How to plot min and max values on graph

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Noah Johnson
Noah Johnson 2020-10-20
评论： Steven Lord 2020-10-20
I've created some formulas and a function that is composed of if and elseif statements and plots a graph of it. However I am needing for the graph to show the max and min of the curve and I am unsure of how to do so.
X = 0:0.1:10;
Y = (X.^(1.01))+4*cos((3/4).*X.*pi) - 2*sin((2/3)*pi.*X)-0.25;
figure(); %creates figure window
plot (Y,'-r+') %plots figure
xlabel('Time (mins)') %x axis label
ylabel('Density Altitude (km)') %y axis label
function minmax_i = min_max(Y) %function code
N = ones(1,7);
for i = 2:(length(Y)-1)
if (Y(i-1) > Y(i)) && (Y(i+1) > Y(i)) %if true, local minimum
minmax_i(N) = i;
N = N+1;
elseif (Y(i-1) < Y(i)) && (Y(i+1) < Y(i)) %if true, local maximum
minmax_i(N) = i;
N = N+1;
end
end
end

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采纳的回答

KSSV
KSSV 2020-10-20
X = 0:0.1:10;
Y = (X.^(1.01))+4*cos((3/4).*X.*pi) - 2*sin((2/3)*pi.*X)-0.25;
% min
[val0,idx0] = min(Y) ;
% max
[val1,idx1] = max(Y) ;
plot(X,Y)
hold on
plot(X(idx0),Y(idx0),'*r')
plot(X(idx1),Y(idx1),'*b')

更多回答（1 个）

Alan Stevens
Alan Stevens 2020-10-20
If you meant you want to highight all the peaks and troughs then the following will do it (assuming you haven't got the findpeaks function available):
X = 0:0.01:10;
Y = (X.^(1.01))+4*cos((3/4).*X.*pi) - 2*sin((2/3)*pi.*X)-0.25;
% Peaks and troughs
% Gradient of curve
dYdX = 1.01*X.^0.01 - 3*pi*sin((3/4).*X.*pi) - (4/3)*pi*cos((2/3)*pi.*X);
signchange = diff(sign(dYdX)); % Where curve turns over
idxpos = signchange<0; % Peak positions
idxneg = signchange>0; % Trough positions
figure(); %creates figure window
plot(X,Y,'r',X(idxpos),Y(idxpos),'bo',X(idxneg),Y(idxneg),'bs'),grid
xlabel('Time (mins)') %x axis label
ylabel('Density Altitude (km)') %y axis label

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