my_atoi.c

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


int my_atoi(const char* s);


int main()
{
	/*
	const char* s = "123456";

	long i;
	i = (long)(s);
	printf("%ld\n", i);
	*/

	char s[100];

	while(1)
	{
		printf("\n:> ");
		fgets(s, sizeof(s), stdin);

		if(s[strlen(s) - 1] == '\n')
			s[strlen(s) - 1] = '\0';

		printf("%d %d\n", atoi(s), my_atoi(s));
	}
	
	return 0;
}



int my_atoi(const char* s)
{
	// 123
	//(s[0] - '0') * 100 + (s[1] - '0') * 10 + (s[2] - '0') * 1 	---> 123
	
	int i = 0, r = 0, sign = 1;

	// 跳过前导空白字符（' ', '\t', '\n'）
	while(s[i] == ' ' || s[i] == '\t' || s[i] == '\n')
		i++;

	// 判断符号
	if(s[i] == '+') i++;
	else if(s[i] == '-') sign = -1, i++;

	while(s[i] != '\0')
	{

		// 碰到第一个非数字字符就停止转换
		if(s[i] < '0' || s[i] > '9')
			break;

		r = r * 10 + s[i] - '0';

		i++;
	}

	return r * sign;
}