Currency Exchange
Currency Exchange

Currency Exchange

Learning Exercise



There are three different kinds of built-in numbers in Python : ints, floats, and complex. However, in this exercise you'll be dealing only with ints and floats.


ints are whole numbers. e.g. 1234, -10, 20201278.

Integers in Python have arbitrary precision -- the number of digits is limited only by the available memory of the host system.


floats are numbers containing a decimal point. e.g. 0.0,3.14,-9.01.

Floating point numbers are usually implemented in Python using a double in C (15 decimal places of precision), but will vary in representation based on the host system and other implementation details. This can create some surprises when working with floats, but is "good enough" for most situations.

You can see more details and discussions in the following resources:


Python fully supports arithmetic between ints and floats. It will convert narrower numbers to match their less narrow counterparts when used with the binary arithmetic operators (+, -, *, /, //, and %). When division with /, // returns the quotient and % returns the remainder.

Python considers ints narrower than floats. So, using a float in an expression ensures the result will be a float too. However, when doing division, the result will always be a float, even if only integers are used.

# The int is widened to a float here, and a float type is returned.
>>> 3 + 4.0
>>> 3 * 4.0
>>> 3 - 2.0
# Division always returns a float.
>>> 6 / 2
>>> 7 / 4
# Calculating remainders.
>>> 7 % 4
>>> 2 % 4
>>> 12.75 % 3

If an int result is needed, you can use // to truncate the result.

>>> 6 // 2
>>> 7 // 4

To convert a float to an integer, you can use int(). Also, to convert an integer to a float, you can use float().

>>> int(6 / 2)
>>> float(1 + 2)


Your friend Chandler plans to visit exotic countries all around the world. Sadly, Chandler's math skills aren't good. He's pretty worried about being scammed by currency exchanges during his trip - and he wants you to make a currency calculator for him. Here are his specifications for the app:

1. Estimate value after exchange

Create the exchange_money() function, taking 2 parameters:

  1. budget : The amount of money you are planning to exchange.
  2. exchange_rate : The amount of domestic currency equal to one unit of foreign currency.

This function should return the value of the exchanged currency.

Note: If your currency is USD and you want to exchange USD for EUR with an exchange rate of 1.20, then 1.20 USD == 1 EUR.

>>> exchange_money(127.5, 1.2)

2. Calculate currency left after an exchange

Create the get_change() function, taking 2 parameters:

  1. budget : Amount of money before exchange.
  2. exchanging_value : Amount of money that is taken from the budget to be exchanged.

This function should return the amount of money that is left from the budget.

>>> get_change(127.5, 120)

3. Calculate value of bills

Create the get_value_of_bills() function, taking 2 parameters:

  1. denomination : The value of a single bill.
  2. number_of_bills : The total number of bills.

This exchanging booth only deals in cash of certain increments. The total you receive must be divisible by the value of one "bill" or unit, which can leave behind a fraction or remainder. Your function should return only the total value of the bills (excluding fractional amounts) the booth would give back. Unfortunately, the booth gets to keep the remainder/change as an added bonus.

>>> get_value_of_bills(5, 128)

4. Calculate number of bills

Create the get_number_of_bills() function, taking amount and denomination.

This function should return the number of currency bills that you can receive within the given amount. In other words: How many whole bills of currency fit into the starting amount? Remember -- you can only receive whole bills, not fractions of bills, so remember to divide accordingly. Effectively, you are rounding down to the nearest whole bill/denomination.

>>> get_number_of_bills(127.5, 5)

5. Calculate leftover after exchanging into bills

Create the get_leftover_of_bills() function, taking amount and denomination.

This function should return the leftover amount that cannot be returned from your starting amount given the denomination of bills. It is very important to know exactly how much the booth gets to keep.

>>> get_leftover_of_bills(127.5, 20)

6. Calculate value after exchange

Create the exchangeable_value() function, taking budget, exchange_rate, spread, and denomination.

Parameter spread is the percentage taken as an exchange fee, written as an integer. It needs to be converted to decimal by dividing it by 100. If 1.00 EUR == 1.20 USD and the spread is 10, the actual exchange rate will be: 1.00 EUR == 1.32 USD because 10% of 1.20 is 0.12, and this additional fee is added to the exchange.

This function should return the maximum value of the new currency after calculating the exchange rate plus the spread. Remember that the currency denomination is a whole number, and cannot be sub-divided.

Note: Returned value should be int type.

>>> exchangeable_value(127.25, 1.20, 10, 20)
>>> exchangeable_value(127.25, 1.20, 10, 5)
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