What is wrong with for loop iteration that is put in a function?

Question

Kenichi 2022-9-11

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1802835-what-is-wrong-with-for-loop-iteration-that-is-put-in-a-function

评论： Kenichi 2022-9-11

Hi! I'm wondering on this code I made:

a = input('Please insert a: ')
b = input('Please insert b: ')
arr = []
for i = a:b-1
    for j = a+1:b
        if and(composite(i), composite(j), composite(i+j))
            arr = [arr; i j]
        end
    end
end
arr

and I got this error message:

Output argument "bool" (and possibly others) not assigned a value in the

execution with "composite" function.

Error in untitled10 (line 6)

if and(composite(i), composite(j), composite(i+j))

I did make a composite function:

that run perfectly but when I put iteration numbers in it, it gives the error message before

function bool = composite(n)
    for i = 2:n
        for j = 2:n
            if i*j == n
                bool = true;
             end
        end
    end
    if n ==2
     bool = false;
    elseif n ==1 
     bool = false;
    end
end

Thank you!

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Stephen23 2022-9-11

在 MATLAB Online 中打开

There are lots of values of n for which the output is not defined: 0, 3, 5, 7, ...

b = composite(7)
Output argument "bool" (and possibly others) not assigned a value in the execution with "solution>composite" function.
function bool = composite(n)
for i = 2:n
    for j = 2:n
        if i*j == n
            bool = true;
        end
    end
end
if n ==2
    bool = false;
elseif n ==1 
    bool = false;
end
end

Kenichi 2022-9-11

Thank you!

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Answer 1

Jan 2022-9-11

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1802835-what-is-wrong-with-for-loop-iteration-that-is-put-in-a-function#answer_1051235

编辑：Jan 2022-9-11

在 MATLAB Online 中打开

function bool = composite(n)
bool = false;  % Create a default value
if n > 2
    for i = 2:n
        for j = 2:n
            if i*j == n
                bool = true;
                return;  % No need for further tests
            end
        end
    end
end
end

Your function composite() computes the same as the much faster Matlab function isprime() except for the input 2. The function composite can be improved: if e.g. i*j > n, the inner loop can be stopped by a break.

This command will fail at next, because Matlab's and() accept 2 inputs only.

if and(composite(i), composite(j), composite(i+j))

Maybe you mean

if composite(i) && composite(j) && composite(i+j)