{"id":5743,"date":"2022-02-12T12:35:04","date_gmt":"2022-02-12T11:35:04","guid":{"rendered":"http:\/\/miro.borodziuk.eu\/?p=5743"},"modified":"2026-01-24T11:26:20","modified_gmt":"2026-01-24T10:26:20","slug":"variables","status":"publish","type":"post","link":"http:\/\/miro.borodziuk.eu\/index.php\/2022\/02\/12\/variables\/","title":{"rendered":"Variables"},"content":{"rendered":"

1. A variable<\/strong> is a named location reserved to store values in the memory. A variable is created or initialized automatically when you assign a value to it for the first time. (2.1.4.1)<\/p>\n

<\/p>\n

2. Each variable must have a unique name – an identifier<\/strong>. A legal identifier name must be a non-empty sequence of characters, must begin with the underscore(_<\/code>), or a letter, and it cannot be a Python keyword. The first character may be followed by underscores, letters, and digits. Identifiers in Python are case-sensitive. (2.1.4.1)<\/p>\n

 <\/p>\n

3. Python is a dynamically-typed<\/strong> language, which means you don’t need to declare<\/i> variables in it.\u00a0 To assign values to variables, you can use a simple assignment operator in the form of the equal (=<\/code>) sign, i.e.,<\/p>\n

