Python Loops

5 Key excerpts on "Python Loops"

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  eBook - ePub

  Hands on Data Science for Biologists Using Python

  • Yasha Hasija, Rajkumar Chakraborty(Authors)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  Computers are frequently used to automate repetitive tasks. Since iteration is frequent in programming, Python has several options to make this easier. The while loop is one form of iteration in Python.

  While loop

  The syntax of a while loop is shown in the figure.

  Figure 2.7     Syntax of a python while loop.

  Loops in Python have a block of statements (recall how Python blocks are defined) which will be repeatedly executed until this meets a stop condition. In the syntax of the while loop, we see a condition which is called a stop condition. The modus operandi of a “while” loop is that the block of statements under the “while” statement will be executed until the stop condition is encountered.

  A simple example of a “while” loop is printing numbers from 0 to 5, as illustrated below:

  Code:

  # Simple while loop a = 0 while a < 6: print(a) a = a + 1

  Output:

  0 1 2 3 4 5

  In the snippet of code above, we can observe that a variable “a” is initiated with value 0. Second, we have a “while” statement where the stop condition is “a” less than 6. The block under the “while” statement contains two statements - one for printing “a” and the other adds 1 increment to “a”. When the block is first executed, the value of “a” is printed, and the same is increased from 0 to 1. Before rerunning the block, the stop condition of the while statement is checked. As “a” = 1 is still less than 6, the block will continue to run again. This cycle of repetitive execution is called a loop execution or iteration, and it stops when “a” will be equal to 6, and, hence, the condition of the loop fails. If we do not establish the stop condition or if we set in a condition that is never going to be false, then the loop becomes an infinite loop and will be executed countless times. To stop this, we have to restart the Python kernel in the Jupyter Notebook. If the asterisk on the left side of the cell persists for a long time, then this may imply that we may have entered an infinite loop. Let us see another example of the “while” loop for finding the longest DNA sequence below:

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  eBook - ePub

  Bioinformatics Algorithms

  Design and Implementation in Python

  • Miguel Rocha, Pedro G. Ferreira(Authors)
  • 2018(Publication Date)
  • Academic Press

    (Publisher)

  while cycle is the following:

  In this case, the statements in the block will be executed while the condition holds true. When the condition switches to false, the loop ends, and the program flow follows with the next statement. This means that, to avoid infinite cycles, the programmer needs to insure that the condition will be false at some point in the program execution.

  In the following example, the value of the variable a is printed, while it is smaller than 100. At each iteration its value is incremented by 10, thus insuring the cycle terminates:

  As an illustration of the potential of while cycles, in the next example, we develop a function that searches if a given element is present in a list of numbers. The function returns the position of the first occurrence of that element, or −1 if the element does not occur in the list.

  2.3.5 Iterative Loop Statements

  If we know in advance that we need to execute a block of statements a fixed number of times, a for loop can be used. This control structure provides an iterative loop through all the elements of an iterator, which can be retrieved from a container variable or using a function that yields these values. An iterable object is any object that can be iterated over, i.e. that provides a mechanism to go through all its elements. Strings or lists are examples of such objects. These objects are particularly suitable to be iterated in for loops. Indeed, iteration through a range of values is one of the most common tasks in programming.

  In the following example, the code iterates through all the characters in a string and increments the value of the variable seq_len for each of them, obtaining in the end the length of the string.

  There are also functions that return iterators, which can be used directly in these loops. Python offers a function range to generate an immutable sequence of integers between a start and a stop value, with an increment step value. The general syntax

  range ( [ start , ] , stop , [ , step ] )

  allows a more compact notation where only the stop value needs to be provided. In that case, the start value is assumed to be zero and step to be one. By considering a step with a negative value, sequences of decreasing values can be generated. Note that in the generated sequence of values, the stop

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  Coding + Math

  Strengthen K–5 Math Skills With Computer Science

  • Nicol R. Howard(Author)
  • 2020(Publication Date)
  • International Society for Technology in Education

    (Publisher)

  Figure 1.2 ). This area has been investigated widely, as examined in twenty-four studies in the Zhang and Nouri (2019) review, and there was evidence of improved ability to use conditionals and control the flow of programs in the Scratch environment. It is possible that a programming context is better suited for teaching conditionals because it is an applied use of the concept to accomplish a programmatic goal. However, conditionals can also be modeled effectively through the use of flowcharts or unplugged programming activities.

