{"id":115,"date":"2021-05-22T16:18:08","date_gmt":"2021-05-22T16:18:08","guid":{"rendered":"http:\/\/juthon.com\/?p=115"},"modified":"2021-05-23T14:58:57","modified_gmt":"2021-05-23T14:58:57","slug":"passage-4-conditional-instruction","status":"publish","type":"post","link":"http:\/\/juthon.com\/index.php\/2021\/05\/22\/passage-4-conditional-instruction\/","title":{"rendered":"Passage 4: Conditional instruction"},"content":{"rendered":"\n

Very often when you write code, you want to perform different actions for different decisions. In programming this is sometimes called variation<\/em>, but more often decision making<\/em>, because it allows the execution of various code fragments only when a certain condition is met (when the user makes a certain decision).<\/p>\n\n\n\n

We will use conditional instructions to perform decision making.<\/p>\n\n\n\n

Jump to Polish version<\/a><\/p>\n\n\n\n

The simplest example of decision making is the check box in the program: \"Continue? y \/ n\". The program only continues when the user presses the letter y.<\/p>\n\n\n\n

It is difficult to imagine modern programming without conditional instructions, which allow the user to make choice<\/em>, and thus give him a real possibility of influencing the \"program flow\" (control flow). So\u2026 let's learn conditional statements!<\/p>\n\n\n

Introduction<\/h2>\n\n\n

If you've been following earlier passages of our series, you already know how to write a program that makes a short dialogue with the user: the program will ask the user for a name, remember it and reply with the greeting: Have a nice day, adding a name it remembered.<\/p>\n\n\n\n

Let's try to expand our program to further ask the user for age. And let's set ourselves an ambitious task. We vary the response of the program depending on the given age.<\/p>\n\n\n\n

Say you are building a program that \"gives a prize\" (whatever that prize might be) that is different for the older person and different for the younger person. If the user is 16 years old or older, the program generates the message: \"You are receiving the X award\". Otherwise (if the user is under 16), the program generates the message: \"You are receiving the Y award\".<\/p>\n\n\n\n

As you can see, we use the word if. The word if is a keyword in the Python programming language. In other words, a word that belongs to the syntax of the language and allows you to build the command structure. With the if keyword, we create a conditional statement in Python.<\/p>\n\n\n\n

Conditional statement \u2014 this is the topic of today's passage of our series. The conditional statement really isn't anything difficult. You will find out about it in a moment. In the future, you will use it hundreds and thousands of times without any problem.<\/p>\n\n\n

The if \u2014 else conditional<\/h2>\n\n\n

The if\u2014else conditional consists of a condition<\/em> and a command<\/em> that is executed when the condition fulfills, and a second command<\/em> that is executed when the condition fails.<\/p>\n\n\n

Conditions<\/h3>\n\n\n

A condition that \"fulfills\" is said to have a value: True.<\/p>\n\n\n\n

A condition that \"fails\"  is said to have a value:  False.<\/p>\n\n\n\n

We usually build conditions using equal (==), greater (>), less (<) operators. Python also defines not equal (! =), no-less (> =), and no-greater (<=) operators.<\/p>\n\n\n\n

An example of checking the truthfulness of the condition (based on a short program with the input command):<\/p>\n\n\n

print ('Enter number a:')
a=input()
print ('Enter number b:')
b=input()

<\/p>\n\n\n

Let's evaluate the truthfulness of the condition a>b<\/strong><\/p>\n\n\n\n

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

Let's be honest: it is not complicated at all, right?<\/p>\n\n\n

Commands<\/h3>\n\n\n

Let's move on to the commands that are executed after the program checks the condition.
We have nothing special to say about these commands\u2026<\/p>\n\n\n\n

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

\u2026They can be any command you wish, be it actions (e.g. order to print a message on the screen) or calculations.<\/p>\n\n\n

Conditional statement (i.e. condition + commands)<\/h3>\n\n\n

Now comes the time to combine the condition with the commands and show what the conditional statement looks like:<\/p>\n\n\n\n

if condition is True:
    statement 1
else:
    statement 2

<\/strong><\/strong><\/p>\n\n\n\n

Translating into human language we read this: if condition is True, execute statement 1, otherwise, execute statement 2.<\/p>\n\n\n

