eval giving the wrong mathematical answer

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Katie Doyle
Katie Doyle 2015-2-22
编辑： John D'Errico 2015-2-22
I'm trying to write a script that I will use for Newton Rhapson approximation. I am testing as I go and I noticed that the eval command is returning the incorrect mathematical answer. I am testing it with x=2 with func=((2*x)*(log(x)))-1
Here is the code:
syms x;
func=input('Enter your function in terms of x: ');
f=@(x)(func);
z=diff(f(x));
f1=@(x)(z);
x0=input('Enter your initial value of x: ');
x=x0
s=char(f1);
y=subs(f1)
eval(y)
Upon entering x=2 and func=((2*x)*(log(x)))-1
The code redturns:
x =
2
y =
2*log(2) + 2
ans =
3.3863
The answer to 2*log(2)+2 should be 2.602
What am I doing wrong?

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回答（1 个）

Roger Stafford
Roger Stafford 2015-2-22
In matlab the 'log' function is considered to be the natural logarithm with base e, not 10. Therefore 3.3863 is the correct answer, not 2.602 .
  1 个评论
John D'Errico
John D'Errico 2015-2-22
编辑：John D'Errico 2015-2-22
And of course, if you really wanted a log to the base 10, use log10.
2*log10(2)+2
ans =
2.6021
Computers are so silly. They do what you tell them to do, instead of what you want them to do.

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