一.字符分类函数
cpp
int main()
{
int ch = getchar();
if (ch >= 'A' && ch <= 'Z')
{
printf("大写\n");
}
else if (ch >= 'a' && ch <= 'z')
{
printf("小写\n");
}
return 0;
}
cpp
#include <ctype.h>
int main()
{
int ch = getchar();
putchar(ch);
if (islower(ch))
{
printf("小写\n");
}
else if (isupper(ch))
{
printf("大写\n");
}
return 0;
}
cpp
//把一个字符串转换成全小写
int main()
{
char arr[] = "I Am A Student";
int i = 0;
while (arr[i])
{
if (isupper(arr[i]))
{
arr[i] = arr[i] + 32;
}
putchar(arr[i]);
i++;
}
return 0;
}二.字符转换函数
cpp
int main()
{
char arr[] = "I Am A Student";
int i = 0;
while (arr[i])
{
if (isupper(arr[i]))
{
//arr[i] = arr[i] + 32;
arr[i] = tolower(arr[i]);
}
putchar(arr[i]);
i++;
}
return 0;
}
cpp
int main()
{
int ch = getchar();
int r = toupper(ch);
printf("%c\n", r);
return 0;
}三.strlen的使用和模拟实现
1.strlen的使用
cpp
#include <string.h>
#include <stdio.h>
int main()
{
//char arr[] = "abcdef";//[a b c d e f \0]
char arr[] = { 'a', 'b', 'c' ,'\0'};//[a b c]
size_t len = strlen(arr);
printf("%zd\n", len);
return 0;
}
cpp
int main()
{
if (strlen("abc") - strlen("abcdef") > 0)
{
printf("大于\n");
}
else
{
printf("小于等于\n");
}
return 0;
}
strlen返回的size_t 类型,size_t是无符号整性,两个无符号整性相减还是无符号整性
cpp
int main()
{
//3-6=-3
if ((int)strlen("abc") - (int)strlen("abcdef") > 0)
{
printf("大于\n");
}
else
{
printf("小于等于\n");
}
return 0;
}2.strlen的模拟实现
cpp
#include <assert.h>
size_t my_strlen1(const char* str)
{
size_t count = 0;
assert(str != NULL);
while (*str)
{
count++;
str++;
}
return count;
}
size_t my_strlen2(const char* str)
{
assert(str);
const char* start = str;
while (*str)
{
str++;
}
return str - start;
}
//写一个函数，不能使用临时变量，求字符串长度 - 递归
size_t my_strlen(const char* str)
{
if (*str == '\0')
return 0;
else
return 1 + my_strlen(str + 1);
}
//my_strlen("abcdef");
//1+my_strlen("bcdef");
//1+1+my_strlen("cdef");
//1+1+1+my_strlen("def");
//1+1+1+1+my_strlen("ef");
//1+1+1+1+1+my_strlen("f");
//1+1+1+1+1+1+my_strlen("");
//1+1+1+1+1+1+0;
int main()
{
char arr[] = "abc";
size_t len = my_strlen(arr);
printf("%zd\n", len);
return 0;
}四.strcpy的使用和模拟实现
1.strcpy的使用
cpp
int main()
{
char arr1[20] = "xxxxxxxxx";
char arr2[] = {'a', 'b', 'c','\0'};
strcpy(arr1, arr2);
printf("%s\n", arr1);
return 0;
}原字符串里面必须要有\0,否则会一直向后面进行拷贝
cpp
int main()
{
char arr1[5] = {0};//目标空间必须足够大
char arr2[] = "hello world";
strcpy(arr1, arr2);
printf("%s\n", arr1);
return 0;
}目标地址必须足够大,不然会出错
cpp
int main()
{
char* p = "abcdefghilmkqwer";//常量字符串，不能修改
char arr2[] = "hello world";
strcpy(p, arr2);//目标空间必须是可修改的
printf("%s\n", p);
return 0;
}2.strcpy的模拟实现
cpp
//模拟实现strcpy函数
void my_strcpy(char* dest, char* src)
{
while (*src != '\0')
{
*dest = *src;
dest++;
src++;
}
*dest = *src;
}
char* my_strcpy(char* dest, char* src)
{
/*assert(dest != NULL);
assert(src != NULL);*/
