Lecture 4: Turn requirements into safe, testable branches

## The general form of an if chain and a switch

~~~cpp
if (<condition>) {
    // runs when <condition> is true
} else if (<other condition>) {
    // runs when the first was false and this one is true
} else {
    // runs when every condition above was false
}

switch (<integer or char expression>) {
    case <constant>:
        // statements
        break;              // without it, execution falls into the next case
    default:
        // statements when no case matched
}
~~~

| Part | Meaning |
| --- | --- |
| `<condition>` | An