Hi there,i have to create a program that shows the symbolic derivative of a polynomial function, and my teacher told that it is all about string manipulation,but i cant find find any valid tutorial or something that shows me how to do it.i've checked but what i have found is those simple manipulation like adding sting, comparing, etc. Can you advise me any web page or book that can help me?

vijayan121 1,152 Posting Virtuoso

You would need to search for chars in strings, extract substrings from strings, convert numbers to strings and strings to numbers.

In general, most programming problems are easy to tackle if we reduce complexity by breaking up the problem into a number of smaller problems, each of which are easy to solve.

What we need to do is something like this:

given a string representation of a ploynomial: `f(x) = "2.4x^4 - 1.2x^3 + 6x^2 + 4.1x + 9.2"`

We need to get the string representation of its derivative: `f'(x) = "9.6x^3 - 3.6x^2 + 12x + 4.1"`

**1.** Device a facility to represent a polynomial in our program.

A term of the polynomial can be represented as a pair with a double coefficient and an int exponent. `typedef std::pair<double,int> term ;`

See: http://www.cplusplus.com/reference/std/utility/pair/

For example, the second term of f(x) is given by `std::make_pair( -1.2, 3 )`

A polynomial can be represented as a sequence of terms `typedef std::vector<term> polynomial ;`

See: http://www.cppreference.com/wiki/container/vector/start

For example, the sequence for f(x) would have the following terms: (2.4,4), (-1.2,3), (6.0,2), (4.1,1), (9.0,0)

**2.** Write a function to get the derivative of a term `term derivative( const term& t ) ;`

This is easy, derivative of (a,b) is (a*b,b-1); for example, derivative of (-1.2,3) would be (-3.6,2)

**3.** Write a function to get the derivative of a polynomial `polynomial derivative( const polynomial& p ) ;`

This too is easy; return a sequence which contains the derivative of every term in p. (You can omit any term which has a coefficient of zero).

**4.** Write a function which gives the string representation of a term `std::string to_string( const term& t ) ;`

use a std::ostringstream to do this. http://notfaq.wordpress.com/2006/08/30/c-convert-int-to-string/

If the term is (-1.2,3), return **"-1.2x^3"**, if the term is (1.2,3) return **"+1.2x^3"**, if the term has a coefficient of zero, return an empty string.

**5.** write a function which gives the string representation of a polynomial `std::string to_string( const term& t ) ;`

Just concatenate the string representation of each term in the polynomial. If the leading sign in the result string is a '-', you may drop it.

**6.**Write a function to convert a string to a term. `term to_term( const std::string& str ) ;`

use a std::istringstream for this.

```
term to_term( const std::string& str )
{
// removing spaces from the string first makes it easier
std::string cpy = str ;
std::string::size_type pos = 0 ;
while( ( pos = cpy.find( ' ', pos ) ) != std::string::npos ) cpy.erase(pos,1) ;
double coefficient ;
char variable ;
char exp_symbol ;
int exponent ;
std::istringstream stm(cpy) ;
stm >> coefficient >> variable >> exp_symbol >> exponent ;
return std::make_pair( coefficient, exponent ) ;
}
```

**7.** write a function to convert a string to a polynomial. `polynomial to_polynomial( const std::string& poly_str ) ;`

Parse **poly_str** to pick out the substring for each term in the polynomial.

Use std::string::find_first_of() http://www.cppreference.com/wiki/string/basic_string/find_first_of

and std::string::substr() http://www.cppreference.com/wiki/string/basic_string/substr

to do this.

Convert each substring to a term and return the sequence of terms.

**8.** Finally, put all these together to get the symbolic derivative of a polynomial string.

```
std::string symbolic_derivative( const std::string& str ) ;
{ return to_string( derivative( to_polynomial(str) ) ) ; }
```

Nick Evan commented: Nice one. +16

alfieclinton 0 Newbie Poster

String class contains many public methods can be used to manipulate strings. Most useful, they often be resolved here.There are two types of string manipulation methods

1. String instance method call through a String object.Equals (string).

2. String class method or static method is called through the String class, String.

pucivogel 0 Newbie Poster

Thanks to "vijayan121" help, i have a clearer idea now about my program,and i have done something like a plan of it, but i'm not very clear about the syntax of the program,i don't really now what to fix and how.

```
#include <iostream>
#include <sstream>
#include <utility>
using namespace std
int main()
{ string b = "4x^3+2x^5+3x^5";
parse(b)
term1= to_term(sub1);
term2= to_term(sub2);
term3= to_term(sub3);
derivative(term1);
derivative(term2);
derivative(term3);
cout<< to_string(derivative(term1))<<endl;
cout<<to_string(derivative(term2))<<endl;
cout<< to_string(derivative(term3))<<endl;
return 0;
}
void parse (const string &a)
{ string sub1 = a.substr(0,4);
cout<<sub1<<endl;
string sub2 = a.substr(4, 5);
cout<<sub2<<endl;
string sub3 = a.substr(9, 5);
<<cout<<sub3<<endl;
}
int to_term( const string& str )
{
string cpy = str ;
string::size_type pos = 0 ;
while( ( pos = cpy.find( ' ', pos ) ) != string::npos ) cpy.erase(pos,1) ;
double coefficient ;
char variable ;
char exp_symbol ;
int exponent ;
std::istringstream stm(cpy) ;
stm >> coefficient >> variable >> exp_symbol >> exponent ;
return std::make_pair( coefficient, exponent ) ;
}
int derivative (const term& t)
{ return (coefficient*exponent,exponent-1);
}
char to_string(const term& t)
{ int c=coefficient;
string sc;
std::stringstream out;
out<<c;
sc=out.str();
int e=exponent;
string se;
std::stringstream out;
out<<e;
se=out.str();
char stringterm= sc+"x^"+se;
return stringterm;
}
```

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