What should you pay attention to?<\/h4>\n\n\n

Note the obligatory colon at the end of the first line (the one with the condition) and after the else. Also pay attention to the so-called indentation. Commands following the colon are indented four spaces from the preceding line.<\/p>\n\n\n\n

Indentation is a very important part of Python syntax. They define the so-called code blocks. More about this in a moment.<\/p>\n\n\n\n

Let's write the first program using the conditional statement:<\/p>\n\n\n

print('Enter number a:')
a=input()
print ('Enter number b:')
b=input()
if a>b:
    print ('The number a is greater.')
else:
    print ('The number b is greater or the numbers are equal.')
<\/p>\n\n\n

Conditional instructions are not difficult, the above program is almost intuitive to read, right? The program asks for two numbers: a and b \u2014 and remembers their value. Then we tell the program: if you find that the number a is greater than b, write \"The number a is greater\", otherwise (else) write \"The number b is greater or the numbers are equal\".<\/p>\n\n\n\n

OK. Maybe it's time to try to write something yourself? How about a little training?<\/p>\n\n\n\n

Exercise 1<\/strong>. Try to write a short script that asks the user to enter two numbers (a and b). When these numbers are equal, the program will display the message: \"The numbers are equal\". Otherwise, the program will display the message: \"The numbers are not equal\".<\/p>\n\n\n\n

Exercise 2<\/strong>. Write a short script asking the user to enter a color. The preferred color is white. Therefore, when the user enters the name \"white\", your program has to write \"Great, you guessed it!\" Otherwise, the program has to write: \"Not that color.\"<\/p>\n\n\n\n

You will find the answers to these two exercises at the end of the article.<\/p>\n\n\n

Additional note:<\/h3>\n\n\n

The number of statements you put after a condition (after the line: \"if condition \u2026\") is by no means limited to one. If your program requires it, you can indicate two or three (or more) commands to be executed when the condition is true. The same goes for commands after the word else. For example, if you want to give four statements after \"if\" and three statements after the else, the schema of the conditional statement would look like this:<\/p>\n\n\n\n

if condition is True:<\/p>\n\n\n\n

    statement 1<\/p>\n\n\n\n

    statement 2<\/p>\n\n\n\n

    statement 3<\/p>\n\n\n\n

    statement 4<\/p>\n\n\n\n

else:<\/p>\n\n\n\n

    statement 5<\/p>\n\n\n\n

    statement 6<\/p>\n\n\n\n

    statement 7<\/p>\n\n\n\n

<\/p>\n\n\n\n

<\/p>\n\n\n

We are expanding our program from previous passages<\/h3>\n\n\n

Let us remind you that we want to expand our program from previous episodes, which asked the user for a name and then greeted him with that name. Now the program will additionally ask the user for age and will react differently depending on the answer given:<\/p>\n\n\n\n

\u2022 if the user specifies the age of 16+, the program will generate the message: \"You are receiving the X award\".<\/p>\n\n\n\n

\u2022 otherwise, the program will generate the message: \"You are receiving the Y award\".<\/p>\n\n\n\n

In order to obtain the answer in two variants, we will of course use the if \u2014 else statement:<\/p>\n\n\n

print ('Enter your name, please:')
a=input ()
print ('How old are you?')
age=int(input())
if age>=16:
    print ('You are receiving the X award.')
else:
    print ('You are receiving the Y award.')
print ('Have a nice day,' + a + '.')

<\/p>\n\n\n

Let's take a look at the script above. The first two lines ask for the first name and remember the user's answer. The answer is stored in the variable a<\/em>. The next two lines are a request for age and (as before) \u2014 remembering the user's answer. The answer is stored in the age<\/em> variable. Since we want the age to be stored as a number, we have wrapped the function input() with the function int(), which converts the text to a number (see Appendix to Passage 2).<\/p>\n\n\n\n

Next we implement the if \u2014 else conditional statement. The commands after \"if\" and after \"else\" are indented by four spaces. The last line of code refers to the variable a<\/em> (which holds the name) and prints the Have a nice day [name]<\/em> greeting.<\/p>\n\n\n

