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Three-way Decisions

An if statement makes a two-way decision. Surely you must sometimes pick from more than just two branches?

We ran into this problem with a previous example program that divided integers into negative and non-negative. It really should pick one of three choices:

• Negative: ... -3 -2 -1
• Zero: 0
• Positive: +1 +2 +3 ...

Two-way decisions can do this. First divide the integers into two groups (using a two-way decision):

• Negative: ... -3 -2 -1
• Zero and Positive: 0 +1 +2 +3 ...

Then further divide the second group (by using another two-way decision):

• Negative: ... -3 -2 -1
• Zero and Positive:
  • Zero: 0
  • Positive: +1 +2 +3 ...

By repeatedly splitting groups into subgroups, you can split a collection into any number of fine divisions.