Constant number is changing value, why?

Question

Krish Desai 2016-3-25

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/275464-constant-number-is-changing-value-why

编辑： dpb 2016-3-25

So I have a 1 that should print out, but instead only on the first line does 1 print out, for the rest the number keeps changing. Why?

dt=5*10e-5;
timenumber=0;
for t=0:dt:2*dt %99 for 100 graphs
    figure;
    atom=1;
     for ind=1:1920
         U=(3*ind+linspace(1,3,3)-3);
         Phi= PHI_disp_RBME_ModePlot_N(U);
         r=xlocs_full(ind);
         omega=(f_disp_RBME(iMode,kPtMode)*2*pi);
         k=kappa_space(kPtMode);
         spatialphase=k*r;
         timephase=omega*t;
         displ=Phi*exp(i*((spatialphase)-(timephase)));
         xOvito(ind)=xOvito(ind)+magFact*displ(1);
         yOvito(ind)=yOvito(ind)+magFact*displ(2);
         zOvito(ind)=zOvito(ind)+magFact*displ(3);
         header0= 'Timestep:';
         header1= 'id';
         header2= 'type';
         header3='x';
         header4='y';
         header5='z';
         headernum='Number of atoms:';
         timenumber=(timenumber+1);
         if ind==1
             fileName=['dump',num2str(t*2000)];
         end;
         fid=fopen(fileName,'w');
         fprintf(fid, [ header0 '\n']);
         fprintf(fid, '%d \n', (timenumber/1920)-1');
         fprintf(fid, [headernum '\n']);
         fprintf(fid, '%d \n', 1920);
         fprintf(fid, [ header1 ' ' header2 ' ' header3 ' ' header4 ' ' header5 '\n']);
         fprintf(fid, '%f %f %f %f \n', [1 xOvito yOvito zOvito]');-------------->The one keeps changing
         fclose(fid);
     end

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Walter Roberson 2016-3-25

Which is the value that keeps changing?

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Answer 1

dpb 2016-3-25

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/275464-constant-number-is-changing-value-why#answer_215037

编辑：dpb 2016-3-25

在 MATLAB Online 中打开

...
for ind=1:1920
   ...
   xOvito(ind)=xOvito(ind)+magFact*displ(1);
   yOvito(ind)=yOvito(ind)+magFact*displ(2);
   zOvito(ind)=zOvito(ind)+magFact*displ(3);
   ...         header0= 'Timestep:';
   fprintf(fid, '%f %f %f %f \n', [1 xOvito yOvito zOvito]'); % The one keeps changing
   ...

Because as the above reduced code snippet shows, you're building arrays from 1:1920 for each of the three variables in the loop and the fprintf argument list uses the variable names without subscripting. So, each pass through the loop you'll get first one set of values, then two for each, then three, etc., ... Since the format is for four elements per output record, they'll wrap and the position of each relatively to each other will change each pass.

You undoubtedly intended to write

fprintf(fid, '%f %f %f %f \n', [1 xOvito(ind) yOvito(ind) zOvito(ind)]');

In summary, the '1' didn't change its value, it simply gets placed in a different location owing to the number of values being printed each pass through the loop.

ADDENDUM

While not related to the issue, you'll save quite a lot if you preallocate the three arrays before beginning the loop over ind --

...
xOvito=zeros(1920,1);  % preallocate 
yOvito=zeros(1920,1);  % preallocate 
zOvito=zeros(1920,1);  % preallocate 
for ind=1:1920
 ...
 xOvito(ind)=xOvito(ind)+magFact*displ(1);
 yOvito(ind)=yOvito(ind)+magFact*displ(2);
 zOvito(ind)=zOvito(ind)+magFact*displ(3);
 ...