Need help filling a Matrix with values from an 'fprintf' Command

Question

Jon Stapchuck 2021-2-24

0
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评论： Jon Stapchuck 2021-2-25

Hello,

Basically I am computing several values and displaying them through each loop iteration using the 'fprintf'.

However, I would like to make the 'fprintf' output be in row i of a matrix, then after each loop, make this row the i+1 row, then continue this trend until a desired number of rows is reached. So my matrix will be of the size: k rows x size(fprintf) columns

Eventually, I would like to plot each individual row (which represents a velocity) vs its corresponding distance.

Here is the code I've been working on:

clc; clear all; 
%Initial Guesses for Length and Num. of Grid Cells
L = 1;     
m = 10;     
IL = m+1;   
Dx = L/m;   
Dt = Dx/2;  
%Initialize Vectors/Arrays
k = 10;       
X = zeros(1,IL);
u = zeros(k,IL);    
num = 100;          
N = zeros(1,num);
%Set Grid Points for Given Range
for i=1:IL
    X(i)=(i-1)*Dx;
end
for j=1:k
%Simplify the Initial Conditions
 for i=1:IL
    if (0<=X(i)) && (X(i)<=1)
        u(j,i)=2-X(i);   
    elseif (X(i)>1)
        u(j,i)=1;
    else
        u(j,i)=NaN;  
    end
 end
 
 %Generate Solution
    for N=1:num
      %Interior Points
         for i=2:(IL-1)
             u(j+1,i) = u(j,i)-Dt/Dx*u(j,i)*(u(j,i)-u(j,i-1));
         end
   
      %Boundary Points
         u(j+1,IL) = 1;
         u(j+1,1)  = 2;
     
         formatSpec = '%15.13f\n';
         fprintf(formatSpec,u(j,i))     %I want to be able to use this output as a row-vector
    
     %Go to next Time-Step
         for i=1:IL
             u(j,i) = u(j+1,i);
         end
    end
end

I tried just setting a variable to the 'fprintf', but that didn't work.

I can provide more details if need be.

Thanks,

Jon

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Answer 1

Walter Roberson 2021-2-24

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编辑：Walter Roberson 2021-2-24

在 MATLAB Online 中打开

variable = sprintf(formatSpec,u(j,i));

See also compose() . In particular, if you use a double-quoted format with compose() then the output is a string object.

         formatSpec = "%15.13f\n";
         variable = compose(formatSpec,u(j,i));
    

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Walter Roberson 2021-2-25

在 MATLAB Online 中打开

Both that sprintf() and that compose() are showing the entire vector. Notice you are passing u(j,i) with j and i both scalar, so the scalar so indexed is the entire vector you are asking to display.

Also, just before that you have

for i=2:(IL-1)
    loop body
end

and when you have a for loop, the loop control variable will be left as the last variable it was assigned, so i will be left at the value IL-1 . That might not be what you were expecting.

Jon Stapchuck 2021-2-25