{"id":125,"date":"2021-06-28T19:59:28","date_gmt":"2021-06-28T19:59:28","guid":{"rendered":"http:\/\/juthon.com\/?p=125"},"modified":"2021-06-29T12:29:12","modified_gmt":"2021-06-29T12:29:12","slug":"passage-5-python-loops","status":"publish","type":"post","link":"http:\/\/juthon.com\/index.php\/2021\/06\/28\/passage-5-python-loops\/","title":{"rendered":"Passage 5: Python Loops"},"content":{"rendered":"\n

We code in Python<\/em><\/p>\n\n\n\n

Hello! How are you. Today's passage is devoted to loops. We'll start with the while loop<\/em>. In programming, loops are used to repeat the same instruction multiple times. We would not be exaggerating when we say that loops are one of the most important elements of programming.<\/p>\n\n\n\n

What are we going to start from? Maybe a little warm-up? Let's write a short applet that will convert the temperature in Celsius degrees to the temperature in kelvins. As you probably remember from physics lessons, the Kelvin scale is shifted 273 units up from the Celsius scale. So 0\u00b0 C is 273 K, 10 \u00b0 C is 283 K etc. Here is our code:<\/p>\n\n\n

a = int (input ('Enter the temperature in Celsius degrees:'))
b = a + 273
print ('You entered the temperature ', a, '\u00b0C', sep = '')
print ('In kelvins this temperature is:', b)
<\/p>\n\n\n

The program is simple, the syntax of all commands is known to us from previous passages of the series[1]<\/a>.<\/p>\n\n\n\n

In the same physics lessons, you should also remember that the temperature of matter cannot be lower than 0 K (absolute zero). This is the temperature corresponding to \u2011273\u00b0 C. We will improve our code by adding a conditional instruction that responds with the message \"Incorrect value\" to the user entering a temperature lower than \u2011273\u00b0 C<\/p>\n\n\n

a = int (input ('Enter the temperature in Celsius degrees:'))
if a <-273:
    print ('Invalid value')
else:
    b = a + 273
    print ('You entered the temperature', a, '\u00b0 C', sep = '')
    print ('In kelvins this temperature is:', b)

<\/p>\n\n\n

Run the above script. Works flawlessly. And yet\u2026 We can't quite be satisfied, can we? When the user enters an incorrect temperature value, the program will scold him and exit. Is this really how a decent program is supposed to work? Of course not. The program is not allowed to shut down after the user gives the wrong answer, but it should ask the user again and again for the correct answer. Until the proper reaction!<\/p>\n\n\n

We get to know the loops<\/h2>\n\n\n

The described situation is the bread and butter of programming. We need the program to repeat an action many times. We will achieve this by using loops.<\/p>\n\n\n\n

The while loop (\"do something while \u2026\") is very similar in its construction to the if conditional, with the difference that as after if<\/em> keyword Python would execute the given statements once, after the while<\/em> keyword Python would execute them \u2014 as they say \u2014 \"in circle \"until the condition is no longer met.<\/p>\n\n\n\n

In our example, the condition is: giving an \"incorrect\" temperature by the user. If the user persists and enters incorrect temperatures again and again, the program will stubbornly inform him that he is making a mistake and ask for the temperature again. The \"game\" will continue until the user finally enters a temperature higher than \u2011273\u00b0 C.<\/p>\n\n\n\n

Let's write the appropriate code.<\/p>\n\n\n

a = int (input ('Enter the temperature in Celsius degrees:'))
while a<-273:
    print ('Invalid value')
    a = int(input('Enter the temperature in Celsius degrees:'))
else:
    b = a+273
    print ('You entered the temperature', a, '\u00b0C', sep = '')
    print ('In kelvins this temperature is:', b)

<\/p>\n\n\n

The script is not much different than the previous one, based on if-conditional-instruction<\/em>. It was enough to replace the word \"if\" with the word \"while\" \u2014 one might say. Almost. In addition to this change, we had to \u2014 inside the while loop \u2014 repeat the first line of code, the one with the input command. Why? It is quite obvious. To have Python repeat the execution of this line of code multiple times. The statements used after the word while \u2014 and indented by four spaces \u2014 are the body of the loop and are repeatedly executed.<\/p>\n\n\n

While loop construction<\/h2>\n\n\n

A while loop consists of two parts: the condition and the loop body. The body of the loop is a set (sequence) of commands. The loop executes the sequence of commands multiple times until the given condition becomes false.<\/p>\n\n\n\n

