计算器应用程序中的单元格包含文本字段和按钮部件。
The cells within our calculator application contain text-field and button widgets.
您可以使用键盘输入这些表达式，然后观察expr的工作，现在您已经有了足够的知识来修改该计算器代码，以添加支持这些函数的按钮。
You can use the keyboard to type these in and see expr work; and you should have enough knowledge now to modify the calculator code to add buttons that support these functions.
创建计算器的GUI，为其呈现布局和按钮。
Creates the GUI for the calculator, rendering the layout and the buttons for it.
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