MATLAB: Decrease precision of decimal no value saved for a variable.

MATLABprecision

Hi, I wrote a script file and in that am solving some equations to get y1, y2. Now y1 = (some equation) and y2 = y1+2. Solving it I am getting y1 = -1.7383e-17 and y2 = 2 but I want it to solve and save values as either y1 = 0, y2 = 2 OR y1 = -1.7383e-17 and y2 = 2-(1.7383e-17) i.e, y2<2. I tried to set precision to 4 in beginning of my script file by giving command: digits(4); but still I am getting the same result. Thanks in advance.

Best Answer

You have overstepped the limits of what double floating-point numbers can represent. Because of that limit on floating-point precision, 2-1.7383e-17 is equivalent to 2:

>> fprintf('%.30f\n',2-1.7383e-17)
2.000000000000000000000000000000

Once easy solution would be to simply round to the same number of decimal places:

>> round([2,1.7383e-17]*1e15)/1e15
ans =
   2   0

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MATLAB: Interchange positions in a vector

You can use circshift

y = [1 2 3 4];
y1 = circshift(y, -1);
y2 = circshift(y, -2);

Result

>> y1
y1 =
     2     3     4     1
>> y2
y2 =
     3     4     1     2

MATLAB: How to rearrange rows in specific intervals

I use the Symbolic Math Toolbox to illustrate the approach and the desired result:

syms x1 y1 z1 x2 y2 z2 x3 y3 z3 real
M = [x1  y1  z1;
    x2  y2  z2;
    x3  y3  z3;
    x1  y1  z1;
    x2  y2  z2;
    x3  y3  z3;
    x1  y1  z1;
    x2  y2  z2;
    x3  y3  z3];
A = reshape(M, 3, 3, []);
B = permute(A, [3 2 1]);
Result = reshape(B, 3, [])'
Result =
[ x1, y1, z1]
[ x1, y1, z1]
[ x1, y1, z1]
[ x2, y2, z2]
[ x2, y2, z2]
[ x2, y2, z2]
[ x3, y3, z3]
[ x3, y3, z3]
[ x3, y3, z3]

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