var = 1<\/pre>\n
 <\/p>\n
4. You can also use compound assignment operators<\/strong> (shortcut operators) to modify values assigned to variables, e.g., <\/code><\/p>\n
var += 1\r\nvar \/= 5 * 2<\/pre>\n
 <\/p>\n
5. You can assign new values to already existing variables using the assignment operator or one of the compound operators, e.g.:<\/p>\n
\n
\n
var = 2\r\nprint(var)\r\n\r\nvar = 3\r\nprint(var)\r\n\r\nvar += 1\r\nprint(var)<\/pre>\n<\/div>\n<\/div>\n
 <\/p>\n
6. You can combine text and variables using the +<\/code> operator, and use the print()<\/code> function to output strings and variables, e.g.:<\/p>\n
\n
\n
var = \"007\"\r\nprint(\"Agent \" + var)<\/pre>\n
 <\/p>\n<\/div>\n<\/div>\n
<\/div>\n
\n
\n
\n
\n
Shortcut operators<\/h1>\n
It’s time for the next set of operators that make a developer’s life easier. Very often, we want to use one and the same variable both to the right and left sides of the =<\/code> operator. For example, if we need to calculate a series of successive values of powers of 2, we may use a piece like this:<\/p>\n
x = x * 2<\/pre>\n
You may use an expression like this if you can’t fall asleep and you’re trying to deal with it using some good, old-fashioned methods:<\/p>\n
sheep = sheep + 1<\/pre>\n
Python offers you a shortened way of writing operations like these, which can be coded as follows:<\/p>\n
x *= 2\r\nsheep += 1<\/pre>\n<\/div>\n
\n
Let’s try to present a general description for these operations.<\/p>\n
If op<\/code> is a two-argument operator (this is a very important condition) and the operator is used in the following context:<\/p>\n
variable = variable op expression<\/pre>\n
It can be simplified and shown as follows:<\/p>\n
variable op= expression<\/pre>\n
Take a look at the examples below. Make sure you understand them all.<\/p>\n
i = i + 2 * j \u21d2 i += 2 * j\r\n\r\nvar = var \/ 2 \u21d2 var \/= 2\r\n\r\nrem = rem % 10 \u21d2 rem %= 10\r\n\r\nj = j - (i + var + rem) \u21d2 j -= (i + var + rem)\r\n\r\nx = x ** 2 \u21d2 x **= 2<\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
<\/div>\n
<\/div>\n
\n
Swaping the values of two variables<\/strong><\/p>\n
Take a look at the snippet:<\/p>\n
\n
\n
variable_1 = 1\r\nvariable_2 = 2\r\n\r\nvariable_2 = variable_1\r\nvariable_1 = variable_2<\/pre>\n<\/div>\n<\/div>\n
If you do something like this, you would lose the value previously stored<\/strong> in variable_2<\/code>. Changing the order of the assignments will not help. You need a third variable that serves as an auxiliary storage<\/strong>.<\/p>\n
This is how you can do it:<\/p>\n
variable_1 = 1\r\nvariable_2 = 2\r\nauxiliary = variable_1\r\nvariable_1 = variable_2\r\nvariable_2 = auxiliary<\/pre>\n
Python offers a more convenient way of doing the swap – take a look:<\/p>\n
variable_1 = 1\r\nvariable_2 = 2\r\nvariable_1, variable_2 = variable_2, variable_1<\/pre>\n<\/div>\n
<\/div>\n
\n
Exercise 1<\/em><\/p>\n
What is the output of the following snippet?<\/p>\n
\n
\n
var = 2\r\nvar = 3\r\nprint(var)<\/pre>\n
<\/div>\n<\/div>\n<\/div>\n
3<\/code><\/p>\n
Exercise 2<\/em><\/p>\n
Which of the following variable names are illegal<\/u> in Python?<\/p>\n
my_var\r\nm\r\n101\r\naverylongvariablename\r\nm101\r\nm 101\r\nDel\r\ndel<\/pre>\n
 <\/p>\n
my_var
\nm
\n101 # incorrect (starts with a digit)
\naverylongvariablename
\nm101
\nm 101 # incorrect (contains a space)
\nDel
\ndel # incorrect (is a keyword)<\/code><\/p>\n
 <\/p>\n
Exercise 3<\/em><\/p>\n
What is the output of the following snippet?<\/p>\n
\n
\n
a = '1'\r\nb = \"1\"\r\nprint(a + b)<\/pre>\n
<\/div>\n<\/div>\n<\/div>\n
11<\/code><\/p>\n
 <\/p>\n
Exercise 4<\/em><\/p>\n
What is the output of the following snippet?<\/p>\n
\n
\n
a = 6\r\nb = 3\r\na \/= 2 * b\r\nprint(a)<\/pre>\n
<\/div>\n<\/div>\n<\/div>\n
1.0<\/code><\/p>\n
2 * b = 6
\na = 6 \u2192 6 \/ 6 = 1.0<\/p>\n<\/div>\n
\n
\n
<\/h2>\n
Ex. 5<\/h2>\n<\/div>\n<\/div>\n
\n
\n
\n
Which of the following variable names<\/strong> are illegal<\/strong> in Python?<\/p>\n
(Select two answers)<\/p>\n
\n
\n
\n
\n
False<\/strong><\/code><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
\n
\n
\n
\n
print<\/strong><\/code><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
\n
\n
\n
\n
_a&b<\/strong><\/code><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
\n
\n
\n
\n
Print<\/strong><\/code><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
\n
\n
\n
<\/div>\n
Correct selection<\/span><\/div>\n
\n
\n
False<\/strong><\/code><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
\n
<\/div>\n<\/div>\n
\n
\n
Correct selection<\/span><\/div>\n
\n
\n
_a&b<\/strong><\/code><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
\n
\n
\n
<\/div>\n<\/div>\n<\/div>\n<\/div>\n
\n
<\/div>\n
\n
\n
    \n
  • Under these rules, print<\/strong><\/code> is a legal<\/strong> name because it is not a reserved word, but the name of a Python 3 built-in<\/strong> function<\/strong>.<\/li>\n<\/ul>\n
      \n
    • Print<\/strong><\/code> is correct<\/strong> because it only uses allowed letters.<\/li>\n<\/ul>\n
        \n
      • The name _a&b<\/strong><\/code> causes a SyntaxError<\/strong><\/code> exception because it contains the illegal<\/strong> &<\/strong><\/code> character.<\/li>\n<\/ul>\n
          \n
        • False<\/strong><\/code> is not ok because it is a Python reserved keyword<\/strong>.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
           <\/p>\n
          Ex. 6<\/p>\n
          What is the expected output of the following code ?<\/p>\n
