# While loop isn't terminating even though the expression is 0

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Kiran Prasad on 24 Jun 2016
Edited: Moe_2015 on 24 Jun 2016
Hi, I'm utterly perplexed as to why the following while loop will not terminate. Any help would be appreciated.
CO2percentout1 = 1:1000;
CO2percentout1(:)=0;
CO2percentout1(1) = CO2_percent_feed-.6*CO2_percent_feed;
CO2percentout2 = CO2_percent_feed-.5*CO2_percent_feed;
CO2percentout3 = CO2_percent_feed-.3*CO2_percent_feed;
i=2;
c1 = 1:49;
c2 = 1:49;
c3 = 1:49;
while abs(CO2percentout1(i) - CO2percentout1(i-1)) > 0.001
[CO2percentout1(i),Algae_in_2,c1] = stage1(CO2percentout2,V1,tot_gas_flow,Height1,C01,Water_flow);
[CO2percentout2,Algae_in_3,c2]=stage2(CO2percentout3,V2,tot_gas_flow, Height2, C02,Water_flow,Algae_in_2);
[CO2percentout3,Algae_Product,c3] =stage3(CO2_percent_feed,V3,tot_gas_flow,Height3,C03,Water_flow,Algae_in_3);
i=i+1;
end;
the values of CO2percentout1 for 30 iterations are the following:
0.400000000000000 0.496842690178588 0.577390642895070 0.658829848072415 0.658142131647976 0.658142519746066 0.658142563142344 0.658142562747119 0.658142562747534 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550 0.658142562747550

Moe_2015 on 24 Jun 2016
Edited: Moe_2015 on 24 Jun 2016
I think what you need to do is the following (by the way, I don't know what your code is doing but I think it should be > 0.001 not <):
i = 1;
CO2percentout1(i) = 0; %or whatever starting value here
difference = 1; %just an arbitrary value above 0.001
while difference > 0.001
i = i + 1;
[CO2percentout1(i),Algae_in_2,c1] = stage1(CO2percentout2,V1,tot_gas_flow,Height1,C01,Water_flow);
[CO2percentout2,Algae_in_3,c2]=stage2(CO2percentout3,V2,tot_gas_flow, Height2, C02,Water_flow,Algae_in_2);
[CO2percentout3,Algae_Product,c3] =stage3(CO2_percent_feed,V3,tot_gas_flow,Height3,C03,Water_flow,Algae_in_3);
difference = abs(CO2percentout1(i) - CO2percentout1(i-1));
end
##### 3 CommentsShow 1 older commentHide 1 older comment
Kiran Prasad on 24 Jun 2016
This worked perfectly! Thank you!!! Could you explain why what I did didn't work?
Moe_2015 on 24 Jun 2016
Edited: Moe_2015 on 24 Jun 2016
You're welcome! The problem was that you were putting i=i+1 as the last line in the while loop when it should have been first. Let's say we were going through the loop the first time (so i = 2). If you have i = i + 1 at the top of the loop, when you're computing CO2percentout1(i) it will be CO2percentout1(3). However, if i = i + 1 is at the bottom of the loop, you already computed CO2percentout1(i) before that line so it will be CO2percentout1(2). So when it goes back to the inequality you have CO2percentout1(3) - CO2percentou1(2) but you haven't computed CO2percentout1(3) yet.

Walter Roberson on 24 Jun 2016
What is the initial value of i, and what is CO2percentout1(i-1) for that initial i ?
With the values you post, I would expect the loop to never get started if i starts out less than about 4, but if it starts out greater than that then because the adjacent values are closer than 0.001, the loop will not stop. You appear to have defined a convergent algorithm and asked it to stop iterating when it does not converge. I suspect you want ">" rather than "<"
Kiran Prasad on 24 Jun 2016
Thanks for your reply! I added the full pertinent section of code and would note that the other variables such as V1,V2, and V3 are user inputs. Also both of you were right about the > that was a typo when I was editing the post. Thanks!

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