<\/figure>\n\n\n\n

Additionally (as can be seen in the diagram above) there is an alternative part that starts with the else. This alternative part \u2014 in the case of the while loop \u2013 shows a little trouble, but more on that in a moment. The while loop is executed (potentially) an infinite number of times as long as Python determines that the given condition is True. When the condition becomes False, the loop ends its work (the programme \"exits\" the loop).<\/p>\n\n\n\n

The commands specified after the else<\/em> are executed once when exiting the loop.<\/p>\n\n\n\n

The variable referenced by the while loop must first be defined in the program (before opening the loop) so that it can be reused inside the loop later. Hence \u2014 in our example, the line a = int (input ('Enter temperature in degrees Celsius:')) appears twice. First in front of the loop, then inside it. We will come back to this issue \u2026<\/p>\n\n\n

An example of using the while loop<\/h2>\n\n\n

Let's illustrate a while loop with the simplest possible example, shorter and perhaps easier than our previous example converting from Celsius to Kelvin.<\/p>\n\n\n\n

This example prints the value of the variable <\/em>i, which is initially zero, and increments by 1 each time the loop is run. The loop runs as long as i is less than 5<\/p>\n\n\n

i = 0
while i <5:
    print ('number:', i)
    i = i + 1

Output:
number: 0
number: 1
number: 2
number: 3
number: 4
<\/p>\n\n\n

From this example it is clear that the loop \u2014 in its first cycle \u2014 refers to the value of the variable given before the loop (i = 0) and then reaches for the updated values (after each loop) with the code line i = i + 1, i.e. \u2014 otherwise speaking \u2013 the values increased by 1 for each circuit. This is the idea behind the while loop: check the initial value in the code before the loop; then update this value and check again. The loop continues to generate print<\/em> until the value of i <5. Therefore, output (understood as a printout on the screen) covers the numbers from 0 to 4.<\/p>\n\n\n\n

Note, however, that within the last cycle of the loop \u2014 after generating the string \"number: 4\" \u2014  the value of the variable is increased once again. Finally, the variable i has the value 5.
If you changed the order of the lines in the code and make your script like this:<\/p>\n\n\n

i=0
while i<5:
    i=i+1
    print('number:', i)
   
your loop will generate this  output:
number: 1
number: 2
number: 3
number: 4
number: 5<\/p>\n\n\n

The advantage is that the last printed value of the variable is equal to the current value calculated by the program. The disadvantage, however, boils down to the fact that we do not meet the condition: \"print values until they are less than 5\". Which version of the program is better? There is no answer to that. It is important that you write code with careful awareness of the result of running it.<\/p>\n\n\n

Exiting the loop<\/h2>\n\n\n

As we remember, when exiting of the loop (the value of condition = False), Python will execute the block indicated by the word else  (the commands following the else keyword and indented by four spaces). Let's extend our program so that \u2014 when we exit the loop \u2014 we print the message \"Good bye!\"<\/p>\n\n\n


i=0
while i < 5:
    print('number:', i)
    i=i+1
else:
    print('Good bye!')

print('And now the rest of the programme.')

Output:
number: 0
number: 1
number: 2
number: 3
number: 4
Good bye!
And now the rest of the programme.<\/p>\n\n\n

The program executes the loop five times, then executes the alternative code (else), and finally proceeds to execute the commands outside the loop \u2014 in the main code (the line \"And now the rest of the programme.\").<\/p>\n\n\n

Exercise<\/h3>\n\n\n

Build a script that converts temperature from Celsius to Kelvin. Add a code to count the number of times the user entered an incorrect temperature value. Write down the command, which prints the message at the end of the script: \"You have entered the wrong temperature i times.\" \u2014 where, of course, in place of the letter \"i\", the actual number of incorrect passes will appear.<\/p>\n\n\n

The Riddle<\/h3>\n\n\n

While browsing (even on the internet) various examples of the use of the while loop in Python, we can find scripts very similar to our script that prints consecutive numbers from 0 to 4.<\/p>\n\n\n

i = 0
while i <5:
    print ('number:', i)
    i = i + 1
print ('Good bye!')
print ('And now the rest of the programme.')<\/p>\n\n

