Cody Problem 14. Find the numeric mean of the prime numbers in a matrix.

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I found difficulties with the 14th Problem on Cody here is my solution:
function out = meanOfPrimes(in)
[a,b]=size(in)
sum = 0;
for i = 1:a
for j = 1:b
if isprime(in(i,j))
in(i,j) = str2num(in(i,j))
sum = sum + in(i,j);
end
end
end
out = sum/2;
end
Unfortunately it doesn't work, can anyone help me with that!
Thanks in advance.
  1 Comment
Geoff Hayes
Geoff Hayes on 31 Mar 2021
Hidd_1 - why do you do the following
in(i,j) = str2num(in(i,j))
Isn't the in array already numeric? Won't this fail if already numeric? What is your intention here?
Also, try to avoid creating variables whose name matches those of in-built MATLAB functions. In this case, that would be sum.

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Accepted Answer

David Hill
David Hill on 31 Mar 2021
Input is not a string.
function out = meanOfPrimes(in)
[a,b]=size(in)
sum = 0;
c=0;
for i = 1:a
for j = 1:b
if isprime(in(i,j))
%in(i,j) = str2num(in(i,j))
sum = sum + in(i,j);
c=c+1;%need to keep track of how many
end
end
end
out = sum/c;%mean is sum of primes/number of primes
end
Alternatively, linear index into matrix
function out = meanOfPrimes(in)
sum = 0;
c=0;
for i = 1:numel(in)
if isprime(in(i))
sum = sum + in(i);
c=c+1;
end
end
out = sum/c;
Or better yet, no loop
out=mean(a(isprime(a)));

More Answers (3)

the cyclist
the cyclist on 31 Mar 2021
After you correct the problem that @Geoff Hayes pointed out in his comment, you still have a mathematical one.
out = sum/2
is not the correct way to calculate the mean value from the sum of the prime values.
Is that enough of a hint?
Also, I'll just point out that you did not need to use the for loops here. You could have applied isprime to the whole input array at once, harnessing the vectorized nature of MATLAB.
  1 Comment
Hidd_1
Hidd_1 on 31 Mar 2021
Edited: Hidd_1 on 31 Mar 2021
Thanks I solved the problem! Here is my solution:
function out = meanOfPrimes(in)
[a,b]=size(in);
s = 0;
k=0;
for i = 1:a
for j = 1:b
if isprime(in(i,j))
s = s + in(i,j);
k = k+1;
end
end
end
out = s/k
end
[s] I wonder how can I solve it using only isprime? [/s]

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Hernia Baby
Hernia Baby on 31 Mar 2021
As Geoff Hayes says, variable 'in' is NOT string type.
You can check the data type with class function.
in = [8 3 5 9];
class(in)
ans =
'double'
ーーーーーーーーーーーーーー
Therefore, it will work well when omitting str2num.
clear,clc;
in = [8 3 5 9];
[a,b]=size(in);
sum = 0;
for i = 1:a
for j = 1:b
if isprime(in(i,j))
in(i,j) = in(i,j);
sum = sum + in(i,j);
end
end
end
out = sum/2
out =
4

Abhinav
Abhinav on 26 Apr 2023
function out = meanOfPrimes(in)
primeTF = isprime(in);
primeTFIndex = find(primeTF);
x = [];
for i = 1:length(primeTFIndex)
x = [x, in(primeTFIndex(i))];
end
out = mean(x);
end

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