# plotting 2 variables as they change with respect to one another

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Joshua Anderson on 15 Apr 2021 at 13:51
Edited: Ahmed Redissi on 15 Apr 2021 at 15:27
I'm trying to plot both A and M and they change simultaneously. If M changes, there is an associated A value within the given ranges. I'm having getting multiple M values to show even though multiple A values are being displayed. Any help would be greatly appreciated.
gamma = 1.4;
%M=1.53
for M= (0.1:0.01:2.19);
for A= (1:0.01:1)
A = 0.5*sqrt((1/M^2)*(2/(gamma+1)*(1+((gamma-1)/2)*M^2))^((gamma+1)/(gamma-1)))
end
end
plot(A,M)
ylabel('Area')
xlabel('Mach number')

Star Strider on 15 Apr 2021 at 14:00
Edited: Star Strider on 15 Apr 2021 at 14:06
The solution is to subscript ‘A’.
Try this:
gamma = 1.4;
%M=1.53
Mv = (0.1:0.01:2.19);
for k1 = 1:numel(Mv)
M = Mv(k1);
A(k1) = 0.5*sqrt((1/M^2)*(2/(gamma+1)*(1+((gamma-1)/2)*M^2))^((gamma+1)/(gamma-1)));
end
figure
plot(Mv,A)
ylabel('Area')
xlabel('Mach number')
grid
However in MATLAB the explicit loop is not necessary, using element-wise operations:
M = 0.1:0.01:2.19;
A = 0.5*sqrt((1./M.^2).*(2/(gamma+1).*(1+((gamma-1)/2).*M.^2)).^((gamma+1)/(gamma-1)));
figure
plot(M,A)
ylabel('Area')
xlabel('Mach number')
grid
This produces the same result as the loop.
See Array vs. Matrix Operations for details.

Ahmed Redissi on 15 Apr 2021 at 14:01
Edited: Ahmed Redissi on 15 Apr 2021 at 15:27
It's not clear why you are iterating over the values of A when you are not using them in the loop. Generaly, you shouldn't iterate over a variable and then change its content inside the loop unless you are incrementing or decrementing.
Does this perhaps solve your issue?
gamma = 1.4;
M = 0.1:0.01:2.19;
A = 0.5*sqrt((1./M.^2).*(2./(gamma+1).*(1+((gamma-1)/2)*M.^2)).^((gamma+1)./(gamma-1)));
plot(M,A)
ylabel('Area')
xlabel('Mach number')