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if elseif giving too many outputs

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Stenila Simon
Stenila Simon 2017-9-25
回答： Walter Roberson 2017-9-25
Hi everyone, I'm trying to finish writing a matlab code that will read an input for a certain altitude, and then calculate a couple of properties based on that altitude. The thing is, for each range of altitude, the values for the variables used in calculating these properties changes, so I have to reassign each variable new values based on the range this input falls under.
Here is my code:
if (H <= 11000)
Hbi = 0;
deltai = 1;
KTi = -6.5;
Tbi = 288.15;
LinP = (deltai.*(1+((KTi/Tbi).*(H-Hbi))).^(expterm/KTi));
ConstP = (deltai.*exp(-((expterm)/Tbi).*(H-Hbi)));
fprintf('Linear pressure ratio = %f\n', LinP);
elseif (H < 20000)
Hbi= 11000;
deltai = 0.22336;
KTi = 0;
Tbi = 216.65;
LinP = (deltai.*(1+((KTi/Tbi).*(H-Hbi))).^(expterm/KTi));
ConstP = (deltai.*exp(-((expterm)/Tbi).*(H-Hbi)));
fprintf('Linear pressure ratio = %f\n', LinP);
end
Please note that "expterm", etc. has already been established, so that's not my issue. There is no error when I run it. When I run it, it asks for the input, and when I put in the input, it gives me like ten thousand different values.

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回答（2 个）

Image Analyst
Image Analyst 2017-9-25
That's because LinP is an array of like ten thousand elements, and this:
fprintf('Linear pressure ratio = %f\n', LinP);
will repeat for every one of those ten thousand elements. LinP is 10,000 values because either H or expterm is 10,000 values.
  2 个评论
Walter Roberson
Walter Roberson 2017-9-25
Note that if H is a vector, then
if (H <= 11000)
is the same as
if all(H(:) <= 11000)
which would be false if even one value of H > 11000.
Stenila Simon
Stenila Simon 2017-9-25
So how would I change it to just evaluate which range the input H lies in and execute the function?

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Walter Roberson
Walter Roberson 2017-9-25
LinP = nan(size(H));
ConstP = nan(size(H));
mask = H <= 11000;
Hbi = 0;
deltai = 1;
KTi = -6.5;
Tbi = 288.15;
LinP(mask) = (deltai.*(1+((KTi/Tbi).*(H(mask)-Hbi))).^(expterm/KTi));
ConstP(mask) = (deltai.*exp(-((expterm)/Tbi).*(H(mask)-Hbi)));
mask = H > 11000 & H < 20000;
Hbi= 11000;
deltai = 0.22336;
KTi = 0;
Tbi = 216.65;
LinP(mask) = (deltai.*(1+((KTi/Tbi).*(H(mask)-Hbi))).^(expterm/KTi));
ConstP(mask) = (deltai.*exp(-((expterm)/Tbi).*(H(mask)-Hbi)));
plot(H, LinP)
You did not have a final "elseif" giving a value for the situation where H was not in either range, so I assigned NaN for that case.

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