lecture9

# lecture9 - Wednesday Febr uar y 5 th Custom Compar i son...

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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)

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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.
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!
Templates In this code, we’ve written several different swap functions that swap the two values passed into the function. // the old way void SwapCircle (Circ &a, Circ &b) { Circle temp; temp = a; a = b; b = temp; } void SwapDog (Dog &d1, Dog &d2) { Dog temp; temp = a; a = b; b = temp; } main() { Circle a(5), b(6); Dog c(100), d(750); SwapCircle (a,b); SwapDog (c,d); } Wouldn’t it be nice if we could write one swap function and have it work for any data type?

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