Why is this logical "&&" passing the "if" statement?

My below code determines valid dates. I am not looking for the answer to the code, but rather just some insight as to why my logical functions are working incorrectly. I've inserted 4/31/2018 as my date and it passes on line checking 31 day months below. The 31 days meets criteria, but the months returns a logic array of zeros so it shouldn't be valid. Yet it continues to stop there when debugging. Why is this?

I'm using whatever the most current online Matlab software is.

function valid = valid_date(year,month,day)
    if ~isscalar(year) || year < 1 || year ~= fix(year)
        valid = false;
        return
    end
    if ~isscalar(year) || year < 1 || year ~= fix(year)
        valid = false;
        return
    end
    if ~isscalar(year) || year < 1 || year ~= fix(year)
        valid = false;
        return
    end
    if year/4 && (year/100 && year/400) && (month == 2 && day <= 29)
        valid = true;
    elseif month == 2 && day <= 28
        valid = true;
    elseif month == [1 3 5 7 8 10 12] && day <= 31 %passing here when it should move to next "elseif"
        valid = true;
    elseif (month == [4 6 9 11]) || (day <= 30)
        valid = true;
    else
        valid = false;
    end
end

I've edited the code and it passes on that line. However in the case of 4/1/2018 it fails on the following line where it should pass.

function valid = valid_date(year,month,day)
    if ~isscalar(year) || year < 1 || year ~= fix(year)
        valid = false;
        return
    end
    if ~isscalar(year) || year < 1 || year ~= fix(year)
        valid = false;
        return
    end
    if ~isscalar(year) || year < 1 || year ~= fix(year)
        valid = false;
        return
    end
    if year/4 && (year/100 && year/400) && (month == 2 && day <= 29)
        valid = true;
    elseif month == 2 && day <= 28
        valid = true;
    elseif (month == [1 || 3 || 5 || 7 || 8 || 10 || 12]) && (day <= 31)
        valid = true;
    elseif (month == [4 || 6 || 9 || 11]) && (day <= 30)
        valid = true;
    else
        valid = false;
    end
end

Updated the code and now runs smooth:

function valid = valid_date(year,month,day)
    if ~isscalar(year) || year < 1 || year ~= fix(year)
        valid = false;
        return
    end
    if ~isscalar(month) || month < 1 || month ~= fix(month)
        valid = false;
        return
    end
    if ~isscalar(day) || day < 1 || day ~= fix(day)
        valid = false;
        return
    end
    if month == 2
        if day < 29
            valid = true
        elseif mod(year,4) == 0 && mod(year,100) == 0
            if mod(year,400) == 0
                if day <= 29
                    valid = true;
                end
            else
                valid = false;
            end
        elseif mod(year,4) == 0
            if day <= 29
                valid = true;
            else
                valid = false;
            end
        else
            valid = false;
        end
    elseif mod(year,4) == 0 && (month == 2 && day <= 29)
        valid = true;
    elseif month == 2 && day <= 28
        valid = true;
    elseif (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) && (day <= 31)
        valid = true;
    elseif (month == 4 || month == 6 || month == 9 || month == 11) && (day <= 30)
        valid = true;
    else
        valid = false;
    end
end

[Torsten]

What do you expect is the result of

if year/4 && (year/100 && year/400)

? It will always return true.

Further, month is a scalar, not a vector.

Thus

elseif month == [1 3 5 7 8 10 12]
elseif (month == [4 6 9 11])

will always return false.

[Andrew]

Thanks for the response! However that first reponse does not always return true, but rather false and therefore my debugger skipped it and passed to the next portion of code successfully. Just a note on this comment. Second part was helpful though.

Answer 1

[the cyclist]

Your code suggests that you think

if year/4

is checking if year is divisible by 4. But that is not what the syntax is doing. I think you want

if mod(year,4)==0

Also,

1 || 3 || 5 || 7 || 8 || 10 || 12

ans = logical
   1

is definitely not doing what you expect.

I think you want something like

ismember(month,[1 3 5 7 8 10 12])

Sorry to sound harsh, but it seems that you are just coding in a way that seems to make sense to you, but doesn't actually correspond to MATLAB syntax.

[Andrew]

Yea my debugger was checking it as false and skipping. I used the following coide to correct the second part:

if (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) && (day <= 31)

I was wondering if there was a line that would make checking if a value is equal to at least one value in a vector easier? Basically I'd like to check if one value matches and return a logical scalar. My code works for my problem, but this is for curiosity.

[the cyclist]

I think I was editing my answer while you were asking this question, to show you exactly that:

ismember(month,[1 3 5 7 8 10 12])

will be true if month is one of those values.

[the cyclist]

You might find the free MATLAB Onramp tutorial useful.