Help writing a for/while loop to identify and sort prime numbers

Question

Devon Von Lichtenstein 2018-4-26

0
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评论： Rik 2020-12-7

I have a class assignment that requires using a for or while loop to identify prime numbers between 10-500. After the numbers have been identified as prime, they need to be classified as "twin" if there is another prime within 2, or "isolated" if there is not another prime within 2. The built-in function "isprime" is not allowed.

I have hit a wall and don't even know where to go from here. So far, I have been able to identify prime numbers, but am stuck on the sorting process.

n = 500; %maximum prime number to find
primes = 2;
for i = 1:n    %code I found to help identify primes 
if mod(i+2,primes) ~= 0
primes = [primes, i+2];
end

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Pierre Lonfat 2018-4-26

编辑：Pierre Lonfat 2018-4-26

在 MATLAB Online 中打开

Maybe this is what you are looking for! Give it a quick run and look at result :)

close all
clear all
clc
%===============================
%set your variables:
numbers=[10:500];    %objective of the assignment
within_left=2;       %nbr of observerations looking from the left or right
within_right=2;      %ex. with 11: (9 10 11(prime) 12 13)
%===============================
% COMPUTATIONS:
%the first existing prime number
primes = 2;   
list=[min(numbers)-within_left:max(numbers)+within_right];
for i = 1:max(list)
    if mod(i+2,primes) ~= 0
        primes = [primes, i+2];
    end
end
for i = 1:length(primes)
    bounds(1,i)=primes(1,i)-within_left;
    bounds(2,i)=primes(1,i)+within_right;
end
% primes numbers and boundaries:
table=[primes(:,min(find(primes>=min(numbers))):min(find(primes>=max(numbers)))-1);...
    bounds(:,min(find(primes>=min(numbers))):min(find(primes>=max(numbers)))-1)];
for i = 1:size(table,2)
    if max(table(2,i)==primes) || max(table(3,i)==primes) == 1
        result{1,i}=table(1,i);
        result{2,i}='twin';
    else result{1,i}=table(1,i);
        result{2,i}='isolated';
    end
end

nassnnf 2020-12-7

can you help me to solve similiar question.the questions is list 50 prime number after 230 in 10x5 matrik

Rik 2020-12-7

It is your homework. What have you tried? Also, this question has two parts: finding 50 prime numbers and storing them in a 10x5 matrix. Which of these two is causing you trouble?

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Answer 1

Rik 2018-4-26

2
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https://ww2.mathworks.cn/matlabcentral/answers/397434-help-writing-a-for-while-loop-to-identify-and-sort-prime-numbers#answer_317300

在 MATLAB Online 中打开

You already have a list of all the primes, so you don't need isprime anymore. What you now want to do is figure out the distance between every prime and it's two neighbor primes. You can do this several ways. Because this is a homework question, I will not give a complete working solution.

n = 500; %maximum prime number to find
primes = 2;
for i = 1:n    %code I found to help identify primes 
    if mod(i+2,primes) ~= 0
        primes = [primes, i+2];
    end
end
inter_prime_distance=diff(primes);