请进来看看(100分)

var
today,yesterday,tomorrow:week;
n:integer;
begin
n:=dayofweek(now);

case n of
1:today:=sun;
2:today:=mon;
3:today:=tue;
4:today:=wed;
5:today:=thu;
6:today:=fri;
7:today:=sat;
end;
if today=low(week) then
yesterday:=high(week)
else
yesterday:=pred(today); [red]//pred(today)是什么意思[/red]
if today=high(week) then
tomorrow:=low(week) [red]//low(week)是什么意思[/red]
else
tomorrow:=succ(today); [red]//succ(today)是什么意思[/red]
button1.Caption:='昨天';
button2.Caption:='今天';
button3.Caption:='明天';
case (sender as tbutton).Tag of
0:button1.Caption:='昨天是'+mday(yesterday);
1:button2.Caption:='今天是'+mday(today);
2:button3.Caption:='明天是'+mday(tomorrow);
end;
end;

pred(today) 是昨天的日期
succ(today) 是明天的日期
low(week)是一个星期的第一天

pred是前导函数，succ是后续函数，比如
开始today是tue,
那么
pred(today) 是mon
succ(today) 是wed

而 low(week) 是week中最小的元素即：sun

我来晚了，pink_wt，DarwinZhang 完全正确！！！

你么是在研究算法吗??

pred(today)是today的前一天
succ(today)是today的后一天
today是你写入的日期

多人接受答案了。