  Loops

  Loops are a mainstay of programming, allowing far more efficiency in programming tasks that must be repeated until a desired result is achieved. For example, if a programmer wants the program to perform a task repeatedly a certain number of times but wants the user to determine that number at runtime, the code would utilize a “for-loop”, accepting user input to set the number of intervals. In Scratch terminology, the coder could use an “ask” block to get user input on how many times the sprite should follow a certain sequence. In that case, the code would contain a “repeat” block to encapsulate the loop containing the blocks in the sequence. This not only prevents the need for repetitive code, but also avoids restricting the loop to some predetermined maximum number of iterations. While loops provide additional power and flexibility to the programmer, they also present a source of potential errors if not used properly.

  Research has shown the use of endless loops in the Scratch environment as a source of many programming errors, where programs get stuck in never-ending loops (Zhang & Nouri, 2019). Varied understanding of different types of loops, particularly nested loops, have often caused confusion, but this confusion has been less prevalent for Scratch users as compared to text-based languages, since loops are easier to identify in a block-based environment (Mladenovi et al., 2017). Perhaps the best analogy for loops in math is summation notation (∑ 2i , e.g., meaning the summation of all values of 2i , starting with 1 and ending with 10); this concept is usually not covered until secondary school. Flowcharting, which will be covered in chapter 4

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  eBook - ePub

  TradeStation Made Easy!

  Using EasyLanguage to Build Profits with the World's Most Popular Trading Software

  • Sunny J. Harris(Author)
  • 2011(Publication Date)
  • Wiley

    (Publisher)

  Chapter 11 Loops In This Chapter

  • Introduction
  • For
  • While

  INTRODUCTION

  Aloop is just as it sounds; it goes around and around until you tell it to stop. A loop defines a group of instructions that will continue executing until a condition is met. The condition being met will cause the loop to stop.

  A FOR loop will execute a predefined number of times. The predefined number of times is the condition. A WHILE loop will continue to execute while (as long as) the condition is true.

  Loops allow you to calculate something over a range of values, such as from 1 to 10. Or to take action only while a condition is met, such as: while it is the month of October, only take short trades. Loops allow you to calculate averages, and summations and even count backward.

  As your programming skills become more proficient, you will find yourself discovering more and more uses for loops. FOR TEMPLATE 1

  FOR Value1 = m TO n BEGIN … END;

  TEMPLATE 2

  FOR Value1 = m DOWNTO n BEGIN … END;

  The FOR command always appears with a BEGIN … END block and is sometimes referred to as a FOR … BEGIN command. The FOR command is always followed by a set of variables that define the starting and ending number of steps to execute the loop. This structure is pretty complex until you get accustomed to it; then it will become second nature, so don’t start worrying yet. Templates for these concepts are shown above. You will specify m and n, and provide code for your task in place of the ... s.

  FOR is always followed by a variable name (Value1 in Figure 11.1

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  Hack Audio

  An Introduction to Computer Programming and Digital Signal Processing in MATLAB

  • Eric Tarr(Author)
  • 2018(Publication Date)
  • Routledge

    (Publisher)

  while loop.

  Examples: Create and run the following m-file in MATLAB.

  % This script compares the process of using a for loop  % with the process of using a while loop. The same  % result can be accomplished with each type of loop by using  % the appropriate syntax.    % for loop example 

  N = 4;               % Number of elements for signal 

  sig1 = zeros(N,1);   % Initialize an output signal 

  for ii = 1:N 

      sig1(ii) = sin(ii*pi/2);  % Fill the output array 

                                % with values from the 

  end                           % sine function 

  sig1                 % Display result on Command Window 

    % while loop example 

  sig2 = zeros(N,1);   % Initialize new output signal 

  jj = 1;              % Declare a counting variable 

  while jj ≤ N 

      sig2(jj) = sin(jj*pi/2);  % Fill the output array 

      jj = jj + 1;              % Iterate counting variable 

  end 

  sig2                 % Display result on Command Window

  Example 5.13: Script: simpleLoop.m

  In audio signal processing, for loops are a common way to iteratively step through each sample of a signal. For many signals in MATLAB, the number of samples can usually be determined. Then, the loop can be set to index through the individual samples or segments of the entire signal.

  5.3.3 Functions

  Functions are a set of commands grouped together and executed each instance the function is called. MATLAB has many built-in functions including sqrt , sin , cos , ceil , and so on. Functions may have input and output variables, but they are only necessary if required by the particular function. Input variables to functions are enclosed in parantheses after the function name. Multiple input variables are separated by commas. Output variables from functions are enclosed in square brackets to the left of an equal sign and function name. Multiple output variables are separated by commas.

  Examples:

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