string.h(cstring)函数库学习笔记

字符串类库，非常实用的一个库，里面的一些函数操作可以省掉很多比较麻烦的实现。下面来一一说明。

（1）strcpy
字符串复制函数，原型是:

char *strcpy(char* dest, const char *src);

很明显的就是将src的内容给了dest，例如：

#include <string.h>
int main()
{
    char a[50]="this is a string.";
    char b[50]="this is another string.";
    strcpy(b,a);
    puts(b);
    char c[50];
    strcpy(c,"Successful!");
    puts(c);
    return 0;
}

运行结果：

this is a string.
Successful!

（2）strcmp

字符串比较函数，原型如下：

extern int strcmp(const char *s1,const char *s2);

作用就是比较s1和s2的大小关系，需要注意的是，这里比较的是两个字符串之间的对应ascii码值的字典序大小，和字符串的长度没有关系，返回的结果是两个字符串的相对大小关系，如果s1

#include <cstring>
int main()
{
    char a[50]="this is a string.";
    char b[50]="this is another string.";
    char c[50]="this is another string.";
    char d[50]="this is d.";
    printf("%d\n",strcmp(a,b));//a<b
    printf("%d\n",strcmp(b,c));//b==c
    printf("%d\n",strcmp(d,a));//d>a
    return 0;
}

运行结果：

-1
0
1

（3）strchr

字符查找函数，原型是：

extern char *strchr(const char *s,char c);

用来查找字符c是否在字符串s中出现过，如果出现过就返回在s中首次出现的地址，如果没有出现过就返回NULL。例如：

#include <cstring>
int main()
{
    char a[50]="asdfghjkl";
    char *b;
    for(char x='a';x<='z';x++)
    {
        b=strchr(a,x);
        if(b!=NULL)
            printf("%c is found at %d\n",x,b-a);//b-a表示返回该字符在字符串中首次出现的下标
        else
            printf("%c is not found\n",x);
    }
    return 0;
}

运行结果：

a is found at 0
b is not found
c is not found
d is found at 2
e is not found
f is found at 3
g is found at 4
h is found at 5
i is not found
j is found at 6
k is found at 7
l is found at 8
m is not found
n is not found
o is not found
p is not found
q is not found
r is not found
s is found at 1
t is not found
u is not found
v is not found
w is not found
x is not found
y is not found
z is not found

（4）strstr
字符串查找函数，函数原型：

extern char *strstr(char *str1, const char *str2);

作用是在str1中查找str2，看看str2是不是str1的子串，如果是的话就返回第一次出现的地址，否则就返回NULL。例如：

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

int main()
{
    char a[50]="this is a string";
    char b[50]="this";
    char c[50]="string";
    char d[50]="substring";
    if(strstr(a,b)!=NULL)
        printf("\'%s\' is the substring of \'%s\'\n",b,a);
    else
        printf("\'%s\' is not the substring of \'%s\'\n",b,a);
    if(strstr(a,c)!=NULL)
        printf("\'%s\' is the substring of \'%s\'\n",c,a);
    else
        printf("\'%s\' is not the substring of \'%s\'\n",c,a);
    if(strstr(a,d)!=NULL)
        printf("\'%s\' is the substring of \'%s\'\n",d,a);
    else
        printf("\'%s\' is not the substring of \'%s\'\n",d,a);
    return 0;
}

运行结果：

'this' is the substring of 'this is a string'
'string' is the substring of 'this is a string'
'substring' is not the substring of 'this is a string'