Keep getting the "Array indices must be positive integers or logical values." error

Question

Tom Oud 2018-12-12

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/435346-keep-getting-the-array-indices-must-be-positive-integers-or-logical-values-error

评论： Tom Oud 2018-12-13

采纳的回答： madhan ravi

在 MATLAB Online 中打开

Trying to simulate a water rocket using matlab, however I can't get my for loop to work properly.

This is what I have so far:

close all
clear all
P0 = 6e5; %Pa
Pout = 1e5; %Pa
V0 = 2; %l
V = 1.5; %l
gamma = 1.4;
rho_w = 998;  %kg/m^3
%Pin = P0*(V/V0).^-gamma;
dt = 0.01;
t = 1:dt:100;
Ve(1) = 0; %m/s
Pin(1) = 6e5; %Pa
v0 = 0; %m/s
v = 0; %m/s
M0 = 0.1; %kg
M = M0 + (V0-V)*rho_w;
for i = 1:100
    Pin(i+1) = Pin(i) - P0(V/V0).^-gamma;
    Ve(i+1) = sqrt(2*(Pin(i) - P0)/rho_w);
    %v = Ve*log(M0/M) + v0;
end

Really hoping one of you can enlighten me, because I'm completely lost here.

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Image Analyst 2018-12-12

V/V0 = 1.5/2, not 0/0

madhan's Answer below lets the code run without error. No guarantee about the equations correctness though - I didn't try to verify them.

Guillaume 2018-12-12

Oh! I didn't see that there were both V and V0 variables (uppercase) as well as v and v0 (lowercase).

That's really begging for trouble! You can be sure at some point the wrong case will be used. Never distinguish variable names just by casing.

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Answer 1

madhan ravi 2018-12-12

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/435346-keep-getting-the-array-indices-must-be-positive-integers-or-logical-values-error#answer_351927

在 MATLAB Online 中打开

  Pin(i+1) = Pin(i) - P0*(V/V0).^-gamma;
                        ^----missed it

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madhan ravi 2018-12-12

编辑：madhan ravi 2018-12-12

在 MATLAB Online 中打开

Ofcourse you need to preallocate the arrays before loop: (Wierd I didn't get any errors)

>> clear all
>> close all
clear all
P0 = 6e5; %Pa
Pout = 1e5; %Pa
V0 = 2; %l
V = 1.5; %l
gamma = 1.4;
rho_w = 998;  %kg/m^3
%Pin = P0*(V/V0).^-gamma;
dt = 0.01;
t = 1:dt:100;
Ve(1) = 0; %m/s
Pin(1) = 6e5; %Pa
v0 = 0; %m/s
v = 0; %m/s
M0 = 0.1; %kg
M = M0 + (V0-V)*rho_w;
for i = 1:100
    Pin(i+1) = Pin(i) - P0*(V/V0).^-gamma;
    Ve(i+1) = sqrt(2*(Pin(i) - P0)/rho_w);
    %v = Ve*log(M0/M) + v0;
end
>> Pin
Pin =
   1.0e+07 *
  Columns 1 through 3
   0.060000000000000  -0.029756411635696  -0.119512823271392
  Columns 4 through 6
  -0.209269234907088  -0.299025646542784  -0.388782058178480
  Columns 7 through 9
  -0.478538469814176  -0.568294881449872  -0.658051293085568
  Columns 10 through 12
  -0.747807704721264  -0.837564116356960  -0.927320527992656
  Columns 13 through 15
  -1.017076939628351  -1.106833351264047  -1.196589762899744
  Columns 16 through 18
  -1.286346174535439  -1.376102586171135  -1.465858997806831
  Columns 19 through 21
  -1.555615409442527  -1.645371821078223  -1.735128232713919
  Columns 22 through 24
  -1.824884644349615  -1.914641055985311  -2.004397467621008
  Columns 25 through 27
  -2.094153879256704  -2.183910290892400  -2.273666702528096
  Columns 28 through 30
  -2.363423114163792  -2.453179525799488  -2.542935937435185
  Columns 31 through 33
  -2.632692349070881  -2.722448760706577  -2.812205172342273
  Columns 34 through 36
  -2.901961583977969  -2.991717995613665  -3.081474407249361
  Columns 37 through 39
  -3.171230818885058  -3.260987230520754  -3.350743642156450
  Columns 40 through 42
  -3.440500053792146  -3.530256465427842  -3.620012877063538
  Columns 43 through 45
  -3.709769288699234  -3.799525700334931  -3.889282111970627
  Columns 46 through 48
  -3.979038523606323  -4.068794935242019  -4.158551346877715
  Columns 49 through 51
  -4.248307758513411  -4.338064170149107  -4.427820581784803
  Columns 52 through 54
  -4.517576993420500  -4.607333405056195  -4.697089816691892
  Columns 55 through 57
  -4.786846228327588  -4.876602639963284  -4.966359051598980
  Columns 58 through 60
  -5.056115463234677  -5.145871874870372  -5.235628286506069
  Columns 61 through 63
  -5.325384698141765  -5.415141109777461  -5.504897521413157
  Columns 64 through 66
  -5.594653933048853  -5.684410344684549  -5.774166756320246
  Columns 67 through 69
  -5.863923167955941  -5.953679579591638  -6.043435991227334
  Columns 70 through 72
  -6.133192402863030  -6.222948814498726  -6.312705226134423
  Columns 73 through 75
  -6.402461637770118  -6.492218049405815  -6.581974461041511
  Columns 76 through 78
  -6.671730872677207  -6.761487284312903  -6.851243695948598
  Columns 79 through 81
  -6.941000107584293  -7.030756519219989  -7.120512930855684
  Columns 82 through 84
  -7.210269342491380  -7.300025754127075  -7.389782165762770
  Columns 85 through 87
  -7.479538577398466  -7.569294989034161  -7.659051400669856
  Columns 88 through 90
  -7.748807812305552  -7.838564223941247  -7.928320635576942
  Columns 91 through 93
  -8.018077047212637  -8.107833458848333  -8.197589870484029
  Columns 94 through 96
  -8.287346282119724  -8.377102693755420  -8.466859105391116
  Columns 97 through 99
  -8.556615517026810  -8.646371928662505  -8.736128340298201
  Columns 100 through 101
  -8.825884751933897  -8.915641163569592
>> 

Tom Oud 2018-12-12

Of course!

P0 = 6e5; %Pa

Pout = 1e5; %Pa

V0 = 2; %l

V = 1.5; %l

gamma = 1.4;

rho_w = 998; %kg/m^3

dt = 0.001;

t = 1:dt:10;

Ve(1) = 0; %m/s

Pin(1) = 6e5; %Pa

v0 = 0; %m/s

v = 0; %m/s

M0 = 0.1; %kg

M = M0 + (V0-V)*rho_w;

for i = 1:numel(t)

k = t(i);

Pin(i+1) = Pin(i) - P0*(V/V0).^-gamma;

Ve(i+1) = sqrt(2*(Pin(i) - Pout)/rho_w);

if Pin(i) < P0

Ve(i+1) = Ve(i);

end

%v = Ve*log(M0/M) + v0;

end

Tom Oud 2018-12-13

Turns out I interpreted my results wrong. Decreasing the timestep now just seems to add more steps in total, instead of making the increment smaller.

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Keep getting the "Array indices must be positive integers or logical values." error

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Keep getting the "Array indices must be positive integers or logical values." error

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