C advanced

Function

Definition and declaration

Examples

sqrt(100.0)
pow(x,y)
strlen(str1)
strcmp (str1,str2) // compare
atoi(str1) // convert "12345" to 12345

usage

// independent
stringPrint();
// part of expression
num = max(a,b)/2;
// as args
num = min(sum(a,b),c);

Can use files file.h to store functions.

#include "file.h"

definition

void delay(int n) {
    for (int i = 0; i<n*100000; i++);
    return; // or none
    }

Declaration

If we want to put funcions after main func, we need to declare those functions before main.

float max(float, float);

int main() {

}
float max(float a, float b) {
}

Execution

pass value

m = max(a,b);

variable field

Global variables are defined outside functions.

#include<iostream>
using namespace std;
int a = 0, b = 0;
void exchange() {
    int p;
    if (a < b) {
        p = a; a = b; b = p;
    }
}

int main() {

    cin >> a >> b;
    exchange();
    return 0;
}

If a function variable has the same name as the global variable, global variable won’t work in this function.

arrays as args

Pass values as well while pass array elements as args.

If we use array name as args, it acts like passing ref. The operation inside will change array values. Pass the array name to the args. The array name is not a variable, but the location in memory.

void change(int a[]) {
    a[0] = 30; a[1] = 50;
}

int main() {
    int a[2] = {3,5};
    change(a);
    cout << a[0] << ' ' << a[1];
}
// will be 30 50

Excercise

// fill the extra width with '0' before the string 
cout << setfill('0') << setw(2) << day << endl;

Recursion

An example: reverse word

int recur() {
    char c;
    c = cin.get();
    if (c != '\n') 
        recur();
    cout << c;
    return 0;
}

Pointer

Get address operator &:

int c = 1;
cout << &c << endl;

Pointer operator * to access the resource:

cout << *&c << endl;
cout << c << endl;

Pointer variable

Definition

A variable to store the pointer of another variable

int *pointer = NULL; // int is base type, which is the type of the variable pointed to, usually use NULL to initialize
int c = 76;
pointer = &c;
pointer = c; //invalid

Content:

*pointer // return the variable c, not value 76

Order:

int *pointer = NULL;
int a = 1;
pointer = &a;
(*pointer)++ // a++, a value
// not equal to 
*pointer++ // same as (*pointer)++

//
pointer++ // will jump over the variable (like an int), move to next bit

Array

The case with pointer to array element makes no difference with the case of a variable.

int a[5] = {1,2,3,4,5};
int *p = &a[3];
// *p is a[3]

int a[5] = {1,2,3,4,5};
cout << a << endl; // an address
cout << *a << endl; // return the value of a[0]
cout << &a[0] << endl; // an address same as the first one

The name of an array represents the address of the first element in array
Array name is the pointer to the first element
Array name is not a variable, cannot assign a value to

Use pointer access array

int a[5] = {1,2,3,4,5};
int *p = NULL;
cout << a << endl;
p = a;

cout << p << endl;  // address
cout << *p << endl; // value of a[0]
cout << (*p)++ << endl; // a[0], "++" is do the front thing first then + 1
cout << *p++ << endl; // a[0]

If we have:

int a[10];
int *pointer;

pointer = a; 
// equivalent to
pointer = &a[0];

pointer + i == a + i == &a[i]
*(pointer + i) == *(a + i) == a[i]
pointer[i] == *(pointer + i)

Assign value to array

int a[10], i, *p = a;
for (i=0;i<10;i++)
    cin >> *p++;
for (p--;p >= a;)
    cout<<setw(3)<<*p--;

a++ is nonsense
p++ will change p
p can point to area other than array elements
- pay attention to the range
*--p: - 1 first then *

Reverse array

int a[10], *p = NULL, *q = NULL, temp;
for (p = a; p < a + 10;p++)
    cin >> *p;
for (p = a, q = a + 9;p < q;p++,q--)
{
    temp = *p; *p = *q; *q = temp;
}

2-D array

int a[3][4] = {1,2,...,12};
int *p;
// store and print row by row
for (p = &a[0][0]; p < &a[0][0] + 12; p++) 
    cout << p << " " << *p << endl;

Access an element

int a[3][4] = {1,2,...,12};
int (*p)[4];    // define a pointer pointing to an array of 4 elements
p = a;  // The first element of a is an array of 4 elements, so p should be the same
// access a[i][j]
cout << setw(4) << *(*(p+i)+j);

p+i == &a[i]
*(p+i) == a[i]
*(p+i) + j == a[i] + j
a[i] + j == &a[i][j]

