Translating isPrime to Code

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From the course by Duke University

Writing, Running, and Fixing Code in C

197 ratings

Duke University

Writing, Running, and Fixing Code in C

197 ratings

Course 2 of 4 in the Specialization Introduction to Programming in C

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.

From the lesson

Compiling and Running

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.

"Hello World"3:15

Planning isPrime3:49

Generalizing isPrime5:11

Translating isPrime to Code2:25

Comparing Output with diff3:52

Meet the Instructors

Andrew D. Hilton
Associate Professor of the Practice
Electrical and Computer Engineering
Genevieve M. Lipp
Assistant Professor of the Practice
Electrical and Computer Engineering/Mechanical Engineering and Materials Science
Anne Bracy
Senior Lecturer
Computer Science, Cornell University

Now we're ready to translate our steps to code.

We'll take our algorithm shown here and write it as comments so

that we can translate line by line.

We start with a function declaration written around our steps.

If you were doing this yourself, you could add the algorithm as comments in your

editor without leaving space for code.

The first thing we need to do is check if N is less than or equal to 1.

We're checking a condition and deciding what to do based on that, so

we want an if statement.

if (N <= 1) we want to do something,

otherwise we want to do everything after it.

If so, we want to answer no, and that is a return statement.

We want to leave this function immediately since we know our answer and

we don’t want to do anything else, so we return 0.

Recall that zero means false or

no, and non-zero numbers, generally one, mean true or yes.

Next we want to count from 2 to N and call each number that we count i.

Counting corresponds to a for loop.

The number we want to count is i so we're going to declare int i = 2.

We want to count to N exclusive, so we chose the conditional statement i < N.

Also we're counting by one so our increment statement is going to be i++.

The body of the loop will be the steps we want to take for

each number that we count.

The first of those is check if N mod i is equal to 0.

This is, again, an if statement because I want to check a condition and

make a decision about what to do next based on the result of that.

N mod i, I write N % i since that is the mod operator.

I want to see if it's 0, so I use ==.

Recall that two equal signs compare for equality.

Then I have curly braces around what I want to do if this condition is true.

If so, answer no is again return 0.

We would know our answer immediately is no, so we would return 0 in this case.

After all of this, if we finish counting, that is, after the body of the for loop,

if we haven't returned and left this function, we answer yes, we put return 1.

This code would work fine, but we're going to do just a little clean up.

Generally, when we're programming, we don't like capital letters for variables.

All capital letters is usually reserved for a constant, so

we'll use lowercase n for this variable.

Now we have it, a function that will compute whether or not n is prime.

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