Trying to create a random date using randi and if-elseif-else statements

I'm currently attempting to write code that can generate a random date. However I am running into issues while writing a conditional statement to ensure that the correct amount of days are in each month. (To ensure I dont get the 31st of Feb as an answer for example.) This is what I have so far, later I wish to generate a day and then assign the month and day into a 1x2 matrix. This always gives me "days = 31" no matter what. I also want to mention that I desire to use a conditional statement for this.

function dates = dategen(N)
    month = 0;
    days = 0;
    month = randi(12)
    if month == 1 || 3 || 5 || 7 || 8 || 10 || 12
        days = 31
    elseif month == 4 || 6 || 9 || 11
        days = 30
    else 
        days = 28
end

Answer 1

[KSSV]

theday = randi(31) ;   % random day out of 31 days 
themonth = randi(12) ;  % random month out of 12 months 
theyears = 1980:2022 ;  % give the list of years you want 
theyear = theyears(randperm(length(theyears),1)) ; % pick random year 
thedate = datetime(theyear,themonth,theday)

thedate = datetime
   01-Oct-1989

Or other option is generate all the dates and pick from it randomly.

thedates = (datetime(1980,1,1):days(1):datetime(2022,4,8))' ;
thedate = thedates(randperm(length(thedates),1))

thedate = datetime
   18-Oct-1985

Answer 2

[Voss]

To understand why your code gives you days = 31 every time, consider the following:

% Case 1: seems to work ok
month = 1;
if month == 1 || 3
    disp('month is 1 or 3')
else
    disp('month is not 1 and not 3')
end

month is 1 or 3

% Case 2: apparently wrong
month = 2;
if month == 1 || 3
    disp('(month is 1) or (3)')
else
    disp('(month is not 1) and (not 3)')
end

(month is 1) or (3)

% Case 3: correct
month = 2;
if month == 1 || month == 3
    disp('(month is 1) or (month is 3)')
else
    disp('(month is not 1) and (month is not 3)')
end

(month is not 1) and (month is not 3)

[Ryan Peen]

Thankyou, now I think I understand. I will instead have to write "month == " after each ||. Otherwise it will recognise it as either 1 or 3

[Voss]

Consider the fact that month == 1 || 3 is the same as 3 || month == 1

The 3 is treated as its own condition which is always true, since 3 is not 0.

What you really mean is month == 1 || month == 3, so yes you have to compare month to the value each time, month == 1 || month == 3 || month == 5 etc.

You could also use ismember:

if ismember(month,[1 3 5 7 8 10 12])
    days = 31;
elseif ismember(month,[4 6 9 11])
    days = 30;
else
    days = 28; % but what about leap years?
end

[Ryan Peen]

I have everything now working, it now can generate a day and month in a 1x2 matrix, thankyou for the help!

The only thing I have to do now is to call the function to generate N amount of dates. I desire to have the output be a Nx2 matrix of N amount of random dates like the example below when calling on the function as "output = dategen(3)"

output =
    28    2
    14    1
    11    7

Is there a simple way to do this when calling the function? Or do I have to repeat the function N times in a loop and somehow append the results into a matrix? This is my current code for context

function dates = dategen(N)
        month = randi(12);
    
        if month == 1 || month ==  3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12
            day = randi(31);
            date = [day, month]
        elseif month == 4 || month == 6 || month == 9 || month == 11
            day = randi(30);
            date = [day, month]
        else
            day = randi(28);
            date = [day, month]
end

[Rik]

Note that for cases like this, the ismember function is very useful.

[Voss]

output = dategen(3)

output = 3×2

1 6 23 2 20 9

function dates = dategen(N)
    
    % initialize Nx2 matrix of zeros:
    dates = zeros(N,2);
    
    % loop N times:
    for ii = 1:N
        
        month = randi(12);
        if month == 1 || month ==  3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12
            day = randi(31);
            date = [day, month];
        elseif month == 4 || month == 6 || month == 9 || month == 11
            day = randi(30);
            date = [day, month];
        else
            day = randi(28);
            date = [day, month];
        end
        
        % set the ii-th row of the output dates to be 
        % the day and month just generated:
        dates(ii,:) = date;
        
    end
    
end