my_printf.c

/*2010/11/18 shihongzhi
 * 实现了printf，itoa，ftoa
 * todo：%f的自定义小数点没有实现
 */
#include <stdio.h>
#include <stdarg.h>
#include <math.h>


const double eps=1e-12;

void my_itoa(int val,char *buf)
{
	const int radix=10;
	char *p=buf;	//start of the val char
	int a;
	char *b;	//start of the digit char
	char temp;
	unsigned int u;
	if(val<0){
		*p++='-';
		val=0-val;
	}
	b=p;
	u=(unsigned int)val;
	while(u>0){
		a=u%radix;
		u/=radix;
		*p++=a+'0';
	}
	*p--='\0';
	while(p>b){
		temp=*p;
		*p--=*b;
		*b++=temp;
	}
}

void my_ftoa(double val,char *buf,int digits)
{
	const int radix=10;
	char *p=buf;
	char *b,*q;
	char temp;
	int a,cur_d,dec;
	long long i_val=(long long)val;
	double dec_val=val-i_val;
	if(val<0){
		*p++='-';
		val=0-val;
	}
	b=p;
	while(i_val>0){
		a=i_val%radix;
		i_val/=radix;
		*p++=a+'0';
	}
	q=p-1;
	
	while(q>b){
		temp=*q;
		*q--=*b;
		*b++=temp;
	}
	if(fabs(dec_val-0.0)>eps)
	{
		*p++='.';
		cur_d=0;
		while(cur_d<digits){
			cur_d++;
			dec_val*=10;
			dec=(int)dec_val;
			dec_val-=dec;
			*p++=dec+'0';
		}
	}
	*p='\0';
}

void my_printf(const char *format,...)
{
	va_list ap;
	char c;
	char ch;
	char *p;
	char ibuf[1000];	//刚开始写成了char *buf 出现了段错误，查了半个小时，悲剧
	char dbuf[1000];
	int i;
	double d; 

	va_start(ap,format);
	while(c=*format){
		if(c=='%'){
		//	c=*(++format);
			format++;
			c=*format;
			switch(c){
				case 'c':
					ch=va_arg(ap,int);
					putchar(ch);
					break;
				case 's':
					p=va_arg(ap,char*);
					fputs(p,stdout);
					break;
				case 'd':
					i=va_arg(ap,int);
					my_itoa(i,ibuf);
					fputs(ibuf,stdout);
					break;
				case 'f':
					d=va_arg(ap,double);
					my_ftoa(d,dbuf,6);
					fputs(dbuf,stdout);
					break;
				default:
					putchar('%');
					putchar(c);
			}
		}
		else{
			putchar(c);
		}
		format++;
	}
	va_end(ap);
}

int main(void)
{
	my_printf("int:%d,float:%f,char:%c,string:%s\n",12,13.1212611214,'a',"hello world");
	my_printf("int:%d,float:%f,char:%c,string:%s\n",123456,1.1,'b',"hello123");
	return 0;
}