Is the else necessary?<\/h3>\n\n\n

You may be wondering if the \"second part of the conditional\", that is, the piece of code that starts with \"else\", is always \u2014 in any case \u2014 needed. After all, we can wish that the program would react only to the user indicating the age of 16+ (awarding a prize), and otherwise it would simply not award a prize \u2014 in short, it would not do anything. In this case, we can use the shortened version of the if statement (without the else part).<\/p>\n\n\n

print ('Enter your name, please:')
a=input()
print ('How old are you?')
age=int(input())
if age>=16:
    print ('You are getting a prize.')
print ('Have a nice day,' + a + '.')<\/p>\n\n\n


If the user enters age 16+, the program will execute the snippet following if<\/em> (a line indented by four spaces). Otherwise, the program will skip this piece of code.<\/p>\n\n\n

Indentation<\/h3>\n\n\n

We promised to say a few more words about indentation in Python. Again, indenting is a very important part of Python syntax as it allows you to \"overmark\" blocks of code. Each programming language must contain (within its syntax) ways to define blocks of code. A code block is a set of commands that are logically or organizationally related to each other. The conditional if statement provides us with the first example to illustrate blocks of code.<\/p>\n\n\n\n

Example<\/p>\n\n\n

print ('Enter number a: ')
a=input()
print ('Enter number b: ')
b=input ()
if a> b:
    print ('The number a is greater.')
    print ('The number a is really bigger.')
    print ('The number a is undoubtedly greater.')
print ('It's nice working with you.')<\/p>\n\n\n

As you can see, after the condition checking \"whether a is greater than b\", we gave three commands, differing in the content of the printed message. There is also a fourth command (\"It's nice working with you\"), but it is not indented by four spaces. What does it all mean? All in all, even without knowing Python's syntax, it's easy to guess. Lines that follow the line with \"condition\" and are indented by four spaces are subject to this condition. To be more precise: they are only executed when the condition is True. That's why we call them \"code block\"[1]<\/a>.<\/p>\n\n\n\n

The end of the block is marked out by the last indented line. The first unintended line means that we are already back in the main code (i.e. outside the conditional block), so the commands are executed regardless of whether the condition is met or not.<\/p>\n\n\n\n

Try it out with the script example above (run the code as a Python file). If you specify number a greater than number b (e.g. 4 and 3), the program will generate the output in the form of four lines:<\/p>\n\n\n

The number a is greater.
The number a is really bigger.
The number a is undoubtedly greater.
It's nice working with you.<\/p>\n\n\n

However, when you enter number a less than number b (such as 3 and 4), only one line will be printed:<\/p>\n\n\n

It's nice working with you.<\/p>\n\n\n

The command: print ('It's nice working with you.') <\/em>\u2013 as \"unintended\" \u2013 is independent of the given condition and is executed in both variants.<\/p>\n\n\n\n

The use of indentation to mark blocks of code is a distinctive feature of Python. And proof of the genius of its creators. Indents provide great readability and clarity of the code. Most programming languages such as C, C ++, and Java use curly braces {} to define a block of code. And parentheses that are opened in different places and then closed in bulk in one (or vice versa) are what novice programmers hate the most.<\/p>\n\n\n\n

Believe us. Guido van Rossum, inventing indentations, saved you from the \"bracket thriller\".<\/p>\n\n\n

Finally something special \"for dessert\"<\/h2>\n\n\n

We decided to throw in a little topic additionally. In programming languages, the goal is to make the syntax as compact as possible. For this reason, various abbreviations are allowed. Experienced programmers praise them, beginners rather swear (until they get used to them).<\/p>\n\n\n\n

For example, it is well established to replace i=i+1 with i+=1<\/p>\n\n\n\n

Typically, you will often meet abbreviations that allow you to replace two lines of code with one line. We'll give you one of these abbreviations right now. As you may have noticed, input() statement is \u2014 almost always \u2014 preceded by a print() statement. Why? If you don't tell the user what to put into the program, how would they know? You have to indicate what the program is waiting for, whether to enter your name, date, or deposit amount, or any other data. Therefore, print and input must form an inseparable pair.<\/p>\n\n\n\n

