Python while else

3 Key excerpts on "Python while else"

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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)

  if statement can be used. The simplest case occurs when two different alternatives arise depending on the value of a condition, being represented by the pseudo-code below:

  In this case, the statements of the first block (below the if ) are executed when the condition is true, while the statements in the second block (below the else ) are executed otherwise. Note that the else block may not exist if there are no statements to execute if the condition is false.

  If there are more than two alternative blocks of code, several elif (with the meaning else if) branches may exist with additional logical conditions, while a single final else clause exists, for the case when all previous conditions fail. The pseudo-code below represents the case with multiple conditions:

  A condition can be either a Boolean variable, a function or operator returning a Boolean result or an expression including those. A common case is the use of comparison operators, presented above in Section 2.2.5 . An example of Python code where the score of an exam is tested in a cascade of if /elif /else statements is provided next:

  A more compact notation to test the logical value of a numerical variable can be used, by including only the name of the variable in the test condition: if var. The first test will hold true, if var is different from zero. Also, a variable with value None always holds false.

  As an additional example, let us define and test a function to calculate the largest numerical value between two inputs.

  2.3.4 Conditional Loops

  If a statement needs to be executed multiple times (zero or more times) depending on a test condition holding true, the use of a while statement can be appropriate. The pseudo-code of the block of statements within a 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.

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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

  Programming with Python for Social Scientists

  • Phillip D. Brooker(Author)
  • 2019(Publication Date)
  • SAGE Publications Ltd

    (Publisher)

  Reread the correct and incorrect versions of the solution with this in mind, to see if you can identify and understand the difference. One general rule of thumb when constructing conditional logic in this way is always to write the logic in such a way that the most exclusive (i.e. the most difficult to satisfy) condition is checked first. And, in a more general sense, pay attention to the order of your conditions – odd things can happen when you don’t!

  IF/ELIF/ELSE in action

  IF, ELIF and ELSE statements are a crucial part of programming in Python and can be used in lots of different ways. As we’ve seen, they can control the values that are assigned to variables, but they can also be used as “flow control” statements which tell your program to do different things depending on the conditions you establish (and more). We’ll see examples of both of these things below, but one key thing to remember is that the structure of your IF/ELIF/ELSE statements is critical to how they operate. As noted, code runs from line 1 down to the bottom of a script, so the sequential order of your lines of code becomes important here – it’s generally a good idea to plan out the logic of your IF/ELIF/ELSE statements before writing the code, so as to prevent against building these kinds of “bugs” into your program.

  Controlling the flow of the program (and planning code with “pseudocode”). Sometimes programs will have lots of bits of code which do lots of different things, but you may not want to just run the code from line 1 to the end each time. Hence, using IF/ELIF/ELSE statements as a way to control the flow of your program (in conjunction with “functions”, which you will learn about later in the book), you can tell Python which jobs you want to perform and which to bypass.

  For instance, suppose you want to do some web scraping on a particular webpage – perhaps you want to look at an influential political commentary blog, and scrape once a day for any new content that might have been posted. So, you already have some metadata – the date of the last time you attempted to scrape the blog, and the dataset of blog content you have already scraped (sorted by date). Using just these bits of information, you could use an IF/ELSE statement to check to see if the blog has new content, and, if it does, add that content to your existing blog dataset:

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