Pointer & string

char a[10];
char *p;
p = a;

StrCPY

#include<iostream>
#include<iomanip>
using namespace std;
int main()
{
    char a[] = "How are you?", b[20];
    char *p1, *p2;
    for (p1 = a, p2 = b; *p1 != '\0'; p1++, p2++)
        *p2 = *p1;
    *p2 = '\0';
}

Print pointer of string

int main() {
    char c[6] = {'h','i','\0'};
    char *pc = c;
    
    cout << c << endl;  // hi
    cout << pc << endl; // hi, not address
}

Application

Assign string value directly to string pointer

#include<iostream>
using namespace std;
int main()
{
    char buf[10] = "ABC";
    char *pc;
    pc = "hello";   // a constant, cannot be modified
    cout << pc << endl; // hello
    pc++;
    cout << pc << endl; // ello
    cout << *pc << endl; // e
    pc = buf;
    cout << pc << endl; // ABC
    return 0;
}

`&` and `*`

a == &a[0]
&a is the pointer to the entire array
- &a+1 will jump over entrie array to outside
- &a will promote the domain of the pointer
*a == a[0]
- *a will decrease the domain of the pointer
- *(&a) == a

#include<iostream>
using namespace std;
int main()
{
    int a[4] = {1,3,5,7};
    cout << a << endl;  // pointer to a[0], address of a[0]
    cout << a + 1 << endl;  // address of a[1]
    cout << &a << endl;  // pointer to a, address of a, not a[0]
    cout << &a + 1 << endl; // address after array a
    cout << *(&a) << endl; // adress of a[0], like a single a
    cout << *(&a) + 1 << endl; // address of a[1]
}

Pointer to 2D array

Definition of 2D array

2D array a[3][4] consists of 3 elements: a[0], a[1] and a[2]
a[0] is the first element in 2D array
a[0] is a 1D array consisting of 4 elements
a == &a[0]
a[0] == &a[0][0]
&a > a == &a[0] > a[0] == &a[0][0] == *a > a[0][0] == **a

3 principles

array name is the pointer to the first element in array
&E promotes the domain of E by one level
*E decreases the domain of E by one level

Pointer and function

Pointer as function args

#include<iostream>
using namespace std;

void Rank(int *q1, int *q2)
{
    int temp;
    if (*q1 < *q2)
    {
        temp = *q1;
        *q1 = *q2;
        *q2 = temp;
    }
}
// Rank function will manipulate a and b directly
int main()
{
    int a, b, *p1, *p2;
    cin >> a >> b;
    p1 = &a; p2 = &b;
    Rank(p1,p2);
}

Use array name as func args

#include<iostream>
using namespace std;
void sum(int *p, int n)
{
    int total = 0;
    for (int i=0;i<n;i++)
        total += *p++;
    cout << total << endl;
}

int main()
{
    int a[4] = {1,2,3,4};
    sum(a, 4);
    return 0;
}

Use multi-D array name as function args

#include<iostream>
using namespace std;
// the definition of arg must have the same type as input a;
// a is a pointer to the first element in 2D array
int maxvalue(int (*p)[3])
{
    int max = p[0][0];
    for (int i = 0; i < 3; i++)
        for (int j = 0; j < 4;j++)
            if (p[i][j] > max)
                max = p[i][j];
            return max;
}
int main()
{
    int a[3][2] = { {1,2},{6,4},{3,5} };
    cout << maxvalue(a);
    return 0;
}

Can use array name as formal parameters. C++ compiler will treat formal parameter array name as pointer variable.

Can constrain functions to prevent modifying array

#include<iostream>
using namespace std;
// array elements are modified
int sum(int array[], int n)
{
    // start from the first element
    for (int i=0;i<n - 1;i++)
    {
        *(array + 1) = *array + *(array+1);
        array++;
    }
    return *array;
}

// array elements will not be modified
int sum(const int array[], int n)
{
    // start from the first element
    for (int i=0;i<n - 1;i++)
    {
    // error, cannot modify constant
        *(array + 1) = *array + *(array+1); 
        array++;
    }
    return *array;
}

int main()
{
    int a[4] = {1,2,3,4};
    cout << sum(a, 4);
    return 0;
}

const

Definition

const float PI = 3.14f;
float const PI = 3.14f;

Usage:

#include<iostream>
using namespace std;

void mystrcpy(char *dest, const char *src)
{...}

int main()
{
    int a = 256;
    const int *p = &a;
    *p = 257;   // error
}

#include<iostream>
using namespace std;
int main()
{
    const int a = 78; const int b = 28; int c = 18;
    const *pi = &a;
    *pi = 58;   // *p cannot be assigned value, cannot modify the value pi points to
    pi = &b;    // valid, can assign value to pi
}

