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lecture9 - templates

lecture9 - templates - Wednesday February 5 th Custom...

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Wednesday, February 5 th   Custom Comparison Operators Templates The Standard Template Library (STL) STL Iterators STL Algorithms (find and find_if) For more recursion review (hint!), go to: http://www.sparknotes.com/cs/recursion/examples/summary.html
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Recursion Challenge The following function iteratively computes x n int exponentiate(int x, int n) { int i, result = 1; for(i=0; i<n; i++) result *= x; return result; } Write a function to do this recursively. 
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Custom Comparison Operators Consider the following  main  function that compares various objects to  each other… Notice that the way we compare two  dogs   (by weight)  is different than the  way we compare two  circles (by  radius) . main() { int i1 = 3, i2 = 5; if (i1 > i2) cout << “i1 is bigger”; Circ a(5), b(6); if (a. radius () > b. radius ()) cout << “a was bigger”; Dog fido(10), spot(20); if (fido. weight () > spot. weight ()) cout << “fido is bigger”; } Wouldn’t it be nice if we could  compare objects like circles and dogs  just like we compare two integers? if (a > b) if (fido > spot) cout << “fido is bigger”; We can! Let’s see how!
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The way to solve this is to  define a  comparison  function  for each data  type… bool operator > (const Dog &a, const Dog &b) { if (a. weight () > b. weight ()) return(true); else return(false); } bool operator > (const Circ &a,const Circ &b) { if (a. radius () > b. radius ()) return(true); else return(false); } Once defined, you can use  the operator in your  program! main() { Dog fido(5), spot(3); if (fido > spot) cout << “fido wins”; Circ c(3), d(2); if (c > d) cout << “c is bigger”; } Custom Comparison Operators And of course, you can  define your own versions  of other comparison  operators too… bool operator == (const Dog &a, const Dog &b) { if (a. weight () == b. weight ()) return(true); else return(false); } bool operator != (const Dog &a, const Dog &b) { if (a. weight () != b. weight ()) return(true); else return(false); } bool operator >= (const Dog &a, const Dog &b) { if (a. weight () >= b. weight ()) return(true); else return(false); } = fido weight 5 spot weight 3 Simply using the >=  operator causes C++ to call  your comparison function! 5 3 All comparison operators must return a  Boolean value:  true  or  false All comparison operators accept two  const reference parameters. These refer to the  two objects being compared.  (While making them const isn’t always  required, leaving it out  can  cause compiler  errors!) Carey says: Similarly,  Dog’s weight() function   must be const. class Circ { public: int radius() const { return(m_rad); } ... private: int m_x,m_y,m_rad; }; Carey says: Oh, and by the way… since  a  and  b  are  const,  Circ’s radius() function  now needs to  be const too or else you’ll get an error!
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Templates In this code, we’ve written  several different  swap  functions  that swap the two values passed  into the function.
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