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Planning isPrime

Course video 10 of 22

Now that you know how to plan an algorithm and translate it to code, you need to learn how to compile and run it! You will learn about the compiler, which takes the code you wrote and translates it into instructions a machine can execute, which you can then run. You will also learn about different options you can give the compiler, as well as different ways to run your program that give you debugging information.

Writing, Running, and Fixing Code in C

课程 2（共 4 门，Introduction to Programming in C 专项课程）

Building on the course Programming Fundamentals, this course will teach you how to write code by first planning what your program should do—an important approach for novice and professional programmers. You will learn how to compile and run your program, and then how to test and debug it. This course builds on the Seven Steps you have already learned and provides a framework for systematically testing for problems and fixing them, so you can find and fix problems efficiently.

查看授课大纲

您将学习的技能

C Programming, Compiler, Debugging

教学方

Andrew D. Hilton
Associate Professor of the Practice
Genevieve M. Lipp
Assistant Professor of the Practice
Anne Bracy
Senior Lecturer

脚本

In this example we're going to plan and

write a function that takes one integer N and determine if it is prime.

We begin with step 1, working at least one instance by hand.

We might start with something like is 7 prime and

you might just say, yes, I know this, 7 is prime.

However, this is not very helpful.

Just knowing the answer does not help us develop a step-by-step

approach to solving the general problem.

Unfortunately, when you already know the answer,

it can be hard to see past it, in which case, there are two things you could do.

First, you could consider how you would convince someone that this is right.

That is, if you said 7 is prime, and I said I don't believe it,

what would you do step-by-step to show me that it is?

Second, you might think about a harder problem

where you don't just know the answer to see the step-by-step approach you take.

If that problem is too big to work through all the way,

you might use the step-by-step approach,

you begin with to inform how you work through a more manageable size example.

As an example of a harder problem let us consider whether 29,393 is prime.

I suspect most of you do not know the answer off the top of your head.

We'll start with step 1.

I divided by 2 and get 14,696 remainder 1.

Dividing by 3 gives 9,797 remainder 2.

Dividing by 4 gives 7,348 remainder 1.

Dividing by 5 gives 5,878 remainder 3.

Dividing by 6 gives 4,898 remainder 5.

Dividing by 7 gives 4,199 remainder 0.

Because my number is evenly divisible by 7,

I will answer no, 29,393 is not prime.

Next I'll write down exactly what I did, and this involves thinking through my

actions and describing the steps to get the answer.

I checked if 29,393 mod 2 is 0, that is, if it's evenly divisible by 2.

It was not.

Then I checked if it was divisible by 3.

If mod 3 is equal to 0, it was not.

Then I checked whether it was divisible by 4, divisible by 5,

divisible by 6, divisible by 7.

The answer was yes, when I found that it was divisible by 7 I answered no.

The next step is generalizing the result of step 2, but

we may wish to make multiple instances of steps 1 and 1 first.

In this problem in particular since we have a yes or no question it may be

very useful to do an instance where we get a yes answer before we try to generalize.

Now we return to our first question, is 7 prime?

Having already thought through how to do it step-by-step,

we may be able to get past a mental road block of just knowing the answer.

7/2 = 3 remainder 1.

7/3 = 2 remainder 1.

7/4 = 1 remainder 3.

7/5 = 1 remainder 2.

7/6 is 1 remainder 1.

So at this point, I'm going to answer yes, 7 is prime.

I tried all the numbers from 2 to 6 and found 7 is not divisible by any of them.

Doing step 2 for the same problem,

I'm going to write down that I checked 7 mod 2 is 0, it is not.

Checked if 7 mod 3 is 0.

It is not.

Checked if 7 mod 4 is 0.

Checked if 7 mod 5 is 0.

Checked if 7 mod 6 is 0.

And after doing all that and verifying that the answer was no for

all of them I answered yes.

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