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IF statement if == "A variable" not working

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Shasha Dra
Shasha Dra on 15 Apr 2024
Commented: Stephen23 on 15 Apr 2024
Trying to do a heat conduction simulation my and my IF statement not working
I have a variable called 'b' which equal to 'maximum timestep - 1' and I want to use and IF to record maximum temperature
The IF statement not working when I use IF == b, but works with IF = 9999

Answers (2)

Fangjun Jiang
Fangjun Jiang on 15 Apr 2024
Edited: Fangjun Jiang on 15 Apr 2024
Most likely it is a floating point data equal comparison problem. b is somewhere near 9999 but not exactly. Use round() or integer data type.
a=1-1/3
a = 0.6667
b=2/3
b = 0.6667
a==b
ans = logical
0

Voss
Voss on 15 Apr 2024
"The IF statement not working when I use IF == b, but works with IF = 99999"
The behavior is not the same when using b vs when using 99999 because b is not exactly equal to 99999.
format long g
T= 1; %simulation time seconds
del_t= 1E-5; %time step size
N = T/del_t %Number of time discretization
N =
100000
b=(T/del_t)-1 % N-1
b =
99999
b looks like it is equal to 99999, but it's not.
b == 99999 % nope
ans = logical
0
It's actually slightly smaller:
fprintf('%.40f',b)
99998.9999999999854480847716331481933593750000
Consider the behavior of the following two loops:
counter = 1;
counter_hit_b = false;
while counter < N
counter = counter+1;
if counter == b
counter_hit_b = true;
end
end
disp(counter_hit_b)
0
counter = 1;
counter_hit_99999 = false;
while counter < N
counter = counter+1;
if counter == 99999
counter_hit_99999 = true;
end
end
counter_hit_99999
counter_hit_99999 = logical
1
The counter is 99999 at some point, but it is never b.
To get the right behavior using b instead of hard-coding 99999, you can define b to be exactly 99999 by using round.
b = round(T/del_t-1)
b =
99999
fprintf('%.40f',b)
99999.0000000000000000000000000000000000000000
b == 99999 % now b is exactly 99999
ans = logical
1

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