The Python developers thought: if so, why not write the message, what we ask the user, right in the input() command? print() will then become redundant and we will save one line of code.<\/p>\n\n\n\n

Indeed, Python does allow this. In the input() command, you can put the so-called prompt<\/em>, i.e. an incentive that tells the user what data we are waiting for.<\/p>\n\n\n\n

Full form (traditional):
print('Enter your name:')
input()<\/td>		Short form:
input('Enter your name:') <\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n

Instead of a summary<\/h2>\n\n\n

In this passage, we discussed the if statement. Instead of a summary, we thought we would present a short post on the if statement by a 13-year-old Python enthusiast. In terms of simplicity and brevity, the post is unbeatable. If you had any doubts about the if statements, they will disappear after reading the post. Enjoy reading \u2026<\/p>\n\n\n

Let's get to know the if statement<\/h3>\n\n\n

We will use the \"if\" statement very often. We use the keyword if<\/em> in it. The if<\/em> keyword in Python means exactly the same as in ordinary \"human\" language, and means that if something is true, the program must execute the command, but if it is not true, it must not execute the command.<\/p>\n\n\n

x = input ('Did you buy me milk as I asked? yes \/ no')
if x == 'yes':
    print ('Thanks') # Don't forget to indent

<\/p>\n\n\n

If we run this file, the output will look like this:<\/p>\n\n\n\n

Did you buy me milk as I asked? yes \/ no<\/em><\/p>\n\n\n\n

We answer yes or no<\/p>\n\n\n\n

If we enter yes<\/em>, the program will answer like this: Thanks<\/em><\/p>\n\n\n\n

What if we type no<\/em>? The program's response will be:<\/p>\n\n\n\n

*emptiness*<\/p>\n\n\n\n

So now we're going to improve our program to react to the second answer too!<\/p>\n\n\n

Else<\/h4>\n\n\n

The else keyword is supposed to perform the second option. As we said a moment ago, if we type no<\/em> after running the script, the program won't respond, so now we'll improve it.<\/p>\n\n\n

x = input ('Did you buy me milk as I asked? yes \/ no')
if x == 'yes':   # don't forget the colon
    print ('Thanks') # don't forget to indent
else:
    print ('Why?') # We also indent<\/p>\n\n\n

If we run this file, the output will look like this:<\/p>\n\n\n\n

Did you buy me milk as I asked? yes \/ no<\/em><\/p>\n\n\n\n

We answer yes<\/em> or no<\/em><\/p>\n\n\n\n

If we enter yes<\/em>, the program will answer like this: Thanks<\/em><\/p>\n\n\n\n

And if we enter no<\/em>, the program will respond as follows: Why?<\/em><\/p>\n\n\n\n

Thanks for reading the article and best regards.<\/p>\n\n\n\n

<\/p>\n\n\n

Answers for self-exercises<\/h2>\n\n\n

Exercise 1<\/p>\n\n\n

print('Enter first number: ')
a=input()
print('Enter second number: ')
b=input()
if a==b:
    print('The numbers are equal.')
else:
    print('The numbers are not equal.')

<\/p>\n\n\n

Exercise 2<\/p>\n\n\n

print('Specify a colour, please: ')
x=input()
if x=='white':
    print('Great, you guessed it!')
else:
    print('This is not the color.')

<\/p>\n\n\n

\n\n\n\n

[1]<\/a> these commands are organizationally related, they are linked by dependence on one and the same condition<\/p>\n\n\n\n

<\/p>\n\n\n\n

<\/p>\n\n\n\n

<\/p>\n\n\n\n

Odcinek\u20114<\/p>\n\n\n

Odcinek 4: Instrukcje warunkowe<\/h1>\n\n\n

Witajcie.<\/p>\n\n\n\n

Dzi\u015b o wariantowaniu w programowaniu. Wariantowanie nazywa si\u0119 tak\u017ce \"decision making\", gdy\u017c umo\u017cliwia realizacj\u0119 r\u00f3\u017cnych fragment\u00f3w kodu tylko w\u00f3wczas, gdy zostanie spe\u0142niony okre\u015blony warunek (gdy u\u017cytkownik podejmie okre\u015blon\u0105 decyzj\u0119). Najprostszy przyk\u0142ad to pole wyboru w programie: \"Czy kontynuowa\u0107? T\/N\". Program kontynuuje dzia\u0142anie tylko w\u00f3wczas, je\u015bli u\u017cytkownik wci\u015bnie literk\u0119 T.<\/p>\n\n\n\n

