[c/c++] programming之路（23）、字符串（四）——strncat，atoi，strcmp，strlen等，以及常用内存函数

一、strncat及自行封装实现

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>
#include<string.h>    //<string.h>是C版本的头文件，包含比如strcpy、strcat之类的字符串处理函数。
                    //<cstring>是C++版本的<string.h>
                    //<string>定义了一个string的字符串类，包含了string类的各种操作，如s.size(), s.erase(), s.insert()等。
                    //但<string>又包含了老的C版本的字符串操作如strcpy、strcat等

void main() {
    char str[30] = "yincheng";
    char str1[20] = "8848.88";
    strncat(str, str1, 4);//从str1中拷贝4个字节到str
    printf("%s
", str);
    system("pause");
}

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>

void mystrncat(char *bc, char *c, int length)
{
    if (bc == NULL || c == NULL || length == 0)
        return;
    char *p = bc;
    while (*p)
        p++;
    //循环结束后，p指向了字符串的末端''
    for (int i = 0; i < length; i++)
    {
        *p = c[i];
        p++;
    }
    *p = '';
}

void main() {
    char str[30] = "task";
    char str1[20] = "list8848.88";
    mystrncat(str, str1, 4);
    printf("%s
", str);
    system(str);
    system("pause");
}

二、atoi

#include<stdio.h>
#include<stdlib.h>

void main() {
    char str[10] = "88e48";
    //int num = atoi(str);//8848
    //int num = atoi(str+1);//848
    int num = atoi(str);//8848

    char s1[10] = "88e48";
    //int n1 = atoi(s1);//88
    int n1 = atoi(s1+3);//48

    char s2[10] = "e8848";
    //int n2 = atoi(s2);//0
    int n2 = atoi(s2 + 1);//8848

    printf("%d
", n2);
    system("pause");
}

 三、strrev（字符串逆转）

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

void mystrrev(char *p) {
    int length = strlen(p);//获取字符串长度
    for (int i = 0; i < length/2; i++)//length必须除以2，不然字符对调之后又调回去了
    {
        char temp = p[i];
        p[i] = p[length - 1 - i];
        p[length - 1 - i] = temp;
    }
}

void main() {
    char str[20] = "yincheng8848a";
    printf("原来字符串 %s
", str);
    //_strrev(str);
    mystrrev(str);
    printf("后来字符串 %s
", str);
    system("pause");
}

 四、strupr和strlwr（大小写转换）

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

void tobig(char *p) {
    while (*p)
    {
        if ((*p)>= 'a'&&(*p) <= 'z')
            *p -= 32;
        p++;
    }
}


void main() {
    char str[20] = "notepad";
    //_strupr(str);//小写转大写
    //toupper用法;    #include<cctype>    ch = toupper(ch);//ch是一个字符
    tobig(str);
    printf("%s
", str);

    char str1[20] = "TASKLIST";
    _strlwr(str1);//大写转小写
    printf("%s
", str1);

    system("pause");
}

 五、strlen

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

int mystrlen(char *p) {
    int i = 0;
    while (*p) {
        i++;
        p++;
    }
    return i;
}


void main() {
    char str[100] = "i love iphone";//str是变量
    char *p = "i love china";//常量
    int len1 = strlen(str);
    int len2 = mystrlen(p);    

    printf("str=%d,p=%d
",len1,len2);

    system("pause");
}

 六、strcat(字符串拼接)

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

char * mystrcpy(char *dst,char *src) {
    char *p = dst;
    if (dst == NULL || src == NULL)
        return NULL;
    while (*src) {
        *dst = *src;
        dst++;
        src++;
    }
    *dst = '';
    return p;
}

char * mystrcat(char *dst, char *src) {
    char *p = dst;
    if (dst == NULL || src == NULL)
        return NULL;
    while (*dst) 
        dst++;
    while (*src) {
        *dst = *src;
        dst++;
        src++;
    }
    *dst = '';
    return p;
}



void main() {
    char str1[10] = "note";
    char str2[10] = "pad";
    char str[20];
    printf("%s
", mystrcpy(str, str1));
    //mystrcpy(str, str1);//复制
    printf("%s
", str);
    //strcat(str, str2);//拼接
    mystrcat(str, str2);//拼接
    printf("%s
", str);
    system(str);
}

void main0() {
    char str1[10] = "note";
    char str2[10] = "pad";
    char str[20];
    sprintf(str, "%s%s", str1, str2);
    system(str);
    system("pause");
}

 七、strchr(查找某字符是否在字符串中)

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

char * mystrchr(char *dst,char ch) {
    if (dst == NULL)
        return NULL;
    while (*dst) {
        if (*dst == ch) 
            return dst;
        dst++;
    }
    return NULL;
}


void main() {
    char str[20] = "i love china";
    char ch='o';
    /*char *p = strchr(str, ch);*/
    char *p = mystrchr(str, ch);
    if(p==NULL)
        printf("没找到
");
    else
        printf("值%c，地址%x
",*p,p);

    system("pause");
}

 八、strcmp(比较两字符串是否相等)