Output:<\/p>\n\n

number: 0
number: 1
number: 2
number: 3
number: 4
Good bye!<\/p>\n\n

And now the rest of the programme.<\/p>\n\n\n

Similar but not the same example as before. The code is slightly different from the previous one, right? The else is missing. The command that followed the else \u2014 and was indented by four spaces \u2014 is now in the main code. The output, however, is the same as before.<\/p>\n\n\n\n

Since the output is identical with \"else\" and without \"else\", does it mean that \u2014 in the case of a while loop \u2014 the else is redundant? The answer is yes. We don't have to use the else, and we can simply place the commands (we want to execute when exiting the loop) in the main code[2]<\/a>.<\/p>\n\n\n\n

Let's ask another question: and is the same simplification \u2013 that is, placing alternate statements directly in the main code \u2013 also applicable to the if statement? The answer is: definitely not. But why exactly? After all, while and if seemed so logically similar to each other \u2026<\/p>\n\n\n\n

Well, only \"seemed\" \u2013 unfortunately. Let's try to unravel this mystery.
<\/p>\n\n\n

Where is the difference<\/h3>\n\n\n

At the beginning of our course, we emphasised that learning programming develops your ability to think logically. Time to check it out \"in action\". Showing that the else has a different role for the if statement than for the while statement is great training for \"gray cells\" of our brains.<\/p>\n\n\n\n

Let's start by drawing a simple diagram. When Python already executed either a conditional if statement or a while loop, it then proceeds to main code statements.<\/p>\n\n\n\n

<\/figure>\n\n\n\n

Execution \u2014 be it a conditional statement or a while loop \u2014 always starts with checking the condition. Python checks to see if, given the current value of a variable, the condition is True or False. We remember this rule, right? The crux of the difference is that in the case of a conditional statement, it is possible to move to subsequent commands in the main code both when the condition is True or False. However, in the case of a loop, the transition to the subsequent commands of the main code is possible only when the condition is False. When the condition is True, Python doesn't reach for any more lines of code. It is spinning in circles (in a loop), that is, it updates the value of the variable and checks whether the condition is True again, and updates again and checks again and again and again.<\/p>\n\n\n\n

If the condition were always True, we would loop forever.<\/p>\n\n\n\n

Since \u2014 in the case of a while loop \u2014 the transition to subsequent commands of the main code is possible only when the condition is False, the program must<\/strong> execute the else part of the code (because, by definition, the alternate part of the code is executed when the condition is False).<\/p>\n\n\n\n

<\/figure>\n\n\n\n

In case of the while loop, the program proceeds to \"subsequent lines of the main code\" only if the condition's value becomes False.<\/strong><\/p>\n\n\n\n

And what is the consequence of taking the value False by the condition? Then the alternative command is executed (the code fragment after the word else).<\/p>\n\n\n\n

What is the lesson from this? In the case of a while loop (unlike the case of the if statement), it is impossible to skip the execution of an alternate command. It is impossible to exit the loop with condition value = True, that is without executing the else part. In programming, the role of the else keyword is to allow the part of code that follows this keyword (and is indented by four spaces) to be omitted<\/strong>, under a certain condition. Since such omission is not possible, in the case of a while loop the else becomes redundant. The commands we want to execute can be written directly in the main code.<\/p>\n\n\n\n

In short, the alternative command is guaranteed to be executed. And since the alternative command will definitely be executed, why write \"else\" in front of it? So let's repeat: using else keyword with a while loop doesn't make much sense. Most programmers omit the else and simply write the set of commands \u2014 to be executed when they exit the loop \u2014 in the main code.<\/p>\n\n\n\n

<\/figure>\n\n\n

Exercise<\/h3>\n\n\n

Write a short program that asks the user the question: 'Do you like candy? y \/ n ' Use the if\u2026 else\u2026  conditional statement, which will differentiate the program's response depending on the typing of the letter y or n by the user. For example, when the user presses y, the program will print the message: 'You like candy', and when the user presses n, the message: 'You don't like candy'. Run program and select y. Restart and select n. Observe the effect.<\/p>\n\n\n\n

Then make changes to the script, by removing the else keyword, and placing the alternative command<\/em> in the main code. Run program and select y. Restart and select n. Observe the effect.<\/p>\n\n\n\n

Now write a new script that will use a while\u2026 else\u2026 loop, instead of the if\u2026 else\u2026 conditional. Run it and make similar test. Make the same changes as before and test again.<\/p>\n\n\n

Summary<\/h2>\n\n\n

From today's passage, remember:<\/p>\n\n\n\n