Functions and Introduction to Recursion-程式設計筆記

Chapter 5 Functions and Introduction to Recursion

Cmath

#include <cmath>

Defining Our Own Fucntion

• Exapmple(square function)

  #include <iostream>
  int square(int); //function prototype declaration
  int main(){
      ...sth
  }
  int square(int y){
      return y*y;
  }

• We can define a function with default value.
• Default value must be the rightmost in a function’s parameter list.

#include <iostream>
int square(int y=1){
    return y*y;
}
int main(){
    square(); //cout 1
    square(2); //cout 4
}

void

• Used when defining a function that doesn't return a value.

  void square(int x){
      std::cout<<x*x;
  }

Standard Library Header Files

Random Number

#include <cstdlib>
rand()

Time Function

#include <ctime>
...
time();/How many seconds has passed since 1970.1.1 00:00:00

Why is rand() not that random?

• You have to set random seed.

  #include <cstdlib>
  ...
  unsigned int seed{0};
  cin>>seed; //input a random number.
  srand(seed);

  #include <ctime>
  #include <cstdlib>
  ...
  srand(time());
  ...

Enumerate

• If i set win=1, lost will be 2, continue will be 3.
• win{1} is not allowed, only win=1 will work.

  //example
  enum Game_Status{WIN, LOST, CONTINUE}; 
  //win=0, lost=1, continue=2.

static using with local variables

• You can see it as a way to make local variable to become a global variable.
• It will only be initialized one time!

  //example
  static int x=1;

  //Example code
  #include <iostream>
  using namespace std;
  void static_num(){
      static int x=1;
      cout<<x<<"\n";
      x++;
      cout<<x<<"\n";
  }
  int main(){
      static_num();
      static_num();
  }

static using with global variables

• The variable used with static is only accessible for the specific file.

extern

• Get a variable from another file. Ex:

  //file_1
  int a=1;

  //file_2
  extern a;

inline

• The cost of defining a function is too big，inline will decrease the cost.
• The concept of inline is expansion
• inline can only be used when doing simple calculation.

  #include <iostream>
  using namespace std;
  inline cube(int x){
      return x*x*x;
  }
  int main(){
      int x=0;
      cin>>x;
      cout<<cube(x); //cube(x)=x*x*x
  }

Pass by reference.

• It will affect the original value of a variable.
• Adding a '&' in front of a variable, with '&' the function will directly put the variable (num) into the function.
• It won't use extra ram space to store a number. (int a=1; int b=a; will use extra ram space)
• The concept is like domain.

  void function(int &number){
      ...
  }

  //example
  #include <iostream>
  using namespace std;
  void cube(int &x){
      x=x*x*x;
  }
  int main(){
      int num=0;
      cin>>num;
      cube(num);
      cout<<num;
  }

Acces to a global variable

#include <iostream>
int x=1;
int main(){
    int x=5;
    std::cout<<::x;
}

Function overloading

• C++ enables several functions of the same name to be defined, as long as they have different signatures.
• The compiler will automatically choose the proper function to execute. Depending on the data type or the amount of the parameter list
  

  #include <iostream>
  void square(int x){
      std::cout<<x*x<<"\n";
  }
  void square(double x){
      std::cout<<x*x<<"\n";
  }
  int main(){
      square(1); //cout 1
      square(1.1); //cout 1.21
  }

Making and using your own library

• Example: Finding max of three numbers.
• .h template
• template <class T> means that every type of data can use this function.
• The way to decalre a template function is the same as normal function.

  template <class T>
  T maximum(T value1, T Value2, T Value3){
      T maximumValue =value1;
      if(Value2>maximumValue){
          maximumValue=Value2;
      }
      if(Value3>maximumValue){
          maximumValue=Value3;
      }
      return maximumValue;
  }

• main program

  #include <iostream>
  #include "maximum.h"
  int main(){
      int x, y, z;
      while(std::cin>>x>>y>>z){
          std::cout<<"Max= "<<maximum(x, y, z);
      }
  }

Recursion

• A function calling oneself.
• Example of factorial of a nonnegative function with recursion.

  #include <iostream>
  unsigned long factorial(unsigned long num){
      if(num<=1){
          return 1;
      }
      else{
          return num*factorial(num-1);
      }
  }

• Example of Fibonacci Series

  #include <iostream>
  unsigned long fib(usigned long num){
      if(num==1||num==0){
          return 0;
      }
      else{
          return fib(num-1)+fib(num-2);
      }
  }