###### tags: `程設筆記`
{%hackmd theme-dark %}
# Chaper 7 Pointers
## What is Pointers?
* Declaration: int *pointer_name;/char *pointer_name;
* Data being stored inside a pointer is the address(unsigned int: hex) of a data in the memory.
* Pointer shoud be initialized. A pointer with null or the value 0 is called null pointer.(0 is the only integer that can be assigned to a pointer variable)
ex:

## How to assign the address of a data to pointers?
* "*" and "&" are inverses of one another.
ex:

#include <iostream>
#include <string>
using namespace std;
int main(){
	int *pointer;
	int a=10;
	pointer=&a;
	cout<<pointer<<"\n"; //cout the address of a.
	cout<<*pointer<<"\n";cout<<the value of a.
	cout<<*&a; //cout the value of a.
}

## The relations between pointers and data
* alter a <=> alter the value of *pointer.
* The address won't be changed.

ex:

#include <iostream>
#include <string>
using namespace std;
int main(){
	int *pointer;
	int a=10;
	pointer=&a;
	cout<<pointer<<" "<<*pointer<<" "<<a<<"\n";
	*pointer=100;
	cout<<pointer<<" "<<*pointer<<" "<<a<<" \n";
}

## PassByReference with Pointers
ex:

#include <iostream>
using namespace std;
void cube(int *pointer){
	*pointer= *pointer * *pointer * *pointer;
}
int main(){
	int length;
	while(cin>>length){
		cube(&length);
		cout<<length<<"\n";
	}
}

## Const and Pointers

const int* pointer1=# //The number pointer1 points to is a const.
int* const pointer2=# //pointer is a const.

* For pointer1:
* For pointer2:

ex(For pointer1):

#include <iostream>
using namespace std;
int main(){
	int a=10;
	const int* pointer;
	pointer=&a;
	cout<<*pointer<<"\n";	//cout 10
	a=100;
	cout<<*pointer<<"\n";	//cout 100
	/*pointer=100;*/ //not allowed
}

ex(For pointer2):

#include <iostream>
using namespace std;
int main(){
	int a=10, b=100;
	int* const pointer=&a;
	*pointer=1000;
	cout<<a<<"\n";
	/*pointer=&b*/ //not allowed
}

## Pointers and Array
* int array and pointers

#include <iostream>
using namespace std;
int main(){
	int a[5]={0, 1, 2, 3, 4};
	int *pointer=a;	//a is the address of a[0]
	for(int i=0; i<5; i++){
		cout<<*(a+i)<<" "; //cout 0~4
	}
	cout<<endl;
	for(int i=0; i<5; i++){
		*(a+i)=0;
	}
	for(int i=0; i<5; i++){
		cout<<*(a+i)<<" "; //cout 0
	}
	cout<<endl;
	cout<<&a[4]-pointer;	//4 the distance(elements) between a[4] and pointer(a[0]). = 2*sizeof(int)
}

* char array and pointers

#include <iostream>
using namespace std;
int main(){
	char a[18]="Helloworld";
	cout<<a<<"\n"; //it won't cout the address of a[0]
	cout<<a+5<<"\n"; //it will cout world
	cout<<*(a+9)<<"\n"; //equals to a[10];
	cout<<(void*)a<<"\n"; //it will cout the address of a[0](H)
	cout<<(void*)a+9<<"\n"; //it will cout the address of a[9](d)
}

* c-string and pointers


* sizeof()
* sizeof() with array or value will return the total size in bytes.

Ex1(with array):

#include <iostream>
using namespace std;
int main(){
    int arr[5]={0, 1, 2, 3, 4};
    cout<<sizeof(arr)<<"\n";  //cout the total size of the array(bytes)
    cout<<sizeof(arr)/sizeof(arr[0]); //cout the total amount of elements in arr.    
}

Ex2(with value):

#include <iostream>
using namespace std;
int main(){
    double a=10.5;
    int b=10;
    cout<<sizeof(a+b); //return 8 since the result of int and double addition is still double 
}    

Ex3(with c-string):

#include <iostream>
using namespace std;
int main(){
    char string[11]="Helloworld";
    cout<<sizeof(string);
}

* void* => a pointer for any genre of data type.
* Before using it, the type of pointer must be determined.

#include <iostream>
using namespace std;
int main(){
    int a=10;
    void* pointer;
    pointer=&a;
    cout<<pointer<<"\n"; //cout the address 
    cout<<*(int*)pointer<<"\n"; //cout the number a correctly
}

* Use pointers to print a 2d array

#include <iostream>
using namespace std;
void print_array(int *a, int rows, int columns){
    for(int i=0; i<rows; i++){
        a=a+i*columns;
        for(int j=0; j<columns; j++){
            cout<<*(a+j)<<" ";
        }
        cout<<"\n";
    }
}
int main(){
    int a[2][2]={{0, 1}, {2, 3}};
    print_array(&a[0][0], 2, 2);
}

* Pointers for 2D array


* Pointers for 3D array


* Arrays of Pointers
* The array stores the address of the first character in each c-string.

Ex1:

    const char * const suit[ 4 ] =
    { "Hearts", "Diamonds",
    "Clubs", "Spades" };

Ex2:

    #include <iostream>
    using namespace std;
    int main(){
        char *suit[2]={"Hello", "World"}; //stores the address of the first character in each string
        for(int i=0; i<sizeof(suit)/8; i++){
            cout<<suit[i]<<" ";
        }
    }    

## Function's Pointer
* Pass the pointer of a function to another fucntion

#include <iostream>
#include <vector>
#include <iomanip>
using namespace std;
void swap(int &a, int &b){
    int temp=a;
    a=b;
    b=temp;
}
void selection_sort(vector <int> &v, void (*swap)(int &a, int &b)){
    for(int i=0; i<v.size()-1; i++){
        for(int j=i+1; j<v.size(); j++){
            if(v[j]<v[i]){
                swap(v[j], v[i]);
            }
        }
    }
    for(int i=0; i<v.size(); i++){
        cout<<v[i]<<" ";
    }
}
int main(){
    vector <int> test={1, 2, 5, 3, 7, 6, 10};
    selection_sort(test, swap);
}

## Selection sort
Reference(How it works):https://www.geeksforgeeks.org/selection-sort/
* Easy to program but inefficient.
Ex:

#include <iostream>
#include <vector>
using namespace std;
void swap_double(int &a, int &b){
	double temp=a;
	a=b;
	b=temp;
}
void sort(vector<int> &vec){
    for(int i=0; i<vec.size()-1; i++){
        for(int j=i+1; j<vec.size(); j++){
            if(vec[j]<vec[i]){
                swap_double(vec[j], vec[i]);
            }
        }
    }
}
int main(){
    vector<int> test={0, 7, 3, 5, 10};
    sort(test);
    for(int i=0; i<test.size(); i++){
        cout<<test[i]<<" ";    
    }
}