How do you print data from a loop into a Matrix?

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Matthew
Matthew 2014-4-17
评论： Matthew 2014-4-18
I am running a program that aims to simulate the flight of a rocket and it involves running a loop that simulates the rocket at x times (.0001,.0002,etc.). I need to be able to take the data from the loop not at each time interval, but every ten time intervals ie .001,.002,etc.
Originally I attempted to place an "if" statement within the loop, defining deltat as a array of values ranging from 0 to 100 at intervals of .001, but I keep receiving an error that claims that the left side is an invalid target for assignment. Is there a better way to go about doing this?
while mwater > 0
Vair = Vbottle - (mwater/rhowater)
Pair = (mairini * R * T)/(Mair * Vair)
mdotwater = .01
mwater = mwater - (mdotwater * deltat)
mtotal = mrocket + mwater + mairini
vwater = C*(2*(Pair-Patm)/(rhowater))^(.5)
Fthrust = mdotwater * vwater
Fgravity = (mtotal * 9.81)
Fdrag = (.5)*(Cd)*(rhoatm)*(Aprojected)*(vrocket)^2
Fnet = Fthrust - Fgravity - Fdrag
arocket = (Fnet)/(mtotal)
deltat = deltat + .0001;
end
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Matthew
Matthew 2014-4-17
编辑：Matthew 2014-4-17
Here is everything I have so far, including the excel file that the code pulls the data from. Sara's method does give me values with which to print the data, but it keeps overwriting itself in the matrix. I need each data point to be saved in a separate cell at each interval.
%Data Input
filename = 'RocketData.xlsx';
num = xlsread(filename);
%Data Definitions
deltat = num(1,1);
printing = num(2,1);
Vbottle = .002;
f = num(4,1);
Dn = num(5,1)*.0254;
Anozzle = num(6,1)*.00064516;
Aprojected = num(7,1)*.00064516;
mpayload = num(8,1)*.001;
mrocket = num(9,1)*.001;
Pairini = num(10,1)*6894.752728;
Cd = num(11,1);
C = num(12,1);
Li = num(13,1)*.0254;
g = 9.81;
c = 343;
Mair = .029;
gamma = num(17,1);
Patm = num(18,1)*101.325;
rhowater = 1000;
Tamb = num(20,1) + 273.15;
R = 8.314;
%Phase 1 Initial Calculated Values
T = Tamb;
rhoatm = (Mair*Pairini)/(R*T);
rhoair = (Mair*Pairini)/(R*T);
Vairini = (1-f)*Vbottle;
mairini = Pairini*Vairini;
mwaterini = f*Vbottle*rhowater;
mtotal = mrocket + mpayload + mwaterini + Mair;
t = 0;
x = 0;
vrocket = 0;
Vini = (1-f)*Vbottle;
Tini = Tamb;
%Phase 1 Data Initial Calculations
mwater = mwaterini;
while mwater > 0
Vair = Vbottle - (mwater/rhowater);
Pair = (mairini * R * T)/(Mair * Vair);
mdotwater = .01;
mwater = mwater - (mdotwater * deltat);
mtotal = mrocket + mwater + mairini;
vwater = C*(2*(Pair-Patm)/(rhowater))^(.5);
Fthrust = mdotwater * vwater;
Fgravity = (mtotal * 9.81);
Fdrag = (.5)*(Cd)*(rhoatm)*(Aprojected)*(vrocket)^2;
Fnet = Fthrust - Fgravity - Fdrag;
arocket = (Fnet)/(mtotal);
deltat = deltat + .0001
if deltat = [0:.001:100]
xlswrite('stuff.xlsx',arocket,1,'A1')
end
end
José-Luis
José-Luis 2014-4-17
编辑：José-Luis 2014-4-17
I'll go off on a tangent here. Using mod on doubles makes me cringe. Matlab uses forced rounding to make it work. At least from my point of view, that is unexpected behavior, and one should always be careful to read the fine print of the functions one will use. Given that Matlab is so vast, that makes for a lot of reading.
I'm done ranting for the day.

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Sara
Sara 2014-4-17
You always write in the same cell, that's the issue. Before the while loop, define a variable to count how many times you print, e.g., counter = 0; Then, each time the if statement is true, increment it:
counter = counter + 1;
Then replace
xlswrite('stuff.xlsx',arocket,1,'A1')
with
xlswrite('stuff.xlsx',arocket,1,['A',num2str(counter)])
In that way you print in a new cell each time.
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Sara
Sara 2014-4-17
Ok, the issue is in deltat = deltat + .0001. If you look at the numbers, they are not what you'd expect for numerical reasons. Put:
kk = 0;
before the while loop, increment it in the while loop (before incrementing deltat):
kk = kk + 1;
and replace the deltat = deltat + .0001 with:
deltat = 1e-4*kk;
That should solve it.

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