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Из курса от партнера University of Toronto

Learn to Program: The Fundamentals

2159 оценок

University of Toronto

Learn to Program: The Fundamentals

2159 оценок

Behind every mouse click and touch-screen tap, there is a computer program that makes things happen. This course introduces the fundamental building blocks of programming and teaches you how to write fun and useful programs using the Python language.

Из урока

While Loops, Lists, and Mutability

This module introduces another way to repeat code (using a while loop), how to properly document your code to help other programmers understand it, Python's list data type, and the concept of mutation.

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Jennifer Campbell
Associate Professor, Teaching Stream
Department of Computer Science
Paul Gries
Associate Professor, Teaching Stream
Department of Computer Science

Now that we have If-statements and loops, the problems that

we are going to tackle are going to get more complicated.

Our function bodies are going to get longer as

well, and are going to need some English explanation.

Here is the up_to_vowel function from the While Loop lecture.

We have added two comments.

A comment begins with a number sign character.

Often called the hash character.

And goes until the end of the line. Python ignores these characters.

Their purpose is to document code, much like doc strings document functions.

The first one describes the variables that are used in this function.

Notice how it links together before a vowel s and i.

Let's explore a little bit about what that means.

we'll do that using the word zymurgy with indices

zero,one,two,three,four,five, and six.

Well, we know that second index is not

included in my sub-string, so that means that

this is s starting an index zero going up to index seven.

What is s from zero up through index six?

That is zymurg.

And it's from zero to five is

zymur. S from zero to four

is zymu. S from zero to three

is zym. S from zero to two is zy.

S from zero to one is z. And here's the fun bit s from

zero to zero, just following this pattern of reducing

the length by one gives me just the empty string.

Well, we start before vowel off at the empty string and I starts off at zero.

So if we plug zero in

[INAUDIBLE]

I, we end up at from, with a S from zero to zero, that's this.

And S from zero through zero is the

empty string, so indeed, before vowel contains all

the characters found in S from zero through

zero and before vowel is the empty string.

This simple initialization, where we assign an empty

string to before value and we assign zero

to i.

Establishes that this description here is correct.

Furthermore, the first time through the loop we're going to add s at index zero to

before vowel and replace what we had. Well now before vowel contains a Z.

I was zero, so i plus 1 is 1, will make i 1.

Now we're on that row when i is 1. And indeed this

description still is true.

The next time through the loop we look at

S at index one, coming up here that's the y.

We take the y and we tack it on to the end of before vowel, and then we increment i.

I is now two, so S, from zero through

two is indeed what we have saved inside before vowel.

Maintaining this sort of relationship between variables

is a bit of a tricky business, but it

really helps to understand what's going on inside this loop.

Coming up with these relationships in the first place is

also challenging but with practice you'll get better and better.

Whenever you write a loop there's going to be a relationship between your variables.

Try and figure out what that relationship is and

you'll have an easier time writing your loop body.

Also, once you figure out that relationship, write it down.

Put it in a comment so that you don't have to

figure it out again the next time you read the code.

This second comment, accumulate the non-vowels at the beginning of the string,

describes what the loop does, much like doc strings don't say how functions work.

This comment does not say how this loop gets the job done.

It just describes the effect.

Comments are intended for programmers and so they don't

need to describe how the loop does its job.

You should try and comment complicated bits of code to explain.

The purpose of it.

And you should also write comments

to describe relationships between your variables.

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