9.13 创建不等的按钮（第一行4个，第二行3个）

CGFloat btnW = SIZE.width/4;

for (NSInteger i=0; i<7; i++) {//7个按钮

UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];

btn.frame = CGRectMake((i%4)*btnW, (i/4)*btnW, btnW, btnW);

[btn setTitle:@"纷纷" forState:UIControlStateNormal];

btn.backgroundColor = [UIColor cyanColor];

[self.view addSubview:btn];

}