How to make a program to check if a number is a prime?

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Marin
Marin 2013-5-14
评论： Walter Roberson 2021-1-20
Hello I'm just starting with matlab and I have to make a program to check if a number is a prime. My pgrogramme looks like that but it ain't working.
function prime(x)
i= 3:2:sqrt(x) ;
if mod(x,2) == 0
disp ('is not a prime')
if mod(x,i) > 0
disp('is not a prime')
else
disp ('is a prime')
end
end
Can you help me make my code work please.
  1 个评论
Waseem Wasi
Waseem Wasi 2019-5-25

First condition is true but in second you to have write if(mod(x,2)==1) then this will be show a prime number

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回答（3 个）

Sean de Wolski
Sean de Wolski 2013-5-14
doc isprime
  2 个评论
Marin
Marin 2013-5-14
I know there's a program to do that, I have to make my own...
Jan
Jan 2013-5-14
@Marin: It is useful to mention such important details in the question already. Otherwise the forum users waste the time for mentioning the built-in functions.

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Jan
Jan 2013-5-14
Do you see the difference between:
if mod(x,2) == 0
disp ('is not a prime')
and
if mod(x,i) > 0
disp ('is not a prime')
In the first test you check, if the modulo returns 0. In the second, you want the result to be non-zero to decide, that x is not a prime. Suspicious.
Then the variable i is a vector. In consequence mod(x, i) replies a vector also. But the if-condition must be a scalar, as the English term "if" implies more or less. The functions all() and any() are useful in such cases.
You find a lot of useful links, when you search in the net for prime number algorithms. E.g. ask WikiPedia for Eratosthenes.
Anmol singh
Anmol singh 2021-1-20
Prime number - a number can devide by itself or 1 so now we can iterate loop 2 to n/2 , if n is devide by any number from 2 to n/2 then it is not prime .
loop : i = 2 <= n/2
if mod(n/i == 0)
disp ('is not a prime');
return ;
end if
i++;
end loop
disp ('it is a prime');
return ;
It is very nice trick to check prime number till n/2 .Not need to iterate 2 to n-1
Refrence :
How to Check number is prime or not
  1 个评论
Walter Roberson
Walter Roberson 2021-1-20
That is not MATLAB code. It is also not C or C++ code. I do not know what programming language it is. Certainly not java as might be hinted by the link to a java blog.
There is no need to iterate to n/2: you can stop iterating at floor(sqrt(n))

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