Prime numbers from 2 till hundred?

How to use control flow statements that returns all the prime numbers from 2 till hundred?

[KSSV]

iwant = primes(100)

[Jan]

What exactly are "control flow statements"?

[Walter Roberson]

"for" and "if" and "elseif" and "while" and "break" and "continue" are control-flow statements, along with "switch" and "case"

[Jan]

@Walter Roberson: Thanks.

@Ahmad Omar: This sounds like a homework question. As usual: Please post, what you have tried so far and ask a specific question.

[Steven Lord]

I'd classify most if not all of the functions in the Loops and Conditional Statements category in the documentation as control-flow statements. [return, parfor, and otherwise are the main additions to Walter's list. I mainly wanted to point out the existence of the category documentation pages.]

[Jan]

Is the question meaningful? Is it possible to calculate prime numbers with control flow statements only, or without control flow statements? Maybe a flow chart is meant?

Answer 1

[Rushil]

Hi Ahmad

I believe that this task can be accomplished using a sieve of eratosthenes using control flow statements like for/if/continue etc. The sieve works in O(n log logn) time, which is usually fast enough for small values of “n” (in this case n=100). Below is the implementation of the algorithm, it stores all the prime numbers in the array “prime”:

n = 100; % range of numbers is [2,n] 
prime = []; 
is_pr = ones(1,n); 
for i=2:n 
  if ~is_pr(i) 
     continue 
  end
  prime = [prime i]; 
  for j=2*i:i:n 
    is_pr(j) = 0; 
  end 
end 
prime 

Since the task is concerned with using control flow statements, the above code finds all the primes in [2,n]. However, a more optimised approach may be to use the MATLAB function “primes”. You can read more about it here:

https://mathworks.com/help/matlab/ref/primes.html

Hope it helped.

larush