Bresenham's line drawing algorithm

/* compiler- Turbo c++ author- Mangilal Sharma =================================*/ 

#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 & y1\n");
    scanf("%d%d", &x1, &y1);

    printf("Enter the end point x2 & y2\n");
    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();
}

/*=============================19Nov.,2010=============================*/