char* ret = dest;
assert(dest && src);
while (*dest++ = *src++)
{
;
}
return ret;
}
int main()
{
char arr1[20] = { 0 };
char arr2[] = "abcdef";
printf("%s\n", my_strcpy(arr1, arr2));
return 0;
}这个地方的*src++,是先算的++,但是是后置++,所以还是返回的src进行解引用
五.strcat的使用和模拟实现
cpp
char* my_strcat(char* dest, const char* src)
{
char* ret = dest;
assert(dest && src);
//1. 找到目标空间的\0
while (*dest)
{
dest++;
}
//2. 拷贝
while (*dest++ = *src++)
{
;
}
return ret;
}
int main()
{
char arr1[20] = "hello ";
char arr2[] = "world";
strcat(arr1,arr2);
my_strcat(arr1, arr2);
printf("%s\n", arr1);
return 0;
}
cpp
char* my_strcat(char* dest, const char* src)
{
char* ret = dest;
assert(dest && src);
//1. 找到目标空间的\0
while (*dest)
{
dest++;
}
//2. 拷贝
while (*dest++ = *src++)
{
;
}
return ret;
}
int main()
{
char arr1[20] = "hello ";
my_strcat(arr1, arr1);
printf("%s\n", arr1);
return 0;
}
库里面可以,但是还是最好不要自己给自己进行追加
六.strcmp的使用和模拟实现
1.strcmp的使用
cpp
int main()
{
int ret = strcmp("bbq", "abcdef");
printf("%d\n", ret);
return 0;
}
cpp
int main()
{
int ret = strcmp("bbq", "abcdef");
if (ret > 0)
printf("大于\n");
else if (ret == 0)
printf("等于\n");
else
printf("小于\n");
return 0;
}2.strcmp的模拟实现
cpp
int my_strcmp(const char* s1, const char* s2)
{
while (*s1 == *s2)
{
if (*s1 == '\0')
return 0;
s1++;
s2++;
}
if (*s1 > *s2)
return 1;
else
return -1;
}
int my_strcmp(const char* s1, const char* s2)
{
while (*s1 == *s2)
{
if (*s1 == '\0')
return 0;
s1++;
s2++;
}
return *s1 - *s2;
}
int main()
{
int ret = my_strcmp("abcdef", "abc");
if (ret > 0)
printf("大于\n");
else if (ret == 0)
printf("等于\n");
else
printf("小于\n");
return 0;
}七.strncpy的使用
cpp
int main()
{
char arr1[20] = "xxxxxxxxxx";
char arr2[] = "hello";
strncpy(arr1, arr2, 7);
return 0;
}
八.strncat函数的使用
cpp
int main()
{
char arr1[20] = "hello\0xxxxxxxxxxx";
char arr2[] = "world";
strncat(arr1, arr2, 7);
return 0;
}
九.strncmp函数的使用
cpp
int main()
{
char arr1[] = "abcdef";
char arr2[] = "abcq";
int ret = strncmp(arr1, arr2, 3);
printf("%d\n", ret);
return 0;
}这就是比较前n个,但是要在有限的范围内进行比较
十.strstr的使用和模拟实现
1.strstr的使用
cpp
//strstr 返回字符串在另外一个字符中第一次出现的位置
int main()
{
char arr1[] = "abcdefabcdef";
char arr2[] = "deq";
char* ret = strstr(arr1, arr2);
if (ret != NULL)
printf("%s\n", ret);
else
printf("找不到\n");
return 0;
}
2.strstr的模拟实现
cpp
char* my_strstr(const char* str1, const char*str2)
{
const char* cur = str1;
const char* s1 = NULL;
const char* s2 = NULL;
assert(str1 && str2);
if (*str2 == '\0')
{
return (char*)str1;
}
while (*cur)
{
s1 = cur;
s2 = str2;
while (*s1 == *s2)
{
s1++;
s2++;
}
if (*s2 == '\0')
{
return (char*)cur;
}
cur++;
}
return NULL;
}
int main()
{
char arr1[] = "abbbcdef";
char arr2[] = "bbq";
char* ret = my_strstr(arr1, arr2);
if (ret != NULL)
printf("%s\n", ret);
else
printf("找不到\n");
return 0;
}