# Python: Combining Operations and Functions

**Logical operations** are expressions, which means they can be combined with other expressions. For example, if we want to check if a number is odd or even. The approach used in programming is to check the remainder of a division by two:

- if the remainder is
`0`

, it's an even number - if the remainder isn't
`0`

, it's an odd number

The remainder of division is a simple but important concept in arithmetic, algebra, number theory, and cryptography. You need to divide the number into several equal groups, and if there's something left over at the end, it's the remainder of the division.

Split some candies equally among individuals:

- 7 candies, 2 people: 2 x 3 + a remainder of 1 - 7 is not a multiple of 2
- 21 candy, 3 people: 3 x 7 + a remainder of 0 - 21 is a multiple of 3
- 19 candies, 5 people: 5 x 3 + a remainder of 4 - 19 is not a multiple of 5

The `%`

operator calculates the remainder of the division:

`7 % 2`

→`1`

`21 % 3`

→`0`

`19 % 5`

→`4`

Let's combine the equality check `==`

and the arithmetic operator `%`

into one expression and write a function that checks if a number is odd or even:

```
def is_even(number):
return number % 2 == 0
print(is_even(10)) # => True
print(is_even(3)) # => False
```

Arithmetic operators have higher priority than logical ones. So, first the arithmetic expression `number % 2`

is calculated and the result is compared to zero, then the result of the equality check is returned.

Now write a function that takes a string and checks if the string starts with the letter `a`

.

Algorithm:

- Get the first character of the argument string, and write it to the variable
- Compare whether the symbol is equal to the letter
`a`

. - Return the result

```
def is_first_letter_an_a(string):
first_letter = string[0]
return first_letter == 'a'
print(is_first_letter_an_a('orange')) # => False
print(is_first_letter_an_a('apple')) # => True
```

To make it clear what's going on here, try saying what's going on in the same way as we decoded the process in the `is_even()`

example.

You now know that comparison operations are used in programming alongside arithmetic operations. But remember that equality is indicated using this symbol: `==`

. This way, you won't confuse this operation with assigning a value to a variable.

## Instructions

Implement a function called `is_international_phone()`

function that checks the format of a given phone number. If the phone number starts with *+*, then it's in the international format.

```
is_international_phone('89602223423') # False
is_international_phone('+79602223423') # True
```

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

If you've reached a deadlock it's time to ask your question in the «Discussions». How ask a question correctly:

- 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 🤨

Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.

## My code is different from the teacher's one 🤔

It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.

In some rare cases, the solution may be adjusted to the tests, but this can be seen immediately.

## I've read the lessons but nothing is clear 🙄

It's hard to make educational materials that will suit everyone. We do our best but there is always something to improve. If you see a material that is not clear to you, describe the problem in “Discussions”. It will be great if you'll write unclear points in the question form. Usually, we need a few days for corrections.

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