Problem 14. Find the numeric mean of the prime numbers in a matrix.
Solution Stats
Problem Comments

12 Comments
2 is prime, so the example solution should be (2+3+5)/3 = 3.33....
+1, the example is wrong :(
*blushing* Oops! At least we got the actual test suite right. Thanks for the notes. Fixed it.
i didn't understand the problem
good one...
I don't understand why this doesn't work for test 3: out=sum(in.*isprime(in))/sum(isprime(in))
it works for all other tests but test 3 requires an answer of 3 and the code gives an answer of 3.0000. Can anyone tell me why?
funny :)
good
Good one
The question topic is hard to understand.
Good Problem for beginners.
What is wrong with
a = find( isprime( in))
out = mean( in( a)) ?
It works perfectly in my own MATLAB environment
Solution Comments

2 Comments
the 3rd, I get result is 3.0000 on my computer, but it failed, i am very confused now.
prime = isprime(in);
out = sum(in.*prime)/sum(prime);
This happens because MATLAB works in floating point arithmetic, and not exact (decimal) arithmetic.
You can do a quick check in your computer on MATLAB  sprintf('%0.18f\n', out), after computing out and you'll see that the value is not exactly 3.

2 Comments
tf=isprime(in);
count=nnz(tf);
tot=sum(sum(tf.*in));
out =tot/count;
why is this code not working
good!

2 Comments
how can the mean of the primes of [ 1 2 3] be 2.5?
The mean of the primes which are 1 & 3 is 2.
1 is not a prime number, so therefore 2+3=5, 5/2=2.5

1 Comment
Why this is happening,
3
Fail
x = [3 3; 3 3];
y_correct = 3;
assert(isequal(meanOfPrimes(x),y_correct))
out = 3.0000
Assertion failed.
Why?

2 Comments
very easy
weird flex, but ok

1 Comment
gj

1 Comment
cheating

1 Comment
I also had problems with test 3, but with this code if've passed all tests:
function out = meanOfPrimes(in) mean = sum(in.*isprime(in))/sum(isprime(in)); out = round(mean,1); end

3 Comments
in= [3 3; 3 3]
in =
3 3
3 3
>> sum(in.*isprime(in))/sum(isprime(in))
ans =
3.0000
The fault is in two respects:
(1) the numerator and denominator for Test 3 will be vectors, not scalars, because that's how sum() works;
(2) the MATLAB algorithm for matrix division is therefore being employed, and it has introduced a truncation error. Your code yields a purported answer of 3 – (4.4409E–16) for Test 3. Of course, if only a few decimal places are displayed, this is shown as "3.0000".
To avoid this problem, reshape the input matrix to a vector. This can be done with the reshape command, but an easier way is to simply index as "in(:)".
This is done in https://au.mathworks.com/matlabcentral/cody/problems/14findthenumericmeanoftheprimenumbersinamatrix/solutions/1142672
Another effective (but somewhat less elegant) way of avoiding the problem is to nest your summations: sum(sum(sum( ... ))). (But you must use at least as many "sum" command as your matrices have dimensions.)
For debugging, try using the whos command.
@David Verrelli
Thanks! It works.

2 Comments
where is the problem here? It works on matlab.
the problem is that you cannot change the function name, try to use meanOfPrimes and it will work!

2 Comments
The code I wrote works on my MATLAB. Why is it that it shows that my solution is incorrect?
mean2 is a function from Image Processing Toolbox. Cody only supports MATLAB, not the toolboxes.

1 Comment
Why did the 3rd test fail?It's working in Matlab.

1 Comment
*slow clap*
you sir, are a c programmer...

1 Comment
I tried using this method and it was success.. but.. :(

2 Comments
no reason this shouldn't work...
The problem is that when the sum function is applied to a matrix it produces a vector instead of a scalar.

1 Comment
This should work, but it fails the third test as it produces a value for ans that is 4.409e16 out due to rounding errors.

3 Comments
Why is this solution not correct? It works in case 3 also if i use it in Matlab. I don't understand this.
In case 3 the argument is a matrix, so the sum functions return vectors instead of scalars (which are what the problem wants). The / operator acting on vectors solves a least squares problem, whose answer happens to be 3 in this case (by chance), but the numerical calculation does not give exactly 3 (try subtracting 3 from the solution).
if you use sum() why won't you use simply mean()?

1 Comment
C'mon, Cody Challenge writers! At least provide a correct example  2 is prime (except where prohibited by law.)
Problem Recent Solvers8064
Suggested Problems

2444 Solvers

Remove the polynomials that have positive real elements of their roots.
1255 Solvers

Back to basics 6  Column Vector
1013 Solvers

Given a matrix, swap the 2nd & 3rd columns
845 Solvers

Simple equation: Annual salary
4071 Solvers
More from this Author96
Problem Tags
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!