Evaluating a function in subitervales and comparing the function' values of each interval with te next interval

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Mohamed Saleh 2018-2-7

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https://ww2.mathworks.cn/matlabcentral/answers/381212-evaluating-a-function-in-subitervales-and-comparing-the-function-values-of-each-interval-with-te-ne

回答： Walter Roberson 2018-2-7

Hi. Can any one help me with the following please. let's say I have these values x=[ 2,3,-1,-4,2,-6,...1000], I need to subdevide these valuse into subintervals let's say each subinterval has 10 elements. Then I want to evaluate a function F(x) in each subinterval of x and compare the valuse of the function of the first interval with the function's value of the next interval and so on. Thankful your favor inadvanced.

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Answer 1

Walter Roberson 2018-2-7

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https://ww2.mathworks.cn/matlabcentral/answers/381212-evaluating-a-function-in-subitervales-and-comparing-the-function-values-of-each-interval-with-te-ne#answer_303815

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The below is what you asked for. It seems unlikely to be what you want though.

   x=[ 2,3,-1,-4,2,-6,...
       1000];
   num_x = length(x);
   for xidx = 1 : num_x-1
     first_x = x(xidx);
     second_x = x(xidx+1);
     sub_intervals = linspace(first_x, second_x, 10);
     Fvals = zeros(1, 9);
     for interval_idx = 1 : 9
       this_subinterval = sub_intervals(interval_idx:interval_idx+1);
       Fvals(interval_idx) = F(this_subinterval);     %evaluate F on sub-interval ?
     end
     for interval_idx = 1 : 9
       fprintf('major interval %g:%g, sub_interval %g:%g, value ', first_x, second_x, sub_intervals(interval_idx:interval_idx+1));
       if Fvals(interval_idx) < F_vals(interval_idx+1)
         fprintf('increases\n');
       elseif Fvals(interval_idx) > F_vals(interval_idx+1)
         fprintf('decreases\n);
       elseif any(isnan(Fvals(interval_idx:interval_idx+1)))
         fprintf('is indeterminate because of nan\n');
       else
         fprintf('stays the same');
       end
     end
   end