Pointer as a return value

int *function(int x, int y)

Example

#include<iostream>
using namespace std;
int *get(int arr[][4], int n, int m)
// or int *get(int (*arr)[4], int n, int m)
{
    int *pt;
    pt = *(arr + n-1) + m - 1;
    return(pt);
}
int main()
{
    int a[3][2] = {1,2,3,4,5,6};
    int *p;
    p = get(a,2,3);
    cout << *p << endl;
}

Local variable

// wouldn't be valid, local variable will be discarded after execution of function
// can use global variable
// better: user static variable
int *getint1()
{
    int value1 = 20;
    return &value1;
}
// static local variable wouldn't be disgarded after execution
int *getint1()
{
    static int value1 = 20;
    return &value1;
}

Struct

Example:

struct student      // struct type named student
{
    int id;
    char name[20];
    char sex;
    int age;
    float score;
    char addr[30];
} st_1, st_2;  // ; is needed
// define two variables at the same time

#include<iostream>
using namespace std;
struct student
{
    int id_num;
    char name[10];
};

int main()
{
    
    student mike = {123,{'m','i','k','e','\0'}};
    student mike2;
    mike.id_num = 20130000 + mike.id_num;
    mike2 = mike;   // just copy
}

Struct and func

args: Pass a copy to the function, unlike array
return: Pass a copy back

#include<iostream>
using namespace std;
struct student
{
    int id_num;
    char name[10];
};

student newone()
{
    student one = {2013, {'m','i','k','e','\0'}};
    return one;
}

int main()
{
    student mike = newone();
}

Struct and pointer

#include<iostream>
using namespace std;
struct student
{
    int id_num;
    char name[10];
};

// pass pointer to function, will modify original variable
void renew(student *one)
{
    one->id_num = 2000000 + one->id_num;
}

int main()
{
    student mike = {123,{'m','i','k','e','\0'}};
    student *one = &mike;
    cout << (*one).id_num << endl;
    // or
    cout << one->id_num << endl;    // -> is an operator
    return 0;
}

Array struct

student myclass[3] = {...};
student *one = myclass;
cout one->id_num<<endl;
// will jump to next struct element
one++;
cout one->id_num<<endl;

Linked list

Definition

Structure

head: pointer to the first node
node: every element in the list
- value in current node
- address of next node
tail: won’t point to other node
- address is NULL

Create dynamically

// create memory space (initial value)
int *pint = new int(1024);  
// erase memory space
delete pint;

int *pia = new int[4];      delete[] pia;

struct student
{
    int id;
    student *next;
};

// step 1
student *head;
head = new student;
student *temp = head;

// step 2
// continue?
// yes
temp->next = new student;
temp = temp->next;
    // goto step 2
// no
temp->next = NULL;

Example

student *create()
{
    student *head, *temp; int num, n = 0;
    head = new student;
    temp = head;
    cin >> num;
    while (num!=-1)
    {
        n++;
        temp->id = num;
        temp->next = new student;
        temp = temp->next;
        cin >> num;
    }
    if (n == 0) head = NULL; else temp->next = NULL; //? temp = NULL?
    return head;
}

Operation

traversal
delete node
- head
- internal
insert node

2-direction linked list

temp->num
temp->next
temp->ahead

AI 2

Algorithm 17

Amazon 1

Authorization 1

Blog 3

Bootstrap 1

C++ 1

CCpp 5

CSS 2

Cloud 3

Code 1

Crawler 1

DNS 1

Database 17

DeepLearning 1

Design 16

Development 1

Docker 1

Express 1

GDB 1

Go 3

Google 4

HTML 3

IOS 1

Java 17

Javascript 4

Jekyll 1

Linux 4

MacOS 2

MachineLearning 16

Markdown 4

Mobile 1

MongoDB 2

Multi-threading 3

NAS 1

Network 11

NeuralNetwork 10

Node 1

OS 8

Public-speaking 1

Python 15

RESTful 1

Rails 9

React 1

Redis 1

Ruby 6

Shell 2

Spring 2

System 16

TCP 1

TDD 1

Thread 2

Vim 1

awk 1

git 1

jQuery 1

media 1

network 1

php 1

Function

Definition and declaration

Examples

usage

definition

Declaration

Execution

variable field

arrays as args

Excercise

Recursion

Pointer

Pointer variable

Definition

Array

Use pointer access array

2-D array

Pointer & string

Application

& and *

`&` and `*`