Display matrix results with message

I'm new to Matlab, and just finishing up a semester of a programming class. This is only the second week of Matlab instruction. I have written a program that takes input from a file with 20 rows and one column of data. It performs calculations on that data, and creates a graph comparing the original data and the calculation, which is also stored in a matrix with one column being the original data, and the second column being the calculation. I can do all of that just fine. I can also print this output in columns to the screen. What I can't figure out is how to loop through the final output and print a message if the calculated data is above a certain value, in this case 125. So I'd like to print each row, and if the calculated data is above 125, also print a message. I had a tough time with looping in C, which was covered during most of this semester, but I figured it out as I progressed. There was a similar requirement on the last assignment which I turned in without having figured it out. Just need some guidance on how to approach this.

Answer 1

[Walter Roberson]

num_rows = size(YourMatrix, 1);
for row_index = 1 : num_rows
    if YourMatrix(row_index, 2) > threshold
       fprintf('Alert on flight deck #%d!\n', row_index);
    end
end

[Tommcgtx]

That works great to print a message under those conditions, and I think I understand what you did, thank you. How do I also print the data along with that? I want to print the output, and when the condition is met, also print that message with it.

[Walter Roberson]

num_rows = size(YourMatrix, 1);
for row_index = 1 : num_rows
    fprintf('Flight deck #%d reports hull damage %g\n', row_index, YourMatrix(row_index, 2));
    if YourMatrix(row_index, 2) => threshold1 & YourMatrix(row_index,2) < threshold2
       fprintf('Yellow alert on flight deck #%d!\n', row_index);
    elseif YourMatrix(row_index,2) >= threshold2
       fprintf('Red alert on flight deck #%d!\n', row_index);
    end
end

[Tommcgtx]

Thank you so much! Based on your first answer, I was able to play with the code and get it to work, fitting the variables etc to my situation. This is what I ended up with:

nrows = size(output, 1);
for ii = 1 : nrows
    if output(ii, 2) < 175
        fprintf('     %5.3f           %8.4f\n',output(ii,:))
    else if output(ii, 2) > 175
        fprintf('\n     %5.3f           %8.4f\n',output(ii,:))
        fprintf('Drag Force Exceeds Limit of 175N\n', ii);      
    end
    end
end

[Walter Roberson]

As a matter of style, we recommend using elseif instead of else if -- except, that is, in cases where you have something like

if condition1
    something1
end
else
    if condition2
        something2
    end
    something3
end

That is, if you have multiple actions for the else, some of which depend upon the condition being tested there but not all do, then it can make sense to use else if -- but even then you would typically want to break it into two lines like I did here.

That does happen sometimes. more of the time you have a structure like

if condition1
    something1
else if condition2
        something2
end
end

which is better written

if condition1
    something1
elseif condition2
    something2
end

[Walter Roberson]

Question for you:

Is it correct that if output(ii,2) is exactly 175, that you do not want to display the data at all, and you do not want to display any message then either?

If you have two cases, one of which is intended to include all the conditions under which the first case does not hold, then do not use if / elseif or if / else if/ -- use if and else

nrows = size(output, 1);
for ii = 1 : nrows
    if output(ii, 2) < 175
        fprintf('     %5.3f           %8.4f\n',output(ii,:))
    else
        fprintf('\n     %5.3f           %8.4f\n',output(ii,:))
        fprintf('Drag Force Exceeds Limit of 175N\n', ii);      
    end
end

I would point out that can be abbreviated to

nrows = size(output, 1);
for ii = 1 : nrows
    fprintf('     %5.3f           %8.4f\n',output(ii,:));
    if output(ii, 2) >= 175
        fprintf('Drag Force Exceeds Limit of 175N\n', ii);      
    end
end

[Tommcgtx]

I don't think the condition exists for exactly equal to 175 in this case. basically the idea is that a pipe crossing through a river can only take so much drag force, so if it exceeds 175N I needed to indicate that. I really appreciate your insight.

[Walter Roberson]

Note that your original code did not output any text if the value was exactly 175. You had if < 175 else if > 175 -- but if the value was exactly 175 then neither of those cases would have been true and nothing would have been output. Whereas the if/else version, the else would include all cases where the first was false, so if the first is < 175 then the else would include >= 175.