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Python 3, an Illustrated Tour Transcripts

Chapter: Numbers

Lecture: Walk-through: Number

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0:00
In this video we're going to look at num test, open it up in your editor and validate that you can run it. If you don't run it from your editor,

0:09
run it from the command line, it should work as well. We see that there is one failure., it's a name error on line 8,

0:14
so I think we're good to go on my machine. Let's look at this first test here, there are 102 floors in the Empire State Building,

0:21
if you have walked up a seventh of them, how many whole floors have you walked up, store the result in floors.

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So the idea here is to say we want to make the distinction between true division and floor division,

0:40
because we're walking up floors that should be a hint that we should use floor division here which gives us this division or whole numbers,

0:49
So, 102 and we've walked up a seventh of those. So in Python 3 we use the double slash to get the floor division. Let's run it and see if it works,

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and I have a typo, I did 107 instead 102, I'm trying again. Okay, so that one looked like it worked.

1:13
So that's the whole number of floors that we walked up. What percentage of floors have you climbed,

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store the result as a string with one decimal of precision in the variable per so what percentage have we climbed?

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I'm going to put it in an fstring here feature Python 3 so need to put curly braces around here. So we have climbed 7 out of 102.

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So 7 divided by 102 will give us the number of floors. Let's run this and see what it says. Okay. So this is what I got here

1:54
per do the reverse of it does it equal to that, no, it says it's not. So what's happening here? I am getting a number that's not formatted correctly.

2:08
So it wants me to format it as a percent. So in order to do that, I need to put a colon here and this says I want one decimal, so I put .1

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and then I put percent there to format it as a percent. Okay, in that case, it looks like it worked. It should be 6.9% doing a little trickery here.

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so you don't just cheat and type in 6.9% Okay, I have (2^64)-1 satoshis, can I divide them wholly by 3.

2:36
How many would each person get store the result in coins? So again, this is floor division, if we want to do whole division. So coins = satoshis//3

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and if we multiply that by 3, if it's integer or a floor division, it should get us back to where we started from.

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So let's see here satoshis divided by 3. It looks like that is indeed the case you can divide them by 3.

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The US population is around 326 million, some number after that how many whole coins would eat US citizen get, store the result in US coins.

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So US coins is equal to and there's a hint, use underscore to make the population easier to read so I'm going to say 326_979_681

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so there's our total population and we are going to divide our coins. So whole coins would each person get, we need to divide satoshis.

3:58
Satoshis divided by that, that should give us the whole coins and let's run that. Okay, that looked like it worked.

4:11
So again, when we do the double slash that's what we call floor division, that gives us a whole integer number.

4:18
Okay, I have .5 pumpkin pies and 1.5 apple pies. I want to use Python to round the number of each pie store the result in pumpkin and apple,

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so pumpkin = round (.5) and apple = round (1.5) So you might think well, I do, I was taught when you round

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if it's .5 you go up to the next number, but apparently that's not always the case generally. Python 3 actually doesn't do that.

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It does what's called banker’s rounding, where it rounds to the nearest even number here. So this one should round to 2 and this one should round to 0

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and that's why the sum of those is 2. Okay, hopefully that gave you a little feel for floor division in Python 3.

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And again, this is the behavior just a single slash in Python 2 and the ability to put underscores in number literals

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just to help you read them better. These are good as commas, placeholders are also good in her and binary literals as well.

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And also, we learned a little bit about round, how round does what's called banker’s rounding

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one of the benefits of that is that it eliminates the bias towards rounding high, they should even out if you have randomly distributed numbers,

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hence that's why bakers like to use banker’s rounding.