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cant find the mistake here

Asked by asaf omer on 1 Jul 2019
Latest activity Edited by Stephen Cobeldick on 1 Jul 2019
hola,
i try to write a function that finds "twins prime" among 2 given values.
twins prime are prime numbers that the difference between them is 2
for example (3,5) , (5,7) and etc.
but somehow function provides me an empty vector.
can some1 please help with that tissue?
function [v]=twins(a,b)
for i =1:a
for j =1:b
if isprime (i) && isprime(j) && i+2==j
v=[v,j,i];
end
end
end
end
thanks!

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3 Answers

Answer by Stephen Cobeldick on 1 Jul 2019
Edited by Stephen Cobeldick on 1 Jul 2019

Well you made a good starting attempt, the main bug with your code are these ranges:
for i = 1:a
for j = 1:b
Using those ranges, do you ever generate pairs of values between a+1 and b ? (hint: no, and you can check this quite easily yourself by simply printing those values in the loops). You need to think about what values you actually need to loop over (hint: a:b)
However your two loop-concept is far too complex, and also very inefficient (think about what the two loops are doing, and how often their loop iterator variable values will differ by two (hint: extremely rarely)). A simplified, much more efficient version of your concept using one loop:
function m = twins(a,b)
m = nan(2,0);
for n = a:b-2
v = [n;n+2];
if all(isprime(v))
m = [m,v];
end
end
end
and tested:
>> twins(3,19)
ans =
3 5 11 17
5 7 13 19
Or no loops:
function m = twins(a,b)
m = [a:b-2;a+2:b];
m = m(:,all(isprime(m),1));
end
and tested:
>> twins(3,19)
ans =
3 5 11 17
5 7 13 19

  3 Comments

asaf omer on 1 Jul 2019
this guy gets it!
can u please explain me about nan and all orders?
in addition, what went wrong in my code?
"can u please explain me about nan and all orders?"
nan is used to define an empty 2x0 numeric array. I do not know what "orders" you refer to.
"in addition, what went wrong in my code?"
I explain this at the start of my answer.
Steven Lord
on 1 Jul 2019
As its documention page states, the all function is used to "Determine if all array elements are nonzero or true".
As Stephen uses it, the if is satisfied if all the elements of isprime(v) are true, or to reword it slightly if all the elements of v are prime.

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Star Strider
Answer by Star Strider
on 1 Jul 2019

Try this:
function v = twins(a,b)
v = [];
for i =1:a
for j =1:b
if isprime (i) && isprime(j) && abs(i-j)==2
v=[v;j,i];
end
end
end
end
The test should be ‘abs(i-j)==2’.

  1 Comment

asaf omer on 1 Jul 2019
provides, same solution as before

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Answer by Fangjun Jiang on 1 Jul 2019

The mistake is that you need to insert this line as the first line inside your function.
v=[];

  1 Comment

asaf omer on 1 Jul 2019
still doesnt work

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