Subs does not calculate result

syms x;
y=x*log(x)-1;
dy=diff(y);
u=y/dy;
du=diff(u);
y1=x-(u/du);

When i run subs(y1,x,0.4), answer is ((2*log(2/5))/5 - 1)/(((5*((2*log(2/5))/5 - 1))/(2*(log(2/5) + 1)^2) - 1)*(log(2/5) + 1)) + 2/5 which is not a useful answer for me. However, when i replace x*log(x)-1 with x^2+2*x+1, it calculates the function for x=0.4 and it shows a numerical result unlike previous attempt.

What could I do?

Answer 1

[Dyuman Joshi]

You can use vpa() to get the answer as a variable precision symbolic number

syms x
y = x*log(x)-1;
dy = diff(y);
u = y/dy;
du = diff(u);
y1 = x-(u/du);

val = subs(y1, x, 0.4)

val = 

vpa(val)

ans = 

0.43341516854082805627269878141879

To directly get the output by plugging in the input value instead of using subs(), define y as a function of x -

Defining y as a function of x will result in subsequent variables being defined via the operations as a functions of x as well.

syms x

y(x) = x*log(x)-1;
dy = diff(y);
u = y/dy;
du = diff(u);
y1 = x-(u/du);


vpa(y1(0.4))

ans = 

0.43341516854082805627269878141879

To convert the value into a numeric data type, use double (the default numeric data type in MATLAB)

double(y1(0.4))

ans = 0.4334

Answer 2

[Steven Lord]

If you want to see or obtain the floating point approximation to the symbolic answer, use the double or vpa functions or change the symbolic preferences using sympref.

two = sym(2)

two = 

2

s = sqrt(two)

s = 

vpa(s, 10) % Display to 10 decimal places

ans = 

1.414213562

format longg
d = double(s) % convert to double precision

d = 
           1.4142135623731

oldprefvalue = sympref('FloatingPointOutput', true); % Change the preferences
disp(s)

1.4142

% Reset the preferences
sympref('FloatingPointOutput', oldprefvalue);
disp(s) % Should behave the same as the second line of code above