Problem 44952. Find MPG of Lightest Cars
Mayla
on 10 Sep 2023
Latest activity Reply by Walter Roberson
on 24 Oct 2023
That's the question:
The file cars.mat contains a table named cars with variables Model, MPG, Horsepower, Weight, and Acceleration for several classic cars.
Load the MAT-file. Given an integer N, calculate the output variable mpg.
Output mpg should contain the MPG of the top N lightest cars (by Weight) in a column vector.
I wrote this code and the resulting column vector has the right values but it doesn't pass the tests. What's wrong?
function mpg = sort_cars(N)
load cars.mat
sorted=sortrows(cars,4)
mpg = sorted(1:N,2)
end
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Hello Mayla,
The reason your code does not pass the test suite is because the variable mpg in your code is of Table data type, whereas the question expects the answer in Numeric data type.
This will be helpful - https://in.mathworks.com/help/matlab/matlab_prog/access-data-in-a-table.html
Also, I would suggest you to check the assert() call while solving problems in Cody to see what and which type of output is expected.
Right.
The most common operating system these days is still Windows, and the most common disk file system on Windows is NTFS. NTFS file systems are case insensitive by default -- so if you have a stored NTFS file named cars.mat and you ask to load Cars.mat then it is likely to succeed.
MacOS file systems HFS+ and the newer APFS, also default to case insensitive -- but it is more common for users to deliberately switch them to case sensitive than is the case for Windows.
Mathworks' online systems such as MATLAB Online and MATLAB Grader run on Linux -- and Linux filesystems default to case sensitive
Filenames are case-sensitive.
N=4
sort_cars(N)
N = 4
Error using load
Unable to find file or directory 'Cars.mat'.
Error in sort_cars (line 3)
load Cars.mat
Im solving this problem but im getting an erro :
The file cars.mat contains a table named cars with variables Model, MPG, Horsepower, Weight, and Acceleration for several classic cars.
Load the MAT-file. Given an integer N, calculate the output variable mpg.
Output mpg should contain the MPG of the top N lightest cars (by Weight) in a column vector.
function mpg = sort_cars(N)
load Cars.mat
cars = sortrows(cars,[ "Weight"],'ascend')
mpg = cars.MPG{1:N}';
end
I did suspect it was related to the recent update, but was unable to find how so.
This problem should be working again as the path in the test suite has been updated. The folder statsdemos in 2023a (and all previous versions?) was changed to statsdata in 2023b, which was recently released.
Hi Image Analyst,
The following is one of the test code, which returns the error I mentioned above:
%%
N = 5
load(fullfile(matlabroot, 'toolbox/stats/statsdemos', 'carbig.mat'));
Model = strtrim(string(Model));
cars = table(Model, MPG, Horsepower, Weight, Acceleration);
save cars.mat cars
assert(isequal(sort_cars(N),[35; 31; 39.1; 35.1; 31]));
Acutally the question did require us to load cars.mat, and I tried to load carbig.mat in the function. My guess is the same as Joshi.
Bojun
Exactly what line of code is trying to load carbig.mat? All I see is a line of code trying to load cars.mat.
To convert a table column(s) into an array, use table2array
s = load('carbig.mat')
t = struct2table(s);
t = sortrows(t, 3, 'descend')
nanRows = isnan(t.MPG);
t(nanRows, :) = []; % Get rid of nans.
% Print out the 10 highest MPG cars
for k = 1 : 10
fprintf('%2d : %s has an MPG of %g.\n', k, t.Model(k, :), t.MPG(k));
end
Hi Bojun,
"but it seems the data cannot be found"
Yes, there seems to be an internal problem with the test suite.
I'll report this to the Cody Team so that they can check what the issue is and resolve it. I'll update you when I get the information.
For now, continue with solving other problems. Thank you for reporting the issue.
Hi Joshi,
I tried to solve this question but it seems the data cannot be found, and the error occurs as following:
function mpg = sort_cars(N)
load cars.mat
sorted = sortrows(cars, 4);
mpg = sorted(1:N,2);
mpg = mpg{:, :};
end
Error using load Unable to find file or directory '/MATLAB/toolbox/stats/statsdemos/carbig.mat'. Error in Test1 (line 3) load(fullfile(matlabroot, 'toolbox/stats/statsdemos', 'carbig.mat'));
Does it mean the data is lost? I tried some example data in the MATLAB IDE but it works well, which outputs a colum vector.
How should I improve? Many Thanks.
Bojun
