how to use conditional statement realizing prime function
显示 更早的评论
Hi.
I want to use code to check if a number isprime. I know isprime () but I don't want to use it because I want to know how to use judgments to do tha.
Here is my code.
%function code
function[a,b] = isPrime(n)
% A number, n, is not prime if any number between 2 and n/2 divides the number without any remainder.
a = 1; % a = 1 means true
b = 0; % b = 0 means false
half = n/2; % get the half nums of n
for i = 2:half
if(mod(n,i)~=0) % if there is remainder
continue; % keep check
elseif(mod(n,i) == 0)
disp(b); % display false
break; % stop loop
end
disp(a);
end
end
% driver code
n = 9;
[x1,x2]= isPrime(n);
disp(isPrime(n));
Sample test:
isPrimedriver
0
0
1
It should output zero and stop looping, why it work twice?
And how to modify my code?
Thank you.
4 个评论
Walter Roberson
2022-6-17
example:
Consider 35. 35/2 is 17 1/2 so we will examine 2 to 17. 35 is not divisible by 2 so continue; 35 is not divisible by 3 so continue. 35 is not divisible by 4 so continue. 35 is divisible by 5 so 35 is not prime
Consider 37. 37/2 is 18 1/2 so we will examine 2 to 18. 37 is not divisible by 2 so continue. 37 is not divisible by 3 so continue. 37 is not divisible by 4 so continue. Not divisible by 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 or 18 either. We have reached the end of the 2 to 37/2 range so 37 is a prime.
Walter Roberson
2022-6-17
if(mod(n,i)~=0) % if there is remainder
continue; % keep check
elseif(mod(n,i) == 0)
Under which circumstances could both of those tests be false? And why did you not put an else clause for the case where both are false?
Shuoze Xu
2022-6-17
Walter Roberson
2022-6-18
编辑：Walter Roberson
2022-6-18
there are exceptions:
- mod(inf,i) is nan
- mod(nan,i) is nan
- problems if n is empty or not a scalar
采纳的回答
Your driver code is calling isPrime two times.
% driver code
n = 9;
The first time, you only get a disp(b) from inside isPrime
[x1,x2]= isPrime(n);
The second time, you get a disp(b) from inside isPrime and then, after isPrime is done, you also disp the returned value from isPrime, which is a
disp(isPrime(n));
%function code
function[a,b] = isPrime(n)
% A number, n, is not prime if any number between 2 and n/2 divides the number without any remainder.
a = 1; % a = 1 means true
b = 0; % b = 0 means false
half = n/2; % get the half nums of n
for i = 2:half
if(mod(n,i)~=0) % if there is remainder
continue; % keep check
elseif(mod(n,i) == 0)
disp(b); % display false
break; % stop loop
end
disp(a);
end
end
更多回答（1 个）
Walter Roberson
2022-6-17
Hint:
prime_detected = true
for appropriate things
if appropriate condition
prime_detected = false;
break;
end
end
2 个评论
Shuoze Xu
2022-6-17
Walter Roberson
2022-6-19
if prime_detected
a = "true";
else
a = "false";
end
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