quadratic function.c

里面有几个用C语言画图的程序

#include<graphics.h> 
#define BKC 9 
#define COLOR 8 
main() 
{int gd=DETECT,gm,p=1,i; 
double a,b,c,x,y,x1,y1,t; 
char ch; 
initgraph(&gd,&gm,""); 
while(p==1) 
{ 
setbkcolor(BKC); 
setcolor(14); 
setlinestyle(0,0,1); 
rectangle(0,0,639,479); 
line(20,460,639,460);  
line(20,0,20,460);  
setlinestyle(1,0,1); 
setcolor(15); 
for(i=20;i<640;i+=20)line(i,460,i,0); 
for(i=460;i>0;i-=20)line(20,i,640,i); 
setcolor(12); 
setlinestyle(SOLID_LINE,0,3); 
line(300,460,300,0); 
line(0,360,640,360); 
setcolor(15); 
gotoxy(1,1); 
printf("Please Enter a:" ); 
scanf("%lf",&a); 
printf("Please Enter b:" ); 
scanf("%lf",&b); 
printf("Please Enter c:" ); 
scanf("%lf",&c); 

gotoxy(300,440); 
printf("y= %5.2f *x^2 + %5.2f *x + %5.2f ",a,b,c); 

setlinestyle(SOLID_LINE,0,1); 
moveto(200,460-(a*(0-100)*(0-100)+b*(0-100)+c+100)); 
for(t=0.0;;t+=0.0002) 
{ 
x=t+200; 
y=a*(t-100)*(t-100)+b*(t-100)+c+100; 
x1=x; 
y1=460-y; 
if(x1>640.0||x1<0||x>640) break; 
if(y1<=480.0 &&y<=480) lineto(x1,y1); 
} 
setcolor(12); 
outtextxy(250,8,"Finished! Continue(Y/N)?"); 
p=-1; 
while(p==-1) 
{ 
ch=getch(); 
if(ch=='y'||ch=='Y') { cleardevice();p=1;} 
else if(ch=='n'||ch=='N') p=0; 
else outtextxy(220,200,"ERROR! Please Press 'Y' or 'N'_" ); 
} 
} 
closegraph(); 
}