syms but not int

If I enter this command:

>> syms x, int(1/(x^2-1))

I get the correct answer:

ans = -atanh(x)

But if I enter this command:

>> int('1/(x^2-1)','x')

I get this error:

Undefined function 'int' for input arguments of type 'char'.

Why is that and how to fix it?

I am running MATLAB R2014a v8.3 with Symbolic Math Toolbox v6.0.

Thanks

Answer 1

[John BG]

Florin

You have the following options:

1.- upgrade to R2016a or R2016b problem solved, whatever you key in, as long as syntactically correct,

   int(1/(x^2-1))
  ans =
  -atanh(x)

or

int(sym('1/(x^2-1)'),'x')
Warning: Support of strings that are not valid variable names or define a
number will be removed in a future release. To create symbolic expressions,
first create symbolic variables and then use operations on them. 
> In sym>convertExpression (line 1536)
  In sym>convertChar (line 1441)
  In sym>tomupad (line 1198)
  In sym (line 177) 
ans =
-atanh(x)

int(sym('1/(x^2-1)'))
Warning: Support of strings that are not valid variable names or define a
number will be removed in a future release. To create symbolic expressions,
first create symbolic variables and then use operations on them. 
> In sym>convertExpression (line 1536)
  In sym>convertChar (line 1441)
  In sym>tomupad (line 1198)
  In sym (line 177) 
ans =
-atanh(x)

What int command does not take in are strings without sym

int('1/(x^2-1)')
Undefined function 'int' for input arguments of type 'char'.

2.- Use command integral with discrete variables, computable vectors, like

clear x % if you leave x from previous sym  x MATLAB holds to already defined syms variable

f=@(x) 1./(x.^2-1)
integral(f,0,10)
Warning: Reached the limit on the maximum number of intervals in use.
Approximate bound on error is   1.6e+00. The integral may not exist, or it
may be difficult to approximate numerically to the requested accuracy. 
> In integralCalc/iterateScalarValued (line 372)
  In integralCalc/vadapt (line 132)
  In integralCalc (line 75)
  In integral (line 88) 
ans =
  -0.586189361714442

the warning has to do with the vertical asymptotes on x=-1 and x=1.

integral(f,2,Inf)

 without asymptotes along the integration line, then no warnings:

 integral(f,2,Inf)
ans =
   0.549306144334055

if you find my answer useful would you please mark my answer as Accepted Answer?

To any other reader, please if you find this answer of any help, please click on the thumbs-up vote link,

thanks in advance for time and attention

John BG

<mailto:[e-mail] [e-mail]>

[Walter Roberson]

Upgrade is not required. R2014a supported int() of symbolic expressions created with sym().

>> ver
----------------------------------------------------------------------------------------------------
MATLAB Version: 8.3.0.532 (R2014a)
MATLAB License Number: XXXXXXXXXX
Operating System: Mac OS X  Version: 10.11.6 Build: 15G1212 
Java Version: Java 1.7.0_11-b21 with Oracle Corporation Java HotSpot(TM) 64-Bit Server VM mixed mode
----------------------------------------------------------------------------------------------------
MATLAB                                                Version 8.3        (R2014a)
Simulink                                              Version 8.3        (R2014a)
Communications System Toolbox                         Version 5.6        (R2014a)
Computer Vision System Toolbox                        Version 6.0        (R2014a)
Control System Toolbox                                Version 9.7        (R2014a)
Curve Fitting Toolbox                                 Version 3.4.1      (R2014a)
DSP System Toolbox                                    Version 8.6        (R2014a)
Global Optimization Toolbox                           Version 3.2.5      (R2014a)
Image Processing Toolbox                              Version 9.0        (R2014a)
MATLAB Coder                                          Version 2.6        (R2014a)
Neural Network Toolbox                                Version 8.2        (R2014a)
Optimization Toolbox                                  Version 7.0        (R2014a)
Parallel Computing Toolbox                            Version 6.4        (R2014a)
Phased Array System Toolbox                           Version 2.2        (R2014a)
Signal Processing Toolbox                             Version 6.21       (R2014a)
Simulink Coder                                        Version 8.6        (R2014a)
Symbolic Math Toolbox                                 Version 6.0        (R2014a)
>> int(sym('1/(x^2-1)'),'x')

ans =

-atanh(x)

[Muhadda Rehan Saleem]

Hellow John BG

I am try Matlab command int('f',0,pi/2), command is working but answer is in fractional form, I want to get answer in floating point form

Answer 2

[Walter Roberson]

int() applies to symbolic expressions. '1/(x^2-1)' is a string, not a symbolic expression.

Even if you go back as far as R14, you will not find any documentation suggesting that int() can be applied to a string.

What you can do is

int(sym('1/(x^2-1)'),'x')

This will produce a warning in recent versions but as of R2016b will still succeed.

[Florin Georgescu]

Thank you for the reply, Walter.

I've tried the command you've suggested. Sadly I get an error:

Undefined function 'sym' for input arguments of type 'char'.

I took the example in my original post from the book MATLAB Differential and Integral Calculus (page 131) published by César Pérez López in 2014. I thought it's a reference book...

[Walter Roberson]

sym was definitely documented for that purpose in R2014a; see http://www.mathworks.com/help/releases/R2014a/symbolic/creating-symbolic-variables-and-expressions.html

I just tried in R2014a,

>> int(sym('1/(x^2-1)'),'x')

ans =

-atanh(x)

MATLAB Version: 8.3.0.532 (R2014a)

Symbolic Math Toolbox Version 6.0 (R2014a)

[Florin Georgescu]

I've installed MATLAB hoping to start learning the basics. But it is very disappointing that right out the box I stumble on such issues. Searching the web showed me this is the most common problem for every new MATLAB user. Is it a configuration issue that MATLAB wasn't capable to set from the start? Apparently all answers point to paths but everything was installed with the default settings. Telling me it works on your computer doesn't really helps me, does it? Otherwise I wouldn't be here stuck with a program incorrectly configured (probably) by default.

[Walter Roberson]

If syms works for you but sym() does not, then you have a corrupt installation.

If syms did not work for you either then I would have speculated that you do not have a license for the Symbolic Toolbox. Actually, that is still a possibility: sometimes when you do not have a license for a toolbox, but it is installed, some of the routines in the toolbox accidentally work anyhow.

[Karan Gill]

Florin, why are you using string input to "int"? It is not a documented input to "int".

[Walter Roberson]

Using a string input to int() has not been documented in any version for which the documentation is still available in Mathworks' archives.