Keep getting the error "Index exceeds the number of array elements (1)"

Can someone please explain why I keep getting this error? Here is my code:

for Count = 1:100
     fprintf('Count = %d\n', Count');
end 
 
Wait = input('Press return to continue', 's');
 
x = 1;  
while x <= 100        % test Count
     disp(x)
     x = x + 1;  % modify Count
end
 
disp('Prime Numbers')
for num = 1:100
    i = isprime(num);
    primenum = num(i);
    disp(primenum);
end 

It is showing me that the error is located at the "primenum = num(i);" I am not sure why.

Answer 1

[Arjun]

Hi @Alexis Williams,

I see that you are getting this error “Index exceeds number of array elements(1)”. It is also mentioned that the potential cause of the error is the line “primenum = num(i);”.

From the flow of your program, it can be interpreted that you want to display all the Prime Numbers between 1 to 100.

Let’s have a look at the problematic code snippet:

for num = 1:100
    i = isprime(num);
    primenum = num(i);
    disp(primenum);
end 

In the code above “num” is a variable which is supposed to be 1x1 dimensional variable i.e. a scalar and indexing scalars is not a good idea. The error creeps in due to this reason only that we are trying to index a scalar variable.

Instead, you can modify the snippet as follows to be error free:

for num = 1:100
    if isprime(num)
        disp(num);
    end
end

This way we make sure that we don’t index any scalar variable and avoid any potential errors.

I hope it will help!