Project: a unit converter and a bill splitter

What you will be able to do

After this lesson you can

• turn a real need into a program that asks, calculates and answers,
• keep a fixed number, such as a conversion factor, in one place,
• test with a number whose answer you can work out, and
• extend a program with a new feature.

Project 1: a unit converter

People travel, cook and shop with different units. A kilometre is not a mile, and a kilogram is not a pound. A converter does the arithmetic. The conversion factors are

• 1 kilometre is 0.621371 miles,
• 1 kilogram is 2.20462 pounds.

Plan in words

1. Ask for a distance in kilometres. Change it to a decimal number.
2. Multiply by 0.621371.
3. Show the answer, rounded to two decimal places, with the units.
4. Do the same for kilograms and pounds.

The program

Type 10 and 70 on two lines in the Input box, and run it. 10 kilometres is about 6.21 miles. 70 kilograms is about 154.3 pounds. Check one by hand: 10 times 0.62 is about 6.2. The program agrees.

Test it

• Type 0 and 0. Both answers must be 0.
• Type 1 for the distance. The answer must be 0.62.
• Type 42.195 (a marathon). The answer is about 26.22 miles.

Keeping 0.621371 in one place means one change fixes every answer, if the number is ever wrong.

Project 2: a bill splitter

Friends share a bill. In India, we do not like to divide a rupee into pieces. So we want to know how much each pays in whole rupees, and what is left over.

Plan in words

1. Ask for the total bill and for the number of friends. Both are whole numbers.
2. Each friend pays the total divided by the friends, as a whole number. We know the sign for it from the calculator course: //.
3. The rupees that are left over are the remainder, %.
4. Show both.

The program

Type 1000 and 3 in the Input box and run it. Each friend pays 333, and 1 rupee is left over. Check: 3 * 333 + 1 = 1000. The rule "quotient times divisor plus remainder" gives the total again, so the answer is right.

Make it your own

Choose one, and do it without looking at a solution.

1. Converter. Add a third conversion: Celsius to Fahrenheit from the last lesson, or centimetres to inches (1 inch is 2.54 centimetres).
2. Converter. Show the answer in a full sentence: 10.0 km is 6.21 miles.
3. Splitter. The one who has the odd rupees pays them. Show what the first friend pays: each + left_over.
4. Splitter. Add a tip in percent, like the tip calculator, before splitting.

Common mistakes

• Using / for a bill in whole rupees. 1000 / 3 is 333.3333333333333.
• Forgetting float() for a distance such as 42.195. With int() it is a ValueError.
• Writing the factor inside the formula many times. Keep it in one variable, factor = 0.621371, so that a change is made in one place.
• Showing an answer without units. The user does not know if it is miles or kilometres.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Project: the tip calculator, if you have not done it.
2. Build. Write the unit converter with a third conversion of your own.
3. Stretch. Open the lab Project: the bill splitter. It gives the program the answers and checks the output.

Remember these

• A converter multiplies by a fixed factor. Keep the factor in a variable.
• // and % split an amount into whole parts and a remainder.
• Test with the answers that you can find by hand, and with zero.
• A good answer has a label and a unit.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many miles is 100 kilometres, to one decimal place?

2. A bill of 500 rupees is split among 7 friends. How much does each pay, and how much is left over?

3. Why keep 0.621371 in a variable?