MATLAB: Error during multiplication (r2009b)

bugnot a bug

Hi all, I am using MATLAB r2009b and when I type the following at the command prompt, I get something very unexpected…

19*2.2 == 41.8

I get an answer of 0, when the answer should be 1! Upon typing 19*2.2 and inspecting the answer, I get 41.800000000000004.

Moreover, mod(41.8, 2.2) gives the correct answer, 0, but mod(41.8/2.2, 1) gives the wrong answer, 0.999999999999996.

Kindly let me know if this is a known issue that has already been discussed and rectified and if my friend discovered just a specific case of this issue.

Thanks in advance for your time!

– Praneeth

Best Answer

Welcome to the world of floating point arithmetics with limited precision. Numbers are represented as DOUBLE using 64 bits. In consequence not all numbers can be represented exactly, and you found out (like thousands of others users before), that 19*2.2 has a slightly different representation as 41.8. See also: FAQ: Why is 0.3-0.2~=0.1.

BTW. it is worth to read the rest of the FAQ also, because it contains the frequent standard problems of other MATLAB beginners.

Related Solutions

MATLAB: How to find the coefficients of a polynomial equation for fit the curve in MATLAB

if "p" and "t" are both independant variables then your problem is in the form of "y=f(x,z)" or "v=f(p,t)".

x=[
    19.4
    71.1
    41.8
    161
    180
];  % t
y=[ 
    100000
    100000
    10000000
    80000000
    100000000
];  % p
z=[
    0.956
    0.9717
    0.959
    0.9725
    0.9999
];  % z
sf = fit([y, x],z,'poly21')
plot(sf,[y,x],z)

look at command window,

sf(x,y) = p00 + p10*x + p01*y + p20*x^2 + p11*x*y

V(t,p) = c0 +c1*T - c3*P +c2*(t^2) - c4*T*P

by comparing you will understant that:

c0=p00

c1=p10

c3=-p01

c2=p20

c4=-p11;

MATLAB: Read nonrectangle csv with brackets

I think reading and parsing must be done in two steps. With R2013a

>> tic, C = cssm( 'cssm.csv' ), toc
C = 
    [11x2 double]
    [40x2 double]
Elapsed time is 0.009517 seconds.
>> C{1,1}(1,:)
ans =
   -8.5857   41.1485
>> C{1,1}(2,:)
ans =
   -8.5857   41.1486
>>

where

function    out = cssm( filespec )
%   out = cssm( 'cssm.csv' )
    fid = fopen( filespec );
    cac = textscan( fid, '%*q%*q%*s%*f%*f%*f%*q%*q%q', 'Delimiter', ',' );
    fclose( fid );
    buf = regexprep( cac{1}, '\] *\,', '];' );  % replace "]," by  "];"
    out = cell( size( buf ) );
    for jj = 1 : length( buf )
        str = buf{jj};
        out{jj} = str2num( str );
    end
end

and cssm.csv contains the two lines of your question.

Best Answer

Related Solutions

MATLAB: How to find the coefficients of a polynomial equation for fit the curve in MATLAB

MATLAB: Read nonrectangle csv with brackets

Related Question