字符串

1. 字符串反转

比如： how are you 转换成 you are how

#include <stdio.h>

void reverse(char * start, char *end){

	if(NULL == start || NULL == end){
		return;
	}

	char tmp;
	while(start < end){
		tmp = *start;
		*start = *end;
		*end = tmp;
		start++;
		end--;
	}
}

//字反转
char * reverse_word(char * src){

	if(NULL == src){
		return NULL;
	}
	
	char *start = src;
	char *end = src;

	while(*end != '\0'){
		end++;	
	}
	end--;
	reverse(start, end);

	end = start;

	while(*start != '\0'){
		
		if(*start == ' '){ //指向空字符
			start++;
			end++;
			continue;
		}else if(*end == ' ' || *end == '\0'){ //end指向空格或字符串结尾
			reverse(start, --end);//注意这里先 -- 再传end
			start = ++end;//注意这里先++，再赋值给start
		}else{ //end指向字符
			end++;
		}
		
	}

	return src;
}

int main(){
	char src[] = "how are you";
	char *p = reverse_word(src);
	printf("%s\n", p);

}

2. 判断是否为ip

#include <stdio.h>

int isValidValue(char *start, char *end){

	int sum = 0;
	while(start <= end){
		if(*start < '0' || *start > '9'){
			return -1;
		}else{
			sum = sum*10 + (*start - '0');
			start++;
		}
	}
	if(sum <0 || sum > 255){
		return -1;
	}	
	
	return 1;
}

int isValidFormat(char *src){

	int num = 0;
	int res = 0;
	char *start = src;
	char *end = src;
	
	while(*start != '\0'){
		printf("%s\n", start);
		if(*start == '.'){
			num++;
			start++;
			end++;
		}else if(*end == '.' || *end == '\0'){
			res = isValidValue(start, --end);
			if(-1 == res){
				return -1;
			}else{
				start = ++end;
			}
		}else{
			end++;
		}
	}

	if(3 != num){
		return -1;
	}
	return 1;
}

int main(){
	char *src = "24.1.2.4.1";
	printf("%d\n", isValidFormat(src));

}

3. 字符串包含

一个长字符串a，一个短字符串b，如何判断b中的字符都在a中？

4. 字符串拼接

#include <stdio.h>
#define N 10

char * strconnect(char src[], char des[]){

	if(NULL == src || NULL == des){
		return NULL;
	}
	char *p = des;
	while(*des != '\0'){
		des ++;
	}

	while(*src != '\0'){
		*des = *src;
		des ++;
		src ++;
	}
	*des = '\0';
	return p;
}

int main(){

	char src[N] = "abcd";
	char des[N] = "1234";
	strconnect(src, des);
	printf("%s\n", des);
}

5. 字符串拷贝

#include <stdio.h>

char * strcopy(const char * src, char * des){

	if(NULL == src || NULL == des){
		return NULL;
	}
	char *p = des;

	while(*src != '\0'){
		*des = *src;
		des++;
		src++;
	}
	*des = '\0';
	return p;
}

int main(){

	const char *src = "abc";//src是指向 字符常量const char类型 的指针
	char *des;
	des = strcopy(src, des);//入参src与形参类型要一致，都是指向const char类型的指针
	printf("%s\n", des);
}

6. 字符串移位包含

字符串s1，s2，判断字符串s2能否被s1循环移位得到的字符串包含。例如，给定s1=‘AABCD’，s2=’CDAA’，则s1向右移动3位变长CDAAB，包含s2

7. 查找文件中字符串总数和位置

8. 字符串转整形，整形转字符串

字符串转整形

#include <stdio.h>

int str2int(char *str){
	if(NULL == str){
		return -1;
	}
	int sign = 1;
	int des = 0;

	if('-' == *str){
		sign = -1;
		str++;
	}else if('+' == *str){
		str++;
	}	

	while(*str != '\0'){
		if(*str >= '0' && *str <= '9'){
			des = des*10 + (*str - '0');
			str++;
		}else{
			return -1;
		}	
	}
	return sign * des;
}

int main(){
	char *str = "-1234";
	int des = str2int(str);
	printf("%d\n", des);
}

整形转字符串，方法一

#include <stdio.h>
#define N 100

char * int2str(int src, char des[]){

	if(0 == src){
		return NULL;
	}

	int i=0, j=0;
	char tmp[N];

	while(src > 0){
		tmp[i] = src % 10 + '0';
		src = src / 10;
		i++;
	}

	i--;
	while(i >= 0){
		des[j] = tmp[i];
		i--;
		j++;
	}
	des[j] = '\0';
	return des;	
}

int main(){
	int src = 1234;
	char des[N];
	int2str(src, des);
	printf("%s\n", des);
}

整形转字符串，方法二

#include <stdio.h>

char * int2str(int src, char *des){
	if(0 == src){
		return NULL;
	}

	char * tmp = des;
	while(src > 0){
		*tmp = src % 10 + '0';
		tmp++;
		src = src / 10;
	}
	*tmp = '\0';
	
	char strTmp;
	char *head = des;
	char *end = --tmp;
	
	while(head < end){
		strTmp = *head;
		*head = *end;
		*end = strTmp;
		head++;
		end--;
	}
	return des;
}

int main(){
	int src = 1234;
	char * des;
	des = int2str(src, des);//这里注意啊，需要把des传进去，不然int2str只能返回局部变量的指针，这种是错误的。
	printf("%s\n", des);
}