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

int mystrcmp(char *p1,char *p2) {
    int l1 = strlen(p1);
    int l2 = strlen(p2);
    if (l1 != l2)
        return -1;
    else {
        for (;*p1;p1++,p2++)
        {
            if (*p1 != *p2)
                return -1;
        }
    }
    return 0; 
}

void main() {
    char str1[10] = "note";
    char str2[10] = "note";
    /*if (strcmp(str1,str2)==0)*/
    if (mystrcmp(str1, str2) == 0)
        printf("相等
");
    else
        printf("不相等
");
    system("pause");
}

void main0() {//无论相等还是不等，都输出不等
    char str1[10] = "note";
    char str2[10] = "note";
    if(str1==str2)//c语言判断字符串不能这样
        printf("相等
");
    else
        printf("不相等
");
    system("pause");
}

 实现排序

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

void main() {
    char str[10][20] = {"apple","google","IBM","Intel","Microsoft","baidu","ali","tencent","360","Sun"};
    printf("排序之前
");
    for (int i = 0; i < 10; i++)
        printf("%s
", str[i]);
    printf("排序之后
");
    //冒泡排序
    for (int i = 0; i < 10-1; i++)
    {
        for (int j = 0; j < 10 - 1 - i; j++)
        {
            if (strcmp(str[j], str[j+1]) > 0) {
                //交换字符串,不可以用赋值号
                char temp[30];
                strcpy(temp, str[j]);
                strcpy(str[j],str[j+1]);
                strcpy(str[j+1],temp);
            }
        }
    }
    for (int i = 0; i < 10; i++)
    {
        printf("%s
", str[i]);
    }

    system("pause");
}

八、strstr(在字符串中查找是否包含另一个字符串)

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

char * mystrstr(char *allstr,char *str) {
    if (allstr == NULL || str == NULL)
        return NULL;
    int all_len = strlen(allstr);
    int str_len = strlen(str);
    for (int i = 0; i <all_len-str_len+1; i++)
    {
        int flag = 1;//标示，假定字符串一开始相等
        for (int j = 0; j <str_len; j++)
        {
            if (allstr[i + j] != str[j]) {
                flag = 0;
                break;
            }
        }
        if (flag)
            return allstr+i; //返回找到的地址
    }
    return NULL;
}

void main() {
    char allstr[40] = "i love china i love cpp";
    char str[10] = "china";
    //char *p = strstr(allstr, str);//检索
    char *p = mystrstr(allstr, str);//检索
    if(p==NULL)
        printf("没找到
");
    else
        printf("值%c，地址%x
",*p,p);
    system("pause");
}

 九、常用内存函数

1.memset

#include<stdio.h>
#include<stdlib.h>
#include<string>
#include<memory.h>//内存操作函数

void main() {
    char str[40] = "i love china i love c i love cpp";
    //第一个参数是内存的首地址，第二个参数是要设置的字符
    //第三个参数是整数，从首地址开始前进多少字节，把这一段设置为该字符
    memset(str, 'A', 10);
    printf("%s
",str);
    memset(str, 'A', 12);
    printf("%s
", str);
    memset(str, '',strlen(str));//清空字符串
    printf("%s
", str);
    system("pause");
}

2.memcpy

#include<stdio.h>
#include<stdlib.h>
#include<string>
#include<memory.h>//内存操作函数

void main0() {
    char str[30] = "yincheng8848";
    char str1[20] = "hello c";
    printf("%s
", str);
    //memcpy:从地址str开始，从str1处拷贝5个字节到str
    memcpy(str,str1,5);
    printf("%s
",str);
    
    system("pause");
}

void main() {
    int a[] = { 1,3,5,7,9 };
    int b[] = { 0,2,4,6,8 };
    memcpy(a,b, 8);//每个元素4个字节，两个元素8个字节
    //memcpy按照内存字节来拷贝，不管是什么类型，都是拷贝二进制类型
    for (int i = 0; i < 5; i++)
    {
        printf("%d
",a[i]);
    }
    system("pause");
}

3.memccpy

#include<stdio.h>
#include<stdlib.h>
#include<string>
#include<memory.h>//内存操作函数

void main() {
    char str[30] = "yincheng8848";
    char str1[20] = "hello c";
    printf("%s
", str);
    _memccpy(str,str1,'l',7);
    //memccpy:如果str1没有字符‘l’，就等价于memcpy，读取7个字符拷贝到str
    //如果有，读取拷贝到该字符就停止
    printf("%s
",str);
    
    system("pause");
}

4.memchr

#include<stdio.h>
#include<memory.h>

void main() {
    char str[30] = "helloyincheng";
    char ch='c';
    char *p =(char *) memchr(str, 'c', 30);
    //从地址str开始，前进30个字节检索
    //如果存在，返回地址，否则返回空
    if (p == NULL)
        printf("没找到
");
    else
        printf("找到字符%c，地址%p", *p, p);
    getchar();
}

5.memicmp

#include<stdio.h>
#include<stdlib.h>
#include<memory.h>

void main() {
    char *buf1 = "ABCDE123";
    char *buf2 = "abcde456";
    int num = _memicmp(buf1, buf2, 5);
    //memicmp判断字符串前面几个字节是否相等
    //忽略大小写，相等返回0，反之非0
    if (num == 0)
        printf("忽略大小写的情况下，相等");
    else
        printf("不等");
    
    getchar();
}