Bressenham's Line Drawing Algorithm

#include<stdio.h> 
#include<conio.h> 
#include<graphics.h> 
#include<math.h> 

void swap(int &x,int &y) 
{ 
    int k=x; 
    x=y; 
    y=k; 
} 

void main() 
{ 
    int gd=DETECT,gm=DETECT,x1,x2,y1,y2,dx,dy,p,k; 
    float m=0; 

    clrscr(); 

    printf("Enter the sarting point x1 & y1n"); 
    scanf("%d%d",&x1,&y1); 

    printf("Enter the end point x2 & y2n"); 
    scanf("%d%d",&x2,&y2); 

    dx=abs(x2-x1); 
    dy=abs(y2-y1); 
    m=(float)(y2-y1)/(x2-x1); 

    initgraph(&gd,&gm,""); 
    cleardevice(); 

    if(fabs(m)>1) 
    { 
        swap(x1,y1); 
        swap(x2,y2); 
        swap(dx,dy); 
    } 

    if((x1>x2)) 
    { 
        x1=x2; 
        y1=y2; 
    } 
    p=2*dy-dx; 
    for(k=0;k<abs(dx);k++) 
    { 
        if(p<0) 
        { 
            p=p+2*dy; 
        } 
        else 
        { 
            if(m<0) y1--; 
            else y1++; 
            p=p+(2*dy)-(2*dx); 
        } 
        if(fabs(m)<=1)putpixel(x1++,y1,15);
        else putpixel(y1,x1++,15); 
    } 

    getch(); 
}