Trudno sobie wyobrazi\u0107 nowoczesne programowanie bez instrukcji warunkowych<\/strong>, kt\u00f3re stwarzaj\u0105 mo\u017cliwo\u015b\u0107 wyboru po stronie u\u017cytkownika, a tym samym daj\u0105 mu realn\u0105 mo\u017cliwo\u015b\u0107 oddzia\u0142ywania na \"przep\u0142yw programu\" (control flow)[1]<\/a>. Zatem\u2026 uczymy si\u0119 instrukcji warunkowych!<\/p>\n\n\n

Wst\u0119p<\/h2>\n\n\n

Je\u015bli \u015bledzicie wcze\u015bniejsze odcinki naszego cyklu, wiecie ju\u017c, jak napisa\u0107 program, kt\u00f3ry prze\u00adprowadzi kr\u00f3tki dialog z u\u017cytkownikiem: poprosi go o podanie imienia, zapami\u0119ta je i odpowie pozdrowieniem: Mi\u0142ego dnia, dodaj\u0105c zapami\u0119tane imi\u0119.<\/p>\n\n\n\n

Spr\u00f3bujmy rozbudowa\u0107 nasz program, aby dalej zapyta\u0142 u\u017cytkownika o wiek. I postawmy sobie ambitne zadanie. Zr\u00f3\u017cnicujemy<\/strong> reakcj\u0119 programu zale\u017cnie od podanego wieku.<\/p>\n\n\n\n

Powiedzmy, \u017ce budujesz program, kt\u00f3ry przyznaje nagrod\u0119, inn\u0105 dla osoby starszej, inn\u0105 dla osoby m\u0142odszej.<\/em><\/p>\n\n\n\n

Je\u017celi u\u017cytkownik ma 16 lat lub wi\u0119cej, program generuje komunikat: \"Otrzymujesz nagrod\u0119 X\".<\/em><\/p>\n\n\n\n

W przeciwnym przypadku (je\u017celi u\u017cytkownik ma mniej ni\u017c 16 lat), program generuje komunikat: \"Otrzymujesz nagrod\u0119 Y\".<\/em><\/p>\n\n\n\n

Jak widzisz, pos\u0142ugujemy si\u0119 s\u0142owem je\u017celi<\/strong>. S\u0142owo je\u017celi (if)<\/strong> to s\u0142owo kluczowe w j\u0119zyku prog\u00adramowania Python. Inaczej m\u00f3wi\u0105c, s\u0142owo, kt\u00f3re nale\u017cy do sk\u0142adni j\u0119zyka i pozwala budowa\u0107 struktur\u0119 polece\u0144. Dzi\u0119ki s\u0142owu kluczowemu if utworzymy w Pythonie instrukcj\u0119 warunkow\u0105 (conditional statement).<\/p>\n\n\n\n

Instrukcja warunkowa<\/strong> \u2013 to w\u0142a\u015bnie jest temat dzisiejszego odcinka naszego cyklu. Instrukcja warunkowa naprawd\u0119 nie jest niczym trudnym. Przekonasz si\u0119 o tym ju\u017c za chwil\u0119. W przysz\u0142o\u015bci b\u0119dziesz z niej korzysta\u0142 setki i tysi\u0105ce razy bez najmniejszego problemu.<\/p>\n\n\n

Instrukcja warunkowa if\u2014else<\/h2>\n\n\n

Instrukcja warunkowa if\u2014else sk\u0142ada si\u0119 z warunku<\/em><\/strong> i polecenia<\/em><\/strong>, kt\u00f3re jest wykonywane gdy warunek spe\u0142nia si\u0119, oraz drugiego polecenia<\/em><\/strong>, kt\u00f3re jest wykonywane gdy warunek nie spe\u0142nia si\u0119.<\/p>\n\n\n

Warunki<\/h3>\n\n\n