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# How to solve the range of x axes and obtain y values?

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Megha on 25 Sep 2020
I wish to obtain figure as shown below:
here, y axis is r and x-axis is unu_f.
I tried to generate unu_f using a range of "r" values from 0:0.2:4 by solving equation z2. But i am getting curve like,
I have tried the following code but it does not produce the required o/p. Can Anyone please help me generating like in panel (a,b)?
clear; close all; clc;
Vau = 43;
theta_u = 71.8;
gamma = 5/3;
Vsu = 47.45;
tic
unu_f = [];
for r = 0:0.2:4
if r>3.9
r = 3.88;
end
syms Unu
A = r*Vau^2*cosd(theta_u)^2;
B = 2*r*Vsu^2/(r+1-gamma*(r-1));
C = r*Vau^2*sind(theta_u)^2;
D = (2*r - gamma*(r-1))/(r+1 - gamma*(r - 1));
x = Unu^2;
z2 = real(double(solve((x - A)^2*(x - B) - C*x*(D*x - A))));
unu_f = [unu_f;z2'];
end
toc
r2 = 0:0.2:3.9;
r1 = 3.88;
r = [r2 r1];
figure;
subplot(2,2,4)
x = abs(unu_f(:,1));
y = r;
yyaxis left
plot(x,y,'ob')
ylim([0 4]); xlim([0 1200]);
ylabel('r (Shock Strength)','FontWeight','bold','FontSize',10);
xlabel('Shock Speed (km/s)','FontWeight','bold','FontSize',10);
box on; grid on;
set(gca,'XMinorTick','on','YMinorTick','on','FontSize',12,'FontWeight','bold')
set(gca,'LineWidth',2.25,'TickLength',[0.010 0.010],'YColor',[0 0 0]);

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### Accepted Answer

Kiran Felix Robert on 8 Oct 2020
Hi Megha,
To plot specific regions of the curve in different plots (to emphasize certain characteristic features), the following method can be used.
Given that, you have corrected characteristic data, you can use Logical indexing to achieve this.
This answer gives you a clear explanation of how logical indexing works.
The Following code gives you an example,
% Sample X vs Y Characteristic [Synthesized Data For Plotting]
x = 1:50;
y = x.^0.5;
% Creating a Specific Case for range 10 to 20
i = x >= 10 & x<20;
y(i) = 20;
% Whole Curve
figure(1);
ax = gca;
plot(x,y); % Plotting Whole
ax.YLim = [0,30];
title('Whole')
% Part 1 Curve
i = x>5 & x < 15; % Selecting Specific Range
B1 = x(i); % Logical Indexing to Select Specific Array
D1 = y(i);
figure(2)
ax = gca;
plot(B1,D1); % Plotting The Specific Array
ax.YLim = [0,30];
title('Part 1')
%Part 2 Curve
i = x>15 & x < 25; % Selecting Specific Range
B2 = x(i); % Logical Indexing to Select Specific Array
D2 = y(i);
f = figure(3);
ax = gca;
p = plot(B2,D2); % Plotting The Specific Array
ax.YLim = [0,30];
title('Part 2')
%Part 3 Curve
i = x>25 ; % Selecting Specific Range
B3 = x(i); % Logical Indexing to Select Specific Array
D3 = y(i);
figure(4);
plot(B3,D3); % Plotting The Specific Array
title('Part 3')
Kiran Felix Robert

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