Xlib: why do draw functions have to be inside a loop?

The following code draws lines correctly:
Code:int main(int argc, char ** argv){
int screen_num, width, height;
unsigned long background, border;
Window win;
XEvent ev;
Display *dpy;
GC pen;
XGCValues values;

/* First connect to the display server */
dpy = XOpenDisplay(NULL);
if (!dpy) {fprintf(stderr, "unable to connect to display\n");return 7;}

/* these are macros that pull useful data out of the display object */
/* we use these bits of info enough to want them in their own variables */
screen_num = DefaultScreen(dpy);
background = BlackPixel(dpy, screen_num);
border = WhitePixel(dpy, screen_num);

width = 400; /* start with a small window */
height = 400;

win = XCreateSimpleWindow(dpy, DefaultRootWindow(dpy), /* display, parent */
0,0, /* x, y: the window manager will place the window elsewhere */
width, height, /* width, height */
2, border, /* border width & colour, unless you have a window manager */
background); /* background colour */

/* create the pen to draw lines with */
values.foreground = WhitePixel(dpy, screen_num);
values.line_width = 1;
values.line_style = LineSolid;
pen = XCreateGC(dpy, win, GCForeground|GCLineWidth|GCLineStyle,&values);

/* tell the display server what kind of events we would like to see */
XSelectInput(dpy, win, ButtonPressMask|StructureNotifyMask|ExposureMask );

/* okay, put the window on the screen, please */
XMapWindow(dpy, win);

/* as each event that we asked about occurs, we respond. In this
* case we note if the window's shape changed, and exit if a button
* is pressed inside the window. We also draw lines whenever a part
* of a window becomes viewable. */
while(1){
XNextEvent(dpy, &ev);
switch(ev.type){
case Expose:
XDrawLine(dpy, win, pen, 0, 0, width, height);
XDrawLine(dpy, win, pen, width, 0, 0, height);
break;
case ConfigureNotify:
if (width != ev.xconfigure.width
|| height != ev.xconfigure.height) {
width = ev.xconfigure.width;
height = ev.xconfigure.height;
XClearWindow(dpy, ev.xany.window);
printf("Size changed to: %d by %d\n", width, height);
}
break;
case ButtonPress:
XCloseDisplay(dpy);
return 0;
}
}
}Moving the XDrawLine calls outisde of the while loop, however, does not work:
Code:int main(int argc, char ** argv){
int screen_num, width, height;
unsigned long background, border;
Window win;
XEvent ev;
Display *dpy;
GC pen;
XGCValues values;

/* First connect to the display server */
dpy = XOpenDisplay(NULL);
if (!dpy) {fprintf(stderr, "unable to connect to display\n");return 7;}

/* these are macros that pull useful data out of the display object */
/* we use these bits of info enough to want them in their own variables */
screen_num = DefaultScreen(dpy);
background = BlackPixel(dpy, screen_num);
border = WhitePixel(dpy, screen_num);

width = 400; /* start with a small window */
height = 400;

win = XCreateSimpleWindow(dpy, DefaultRootWindow(dpy), /* display, parent */
0,0, /* x, y: the window manager will place the window elsewhere */
width, height, /* width, height */
2, border, /* border width & colour, unless you have a window manager */
background); /* background colour */

/* create the pen to draw lines with */
values.foreground = WhitePixel(dpy, screen_num);
values.line_width = 1;
values.line_style = LineSolid;
pen = XCreateGC(dpy, win, GCForeground|GCLineWidth|GCLineStyle,&values);

/* tell the display server what kind of events we would like to see */
XSelectInput(dpy, win, ButtonPressMask|StructureNotifyMask|ExposureMask );

/* okay, put the window on the screen, please */
XMapWindow(dpy, win);
XDrawLine(dpy, win, pen, 0, 0, width, height);
XDrawLine(dpy, win, pen, width, 0, 0, height);

/* as each event that we asked about occurs, we respond. In this
* case we note if the window's shape changed, and exit if a button
* is pressed inside the window. We also draw lines whenever a part
* of a window becomes viewable. */
while(1){
XNextEvent(dpy, &ev);
switch(ev.type){
case ConfigureNotify:
if (width != ev.xconfigure.width
|| height != ev.xconfigure.height) {
width = ev.xconfigure.width;
height = ev.xconfigure.height;
XClearWindow(dpy, ev.xany.window);
printf("Size changed to: %d by %d\n", width, height);
}
break;
case ButtonPress:
XCloseDisplay(dpy);
return 0;
}
}
}Why is this?