              y = 'R2D2'\r\n    def my_func(x):\r\n        global y\r\n        y = x\r\n        return y\r\n     \r\n    x = 'Yoda'\r\n    my_func('C3PO')\r\n    print(y)<\/pre>\n
          \n
          \n
          \n
          \n
          A.\u00a0Yoda<\/code><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
          \n
          \n
          \n
          \n
          B. R2D2<\/code><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
          \n
          \n
          C. <\/span>C3PO<\/code><\/div>\n<\/div>\n<\/div>\n
          \n
          \n
          <\/div>\n
          \n
          \n
          D. The code is erroneous.<\/p>\n<\/div>\n
          Explanation:<\/strong><\/em><\/p>\n
          When you create a variable inside a function, that variable has a local scope<\/strong>, and can only be used inside that function.<\/p>\n
          With the\u00a0 global<\/code> keyword, the scope of that variable will be extended to outside <\/strong>the function too.<\/p>\n
          The call to my_func('C3PO')<\/code>\u00a0 will assign 'C3PO'<\/code> to the global <\/strong>variable y<\/code> and print(y)<\/code> will return C3PO<\/code>.<\/p>\n<\/div>\n<\/div>\n<\/div>\n
           <\/p>\n
           <\/p>\n
          \n
          \n
          \n
          Scenario<\/h2>\n
          Miles and kilometers are units of length or distance.<\/p>\n
          Bearing in mind that 1<\/code> mile is equal to approximately 1.61<\/code> kilometers, complete the program in the editor so that it converts:<\/p>\n
            \n
          • miles to kilometers;<\/li>\n
          • kilometers to miles.<\/li>\n<\/ul>\n
            Do not change anything in the existing code. Write your code in the places indicated by ###<\/code>. Test your program with the data we’ve provided in the source code.<\/p>\n
            Pay particular attention to what is going on inside the print()<\/code> function. Analyze how we provide multiple arguments to the function, and how we output the expected data.<\/p>\n
            Note that some of the arguments inside the print()<\/code> function are strings (e.g., \"miles is\"<\/code>, whereas some other are variables (e.g., miles<\/code>).<\/p>\n<\/div>\n
            kilometers = 12.25\r\nmiles = 7.38\r\n\r\nmiles_to_kilometers = 1.61 * miles\r\nkilometers_to_miles = kilometers \/ 1.61\r\n\r\nprint(miles, \"miles is\", round(miles_to_kilometers, 2), \"kilometers\")\r\nprint(kilometers, \"kilometers is\", round(kilometers_to_miles, 2), \"miles\")<\/pre>\n<\/div>\n<\/div>\n
             <\/p>\n
            Scenario<\/h2>\n
            Take a look at the code in the editor: it reads a float<\/code> value, puts it into a variable named x<\/code>, and prints the value of a variable named y<\/code>. Your task is to complete the code in order to evaluate the following expression:<\/p>\n
             3x3<\/sup> – 2x2<\/sup> + 3x – 1 <\/span><\/p>\n
            The result should be assigned to y<\/code>.<\/p>\n
            Remember that classical algebraic notation likes to omit the multiplication operator – you need to use it explicitly. Note how we change data type to make sure that x<\/code> is of type float<\/code>.<\/p>\n
            Keep your code clean and readable, and test it using the data we’ve provided, each time assigning it to the x<\/code> variable (by hardcoding it). Don’t be discouraged by any initial failures. Be persistent and inquisitive.<\/p>\n
             <\/p>\n
            Test Data<\/h2>\n
            Sample input<\/p>\n
            x = 0
            \nx = 1
            \nx = -1<\/code><\/p>\n
            Expected Output<\/p>\n
            y = -1.0
            \ny = 3.0
            \ny = -9.0<\/code><\/p>\n
             <\/p>\n
            Solution:<\/p>\n
            x = -1\r\nx = float(x)\r\n\r\ny = 3*x**3 - 2*x**2 + 3*x - 1\r\n\r\nprint(\"y =\", y)<\/pre>\n
             <\/p>\n
             <\/p>\n","protected":false},"excerpt":{"rendered":"
            1. A variable is a named location reserved to store values in the memory. A variable is created or initialized automatically when you assign a value to it for the first time. (2.1.4.1)<\/p>\n","protected":false},"author":2,"featured_media":5936,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[100],"tags":[],"_links":{"self":[{"href":"http:\/\/miro.borodziuk.eu\/index.php\/wp-json\/wp\/v2\/posts\/5743"}],"collection":[{"href":"http:\/\/miro.borodziuk.eu\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/miro.borodziuk.eu\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/miro.borodziuk.eu\/index.php\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/miro.borodziuk.eu\/index.php\/wp-json\/wp\/v2\/comments?post=5743"}],"version-history":[{"count":16,"href":"http:\/\/miro.borodziuk.eu\/index.php\/wp-json\/wp\/v2\/posts\/5743\/revisions"}],"predecessor-version":[{"id":6286,"href":"http:\/\/miro.borodziuk.eu\/index.php\/wp-json\/wp\/v2\/posts\/5743\/revisions\/6286"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/miro.borodziuk.eu\/index.php\/wp-json\/wp\/v2\/media\/5936"}],"wp:attachment":[{"href":"http:\/\/miro.borodziuk.eu\/index.php\/wp-json\/wp\/v2\/media?parent=5743"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/miro.borodziuk.eu\/index.php\/wp-json\/wp\/v2\/categories?post=5743"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/miro.borodziuk.eu\/index.php\/wp-json\/wp\/v2\/tags?post=5743"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}