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Start with Holy Wars

Cathedral v Bazaar

About that "given enough eyeballs, all bugs are shallow" thing ....

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Wednesday

Discussion of how to explain what a calculator does

• pressing the first digit, following an operation: save some things, clear the display, and put the new digit there
• pressing another digit: put them into the display
• pressing operator: clear the display, storing it in prev_display
• pressing equals: combining prev_display and current display with the operator.

expression evaluation: ~/271/expressions (code should be in pld.cs.luc.edu/courses/271/spr18/demos/expressionsij.zip)

    java expr_eval     (traced version is expr_exp)

    java expr_assign

Notes: pld.cs.luc.edu/courses/271/spr18/mnotes/recursion.html#exprtrees

Demo of command-line expressions

With a calculator interface, things are a little different. Generally, the '(' key clears the display and starts a new subexpression. Operator keys might display the current operator on the screen. The ')' key acts like the '=' key for the current expression, displaying the results so far. galculator takes this approach.

To put it another way, there are these states:

• ENTER_NUMBER
• OPERATOR
• EQUALS

In ENTER_NUMBER state, additional digits pressed go into the display.

We go into OPERATOR state when an operator key is pressed. We save and clear the display, and save the operator. If a digit is pressed in OPERATOR state, we go into ENTER_NUMBER state again.

We go into EQUALS state when the "=" key is pressed. We retrieve the last stored operator and apply it to the previously saved display and the current display. The display is updated to show the result.

The other alternative is to display the entire expression on the display line. qalculate does this.