# Python: Expressions in definitions

We've already figured out how to work with variables to store and reuse information. But they also help simplify complex calculations. For example, currency conversion or making up a new word. Let's look at how to do it in practice.

Let us imagine that we need to convert euros into rubles via dollars. Banks often do this kind of conversion via an intermediate currency when shopping abroad.

First, convert 50 euros into dollars. Suppose that one euro is \$1.25:

``````dollars_count = 50 * 1.25
print(dollars_count)  # => 62.5
``````

Here we write expression into the variable `dollars_count = 50 * 1.25` to the right of the equals sign. The interpreter will calculate the result (`62.5`) and write it to a variable. The interpreter doesn't care what form the data is written in: `62.5` or `50 * 1.25`. From its perspective, both are expressions to be calculated. It does the calculations and comes up with the same value, `62.5`.

Any string is an expression. String concatenation (concatenation of variable values) is also an expression. When the interpreter sees an expression, it processes it and generates a result, the value of the expression.

Here are some examples of an expression. We've written the total values in the comments to the right of each expression:

``````62.5             # 62.5
50 * 1.25        # 62.5
120 / 10 * 2     # 24.0
int('100')       # 100

'hello'          # hello
'Good' + 'will'  # Goodwill
``````

In the places where an expression is expected, you can put any calculation. It can be not only mathematical, but also string-like concatenation. The program will remain functional.

Programs consist of many combinations of expressions. Based on the above, consider whether this code would work:

``````who = "dragon's " + 'mother'
print(who)
``````

This code will display the string `dragon's mother`. If you want to check it yourself, run the code on repl.it and experiment with it.

Variables can be used to write even more complex calculations. Now, back to our currency program. Let's write the dollar value in rubles as a separate variable. Let's calculate the value of 50 euros in dollars by multiplying it by `1.25`. Suppose that 1 dollar is 60 rubles:

``````rubles_per_dollar = 60
dollars_count = 50 * 1.25  # 62.5
rubles_count = dollars_count * rubles_per_dollar  # 3750.0

print(rubles_count)
``````

Now let's add text to the output using concatenation:

``````rubles_per_dollar = 60
dollars_count = 50 * 1.25  # 62.5
rubles_count = dollars_count * rubles_per_dollar  # 3750.0

# The str() function turns a number into a string.
# There will be a separate lesson about these transformations.
print('The price is ' + str(rubles_count) + ' rubles')
# => The price is 3750.0 rubles
``````

Any variable can be part of any expression. At the moment of calculation, the name of the variable will be replaced by its value.

The interpreter calculates the value of `dollars_count` before this variable is used in other expressions. When it comes time to use a variable, Python already knows the value because it's calculated it.

Variables can be used to perform complex calculations and to provide a detailed output with the resulting value. But you can also get new expressions by combining two or more variable values. Concatenation is responsible for this.

## Instructions

Write a program that takes the original number of euros written in the variable `euros_count`, converts euros to dollars, and prints it to the screen. Then, it should convert the result to rubles and print it on a new line.

Example output for 100 euros:

``````125.0
7500.0
``````

For the purposes of the exercise, we'll say that:
- 1 euro = \$1.25 долларов
- 1 dollar = 4,50 zl

The exercise doesn't pass checking. What to do? 😶

• Be sure to attach the test output, without it it's almost impossible to figure out what went wrong, even if you show your code. It's complicated for developers to execute code in their heads, but having a mistake before their eyes most probably will be helpful.
In my environment the code works, but not here 🤨

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My code is different from the teacher's one 🤔

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