# I want to plot a graph for a function which varies according to the range of the variable (Like a piecewise function)

2 vues (au cours des 30 derniers jours)
sai venkat le 2 Juil 2021
Commenté : sai venkat le 2 Juil 2021
function trial2
c=2800;c1=1200;
pshi_i=-180:0.5:180;
t_i=50-(5/18)*pshi_i;
pshi=-180;
if pshi>=-55||pshi<=55
Pt_i=c*cosd(pshi);
pshi=pshi+0.5;
elseif pshi<-55||pshi55
Pt_i=0;
pshi=pshi+0.5;
end
plot(t_i,Pt_i,'*')
end
I tried with this code but the function just stick to the first value of function i give irrespective of the condition mentioned(has become a constant function rather than a varying piecewise function). Have been trying for the past 2 days but the result is the same.
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### Réponse acceptée

Yazan le 2 Juil 2021
That is because your first condition is the most general (or inclusive). The second condition is satisfied when the tested value is less than -55, but any value less than -55 is less than 55, so the first condition is satisfied (note that you are using the operator OR in the conditions) and Matlab never tests the second condition.
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sai venkat le 2 Juil 2021
I have tried using for loop but it just gives out the final value of the function for the final value of the variable. Can you suggest the best way of approach for this problem.
Yazan le 2 Juil 2021
It is not about the looping. Your conditions are not well-posed. Just write your conditions mathematically first for us to be able to help.

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### Plus de réponses (1)

Steven Lord le 2 Juil 2021
You can operate on a vector of values using logical indexing with a mask for each of your conditions.
% x data
d = 0:360;
% Preallocate the y data
y = zeros(size(d));
% Now select each quadrant in turn
quadrant1 = (0 <= d) & (d <= 90);
y(quadrant1) = sind(d(quadrant1));
quadrant2 = (90 <= d) & (d <= 180);
y(quadrant2) = 1 + cosd(d(quadrant2));
quadrant3 = (180 <= d) & (d <= 270);
y(quadrant3) = 1 + sind(d(quadrant3)) + cosd(d(quadrant3));
quadrant4 = (270 <= d) & (d <= 360);
y(quadrant4) = 2*(1 + sind(d(quadrant4)) - cosd(d(quadrant4)));
plot(d, y)
xline(90, 'r:'); xline(180, 'r:'); xline(270, 'r:')
You can see that the function being plotted changes at multiples of 90 (the red dotted lines).
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sai venkat le 2 Juil 2021
Yeah. Now I understand how to operate on such kind of functions. I will try to use this method in my code. Thankyou.

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