Using 'for loop' to plot

Question

Grace 2022-12-2

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1869002-using-for-loop-to-plot

评论： Grace 2022-12-8

采纳的回答： Karim

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I have three set of data (y1, y2 and y3). And I want to only plot a certain range within each data set.

I want to perform this task in two ways. I succedded in the first method to plot the graph without a 'for loop'.

But in the second method, I want to use a 'for loop'. However, I am a bit confused. Can you help me out. Thanks

clear all; close all;clc

x1=linspace(0, 11, 25);y1=2*x1.^2;

f1=max(find(x1<=2));f2=min(find(x1>=10));

x2=linspace(-2, 15, 17);y2=x2.^2;

f3=max(find(x2<=3));f4=min(find(x2>=14));

x3=linspace(-5, 25, 21);y3= (7.*x3)+ 5;

f5=max(find(x3<=5));f6=min(find(x3>=21));

%% Method 1

plot(x1(f1:f2),y1(f1:f2), 'r');hold on;plot(x2(f3:f4),y2(f3:f4), 'k');hold on;plot(x3(f5:f6),y3(f5:f6),'b')

xlabel('X');ylabel('Y')

return

%% Method 2

A=[y1 y2 y3];

B=[x1 x2 x3];

figure;

for i=1:length(A);

for j=1:length(B);

f1=max(find(x1<=2));f2=min(find(x1>=10));

f3=max(find(x2<=3));f4=min(find(x2>=14));

f5=max(find(x3<=5));f6=min(find(x3>=21));

plot(B(i),A(i))

hold on;

end

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Walter Roberson 2022-12-2

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Note that you can do things like

mask = x1 > 2 & x1 <10;
plot(x1(mask), y1(mask))

Grace 2022-12-3

Ok. noted.

Do you have some suggestion to help me with the 'for loop' to perform the operation in Method 2?

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Answer 1

Karim 2022-12-3

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It's not really clear why you would like to use a loop since the vectorized method is a lot cleaner from a code perspective. However, below you can see one method to use a for loop with the concatinated vectors A and B.

x1 = linspace(0, 11, 25);

y1 = 2*x1.^2;

i1 = x1 >=2 & x1 <= 10; % set up index for plot 1

x2 = linspace(-2, 15, 17);

y2 = x2.^2;

i2 = x2 >=3 & x2 <= 14;

x3 = linspace(-5, 25, 21);

y3 = (7.*x3)+ 5;

i3 = x3 >=5 & x3 <= 21;

%% Method 1 - vectorized

figure

hold on

plot(x1(i1), y1(i1), 'r')

plot(x2(i2), y2(i2), 'k')

plot(x3(i3), y3(i3), 'b')

hold off

xlabel('X')

ylabel('Y')

title("Method 1 - vectorized")

grid on

%% Method 2 - using a loop

A = [y1 y2 y3];

B = [x1 x2 x3];

figure

hold on

for i = 1:length(A)

if i <= length(x1)

% we are using the values for x1 y1

minVal = 2;

maxVal = 10;

colVal = 'r';

elseif i <= (length(x1)+length(x2))

% we are using the values for x2 y2

minVal = 3;

maxVal = 14;

colVal = 'k';

else

% we are using the values for x3 y3

minVal = 5;

maxVal = 21;

colVal = 'b';

end

if B(i) >= minVal & B(i) <= maxVal

scatter(B(i),A(i),colVal,'filled')

end

hold off

xlabel('X')

ylabel('Y')

title("Method 2 - in a loop")

grid on

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Answer 2

Walter Roberson 2022-12-3

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Of course you can do it with a loop.

I am not clear as to why you are constructing an x range that is not just the inside of the range that you are masking off, but whatever...

xlow = [0, -2, -5];

xhigh = [11, 15, 25];

masklow = [2, 3, 5];

maskhigh = [10, 14, 21];

colors = {'r', 'k', 'b'};

polycoeffs = [2 0 0; 1 0 0; 0 7 5];

for K = 1 : length(xlow)

x = linspace(xlow(K), xhigh(K));

y = polyval(polycoeffs(K,:), x);

mask = masklow(K) < x & x < maskhigh(K);

plot(x(mask), y(mask), colors{K});

hold on

end

hold off

xlabel('X');

ylabel('Y')

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Grace 2022-12-8

Thank you

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