Array indices must be positive integers or logical values. Error in Untitled2 (line 11) '_', Station_ids (i),...

it keeps saying Unrecognized function or variable 'my_url'.

Error in LabFEECS (line 10)

gauge_data = webread(my_url);

and

Array indices must be positive integers or logical values.

Error in Untitled2 (line 11)

'_', Station_ids (i),...

and this is my code

my_url = strcat('https://', base_url, province, '/', frequency, '/', ...

province, '_', Station_ids (i), ...

'_', frequency, '_hydrometric.', file_type);

[Walter Roberson]

For the first part, we would need to see more of the code -- and we would also need to know how you are invoking the file.

For the second part: the code has not assigned a value to the variable i at that point, so as far as the code knows, i is still the function that returns sqrt(-1)

Answer 1

[Siraj]

Hi!

It is my understanding that you’re trying to fetch data from a web service using MATLAB's “webread” function, which requires a correctly formatted URL as input.

You're building the URL called “my_url” by combining parts of a web address with an element from the "Station_ids" array using the “strcat” function.

The error messages that you’ve shared indicate that there are issues with how “my_url” is being created and subsequently used in the “webread” function. The key problem is that the index “i” you're using to access elements in the "Station_ids" array is not a valid index. In MATLAB, array indices need to be positive integers or logical values.

As mentioned in the first comment “i” by default takes a complex value. To confirm this execute the following in the Command Window

disp(i)

   0.0000 + 1.0000i

If “i” is not a positive integer, MATLAB will not be able to use it to index into “Station_ids”, and as a result, “my_url” won't be created.

To resolve this, ensure that “i” is set to a valid index within the bounds of the “Station_ids” array before you use it to build “my_url”. Here's how you might correct the code:

% Setting up some variables for the URL construction
base_url = 'example.com';
province = 'SomeProvince';
frequency = 'daily';
file_type = 'csv';
Station_ids = [101, 202, 303]; % Example station IDs
 
% Ensure i is a positive integer within the bounds of Station_ids
for i = 1:length(Station_ids)
    % Constructing the URL
    my_url = strcat('https://', base_url, province, '/', frequency, '/', ...
     province, '_', num2str(Station_ids(i)), ...
     '_', frequency, '_hydrometric.', file_type);
 
    % Now you can use my_url in webread
    disp(my_url)
    % gauge_data = webread(my_url);
    % Process the gauge_data as needed
end

https://example.comSomeProvince/daily/SomeProvince_101_daily_hydrometric.csv
https://example.comSomeProvince/daily/SomeProvince_202_daily_hydrometric.csv
https://example.comSomeProvince/daily/SomeProvince_303_daily_hydrometric.csv

For more information on the “webread” function, you can visit the following link

https://www.mathworks.com/help/matlab/ref/webread.html

Hope this helps.