Unexpected printf() behavior in Octave for-loop

Question

I just recently switched from MATLAB to Octave because my license ran out and I wanted to give it a shot before renewing, so I can't check the behavior in MATLAB right now. I am not sure if I am making a grave mistake here, but I am having some headache over a for-loop in Octave.

The loop was supposed to find the first "large" change ( 0.08 ) in the values of one of the columns (column 5 in this case) of a csv-file I previously read with dlmread() , and then return to me the index of the first change so I can discard all the data before this first big change occurs for my further computations.

While trying to solve this I remembered that one should avoid for-loops in MATLAB (and probably Octave as well), and I managed to get the result I want by just doing

idx = find(diff(gpsdata(:,5)) > 0.08, 1);

which is obviously much cleaner (and probably faster too?) and I will stick with it; however, I am still wondering why the for-loop is showing this weird behavior.

Is there a mistake in my code, or is that just a sign of why for-loops should be avoided in Octave/MATLAB? Are for-loops parallelized in Octave/MATLAB, is that why this is happening?

Code

for (jj = 1:(size(gpsdata, 1) - 1))
  if (gpsdata(jj, 5) + 0.08 < (gpsdata(jj+1, 5)))
    idx = jj;
    disp(["jj=" num2str(jj) ", idx=" num2str(idx)]);
    printf("Found index %d, at %f s real time\n", num2str(idx), gpsdata(idx, 2));
    break;
  end
end

Actual Output:

jj=380, idx=380
Found index 51, at 56.000000 s real time
Found index 48, at 19.770000 s real time

Expected Output:

jj=380, idx=380
Found index 380, at 19.770000 s real time

The disp() call produced the correct output, while the printf() is somehow executed twice on top of returning wrong values, with only the fourth of the 4 arguments being correct (the 19.770000 is the expected output here); the other 3 values ( 51 , 56.000000 , and 48 ) are wrong.

Data for Reproducing

The data I am using for gpsdata can be found here , and read in with data=load("~/gpsdata.mat"); followed by gpsdata=data.gpsdata; .

Answer 1

As pointed out by @rahnema1, I was passing a string instead of an int by doing printf("%d", num2str(idx)) so Octave was printing the ascii codes for "380" , which happen to be 51 , 56 , 48 .

Doing printf("%d", idx) yields the expected behavior.

Thanks for catching that!