective. The individuals in the group have the same properties, and when they are combined, they form something new.

  Consider the example of cars in a car yard. We can consider the car yard as the collection or iterable and the individual cars as the constituent members of the car yard. If you were shopping for a car, you would probably have a couple of qualities that you are looking for. You walk into the car yard and start going from car to car looking for one that satisfies your conditions or comes as close as possible to satisfying your conditions. The act of going from car to car and repeating the aforementioned inspection operation is basically what looping is.

  Loops allow us to deconstruct iterables and perform operations on their constituent members or even convert them into new data structures. The possibilities are endless once you start using loops.

  The for Loop

  The for loop in Python is also referred to as the for…in loop. This is due to its unique syntax that differs a bit from for loops in other languages.

  A for

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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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  eBook - ePub

  Python Made Simple

  Learn Python programming in easy steps with examples

  • Rydhm Beri(Author)
  • 2019(Publication Date)
  • BPB Publications

    (Publisher)

  if…else statement that is in the else block. If the condition evaluates to true, then the second number is largest will be displayed; otherwise, the third number is largest displayed. After printing appropriate messages, it prints the end of the program message.

  Looping statements

  Looping statements are also known as iteration control statements. An iterative control statement is a control statement that provides the repeated execution of a set of instructions. With decision making statements, we can make decisions on which statement(s) should be executed when the condition is true or false. Sometimes, there is a need to execute the set of statements repeatedly until a certain condition is true. So, looping statements provide the facility to execute the set of statements repeatedly until the condition becomes false. There are mainly two types of looping statements, which are explained in the following sections.

  The while loop

  The while loop is the most basic looping statement available in Python. It allows you to execute the set of statements repeatedly as long as the condition evaluates to true. It is mostly used in those cases where the programmer does not know in advance how many times the loop will be executed. The general form of the while loop is as follows:

  while condition:    statement(s)

  In the preceding representation, the condition is that any valid Python test expression that may use a relational or logical operator which must be followed by a colon (: ) symbol. Here, the statement(s) represent one statement or group of statements. In the while loop, first the test condition is checked and if it evaluates to true, then the statements contained in the block of the loop are executed. The execution continues as long as the condition remains true. When the condition becomes false, the while statement ends and the control moves to the next statement in the program.

  The while loop is also called entry-controlled loop because the condition is checked before the start of execution of the loop. The decision regarding whether the body of the loop is executed or not depends on the test condition which is evaluated at the entry point of the while loop. The following flowchart shows the flow of the while

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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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  Introduction to Engineering and Scientific Computing with Python

  • David E. Clough, Steven C. Chapra(Authors)
  • 2022(Publication Date)
  • CRC Press

    (Publisher)

  mid-test loop. Depending on when the end condition is met, there may be 1, 10, 100, or 1,000 repetitions.

  The second loop scenarios are illustrated in Figure 4.4 where the number of loop repetitions is known ahead of the loop being entered. The first is the list-driven structure, and this is an important feature of Python1 . It is based on a list of items where the loop is repeated for each item in the list in sequence.

  The second is the count-controlled loop, a structure that has been around since the earliest programming languages. This loop is characterized by three parameters: a start value, end value, and step value. After each loop iteration, the step value is added to the loop counter, and, unless the end value is equaled or exceeded, the loop is repeated. In this way, it is possible to calculate ahead of loop entry how many times the loop will be repeated. In fact, once the loop is being executed, modifying the step and stop values will not affect the number of repetitions, since they are precalculated.

  In a following section of this chapter, you will learn how to implement the repetition structures in Python. It will be important to reference the figures shown here to keep in mind a good picture of the structures. Also, you will learn some variations and combinations of the structures. This will also be the case for the selection structures.

  4.2 Implementing Decision Structures with Python

  Learning the general programming structures is important because it prepares you to implement these with different programming languages. You can approach a new language, such as Python, with a question, “Since I already know what a Two-way If is, how do I implement a Two-way If in Python?” In other words, once you learn a procedural programming language like Python, you are never starting from scratch.

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  Machine Learning for Decision Sciences with Case Studies in Python

  • S. Sumathi, Suresh Rajappa, L Ashok Kumar, Surekha Paneerselvam(Authors)
  • 2022(Publication Date)
  • CRC Press

    (Publisher)

  Print (i);
• While loop: Repeats the group of a statement if the given condition is True. For example, add the first five natural numbers.
  I = 0; While y<6: y = y+y I++;
• Nested loop: To use the loop inside another loop is called a nested loop. Let’s take an example of distributing four chocolates to each student of the class. The nested loop will work here, as we have to use two loops, that is, for chocolate and students. chocolate = [“Cadbury”, “kitkat” “ mars”, “ galaxy”]; students = [“Alice”, “ Jhon”, “ Nick” ]; For students in students : For chocolate in chocolate : Print (students + “got” + chocolate);

2.4.3.3 Loop Control Statements

To change the sequence of executing a code, Python provides multiple control statements. Following are the loop control statements:

1. Break statement: To exit from a loop, break statements are used. Let’s take the example of 20 enrollment numbers, and the user wants to print only ten enrollment numbers. x=1; While x< = 20: Ifx ==10: break Print (x) x++;
2. Continue statement: It is similar to the break statement, it exits the loop, but the loop itself is not exited. Let’s consider the same previous example and replace the break with continue. x = 1; While x < = 20: if x ==10: continue Print (x) x++;
3. Pass statement: It is also called a null operator. Nothing happens when it is executed. Consider a project that two programmers complete; one falls sick and cannot come on a specific day. The problem is that another programmer cannot hold the work. In this case, all the functions that the sick programmer wrote will be left empty using the pass statement. Let’s consider the same previous example with a pass statement. x=1; While x < = 20: if x ==10: pass print “10 not included” Print (x) x++;