why is this "if" statement is never true?

Can someone please explain why this "if" statement is never true? It is supposed to allow me to take the elements at every even integer of Time (i.e. 2 4 6...) while Time is increasing by 0.1 every time through the while statement.

I have gone through the program step by step and have actually viewed my j = 2, and Time = to 2.0000 and it still goes to the "else" command. I've also written the same if statement outside of this script and it works.

Any ideas? Your help is appreciated!

dt = 0.1;
i = 1;
j = 0;
Time = 0;
TimeMax = 20000;

while Time < TimeMax 

Mass0 = ...
Altitude0 = ...
Velocity0 = ...
Gamma0 = ...
Radius0 =...
Nu0 = ...

Time = Time + dt;

j = 2+j;

 if j == Time
    i = i+1;
    Mass(i)= Mass0;
    Altitude(i) = Altitude0;
    Velocity(i) = Velocity0;
    Gamma(i) = Gamma0;
    Radius(i)=Radius0;
    Nu(i) = Nu0;
 else
     j = j-2;
 end 
end

[Youssef Khmou]

is the variables Mass, Velocity... are constant then make them outside the "while" loop to gain memory .

Answer 1

[Walter Roberson]

http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

[Brooke]

actually, this is what I needed. the mod doesn't handle the equivalence properly. I accepted his answer to soon. my apologies. thank you for your kind help!

[Youssef Khmou]

ok, we can work on it until it work correctly, no problem !

[Walter Roberson]

The basic problem was floating point comparisons and round-off, that adding 10 copies of 0.1 does not give you exactly 1. If you know your dt is 1/P for an integer P, then add 1 to your counter each instance instead of adding 1/P, and if you need the time the counter represents, divide the counter by P; even then better to compare the integer counter to (time_limit * P) rather than comparing (counter / P) to time_limit

[Youssef Khmou]

Correct !, we found that error , thanks

[Image Analyst]

Go ahead and mark it Accepted then.

Answer 2

[Brooke]

thank you so much! I was getting so frustrated over here. Many thanks!

[Youssef Khmou]

how is the equivalence you want? if my answer is not the right one, delete it to get better solution !

Answer 3

[Youssef Khmou]

Hi, i saw that the answer is not satisfactory, because we evaluate for 20000 points in Time and get only 20 values in Mass, Veloc... the reason is because i made mistake putting Mod(j,Time) while i should put " Mod(Time,j)" or Mod(Time,2) , with this alteration we get the logical lenght for Mass perxample which is 10001, and also changing dt to 1 :

 dt = 1;
 i = 1; 
 j = 0;
 Time = 0;
 TimeMax = 20000;

 Mass0 =2;
  Altitude0 =58;
 Velocity0 = pi;
 Gamma0 =77;
 Radius0 =7.25;
 Nu0 = 77;
while Time < TimeMax
Time = Time + dt;

 j = 2+j;

 if mod(Time,2)==0 %  if mod(time,j)==0
    i = i+1;
    Mass(i)= Mass0;
    Altitude(i) = Altitude0;
    Velocity(i) = Velocity0;
    Gamma(i) = Gamma0;
    Radius(i)=Radius0;
    Nu(i) = Nu0;
 else
     j = j-2;

 end 
end

Is that the equivalence you talked about ?