Using an if and for loop saving a part of a table

Question

Zach 2023-6-5

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https://ww2.mathworks.cn/matlabcentral/answers/1978779-using-an-if-and-for-loop-saving-a-part-of-a-table

回答： Shishir Reddy 2023-6-16

I have two matrices of tables. All the tables in y are in x. I am trying to pick out the times when x and y are happening the same time. I have it set up but I am not sure how to save part of the table. I think I am close but what am I missing? Can I do it like this: X_new{a,b}=X{a,1}(initial); or do I need to log the overlap then save the table afterwards.

Here is the code (the bolded part is the most critical part):

for a=1:Input1
    for b=1:Input2
        
        length_x=length(X{a}.time);
        length_y=length(Y{a,b}.Time);
        up=1;
        
        for initial=1:length_X %Looking through data
            
            if Y{a,b}.Time(count)==X{a,1}.time(initial)
                
                X_new{a,b}=X{a,1}(initial);
                count=count+1;
            end
            
        end
        
    end
end

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KSSV 2023-6-6

Read about ismember

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Answer 1

Shishir Reddy 2023-6-16

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https://ww2.mathworks.cn/matlabcentral/answers/1978779-using-an-if-and-for-loop-saving-a-part-of-a-table#answer_1257164

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Hi Zach,

As per my understanding, you want to pick out the times when x and y are happening the same, in better possible way.

However there are a couple of minor issues in your code that need to be addressed.

The variable ‘length_X’ should be ‘length_x ‘(lowercase 'x') to match the variable assignment in the line above.
You need to initialize the variable ‘count’ before entering the loop. Add ‘count = 1;’ before the inner loop.
To save the part of the table where the times match, you can't directly assign it as X_new{a,b} = X{a,1}(initial). Instead, you can use the table function to create a new table with the desired rows.

Here’s the code using ismember() function:-

X_new = cell(Input1, Input2); % Initialize X_new cell array
for a = 1:Input1
    for b = 1:Input2
        x_times = X{a, 1}.time;
        y_times = Y{a, b}.Time;
        
        % Find the common times using ismember
        common_times = x_times(ismember(x_times, y_times));
        
        % Create a new table with the common times from X{a,1}
        if ~isempty(common_times)
            X_new{a, b} = X{a, 1}(ismember(x_times, common_times), :);
        end
    end
end

In this modified code, we first extract the time values from ‘X{a,1}’ and ‘Y{a,b}’ into ‘x_times’ and ‘y_times’, respectively. Then, we use the ‘ismember’ function to find the common times between ‘x_times’ and y_times. The result is stored in the ‘common_times’ variable.
Next, we check if there are any common times (‘~isempty(common_times)’), and if so, we use ‘ismember’ again to select the corresponding rows from ‘X{a,1}’ using ‘ismember(x_times, common_times)’. This gives us a new table with the rows that have matching times. We assign this new table to ‘X_new{a, b}’.
Note that ‘X_new{a, b}’ will be empty (i.e., ‘[]’) if there are no common times between x_times and y_times
Using ismember simplifies the code and avoids the need for nested loops and manual comparisons. It directly finds the matching times and extracts the corresponding rows from X{a,1} using logical indexing.