Error in the for loop or equality sign

2019 5 21
1 个回答
更新时间：2019 5 21
3 次查看（30 天）

0 个投票

Hello,
I am using following code to check the discharges in the pipelines connecting three reservoirs. But the Loop is not working I guess. Could anybody tell the error?
Ela=100;
Elb=70;
Elc=30;
ka=1563.085;
kb=38.10;
kc=12729.55;
x=70:0.01:75;
for i=1:x;
Qa=sqrt((Ela-x)/ka);
Qb=sqrt((x-Elb)/kb);
Qc=sqrt((x-Elc)/kc);
if Qb+Qc==Qa
disp(Qa)
disp(Qb)
disp(Qc)
break
else
continue
end
end

5 个评论
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Alex Mcaulley
Alex Mcaulley 2019-5-21
What do you mean by "is not working"? What do you expect?
Walter Roberson
Walter Roberson 2019-5-21
x=70:0.01:75;
for i=1:x;
When you execute that for loop what do you expect the first value of i to be? What do you expect the second value of i to be? What do you expect the last value to be?
Faisal Baig
Faisal Baig 2019-5-21
I want to check different values of Qa, Qb and Qc for given range of x until the sum Qb+Qc becomes equal to Qa. At that point, I want the code to show the values of Qa, Qb and Qc which satisfies the condition. But when I run the code, only I get the matrix of Qa, Qb, Qc agaisnt the given range of x and my condition (Qa=Qb+Qc) is not met somehow.
Walter Roberson
Walter Roberson 2019-5-21
Please answer my question about the expected values of i
Faisal Baig
Faisal Baig 2019-5-21
I can use
'for x= 70:0.01:75'
instead of
x= 70:0.01:75;
for i = 1:x;
So basically, I just want to give range of my values to find Qa, Qb and Qc. i doesn't matter

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回答（1 个）

Jos (10584)
Jos (10584) 2019-5-21

1 个投票

Welcome to the world of floating point arithmetic, where
if 0.1+0.2 == 0.3
disp('0.1+0.2 equals 0.3')
else
disp('0.1+0.2 does not equal 0.3')
end
More info here:
https://matlab.fandom.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F
You want to check with a tolerance:
if abs(Qa+Qb-Qc) < 1e-10
disp('Qa+Qb practically equals Qc')
end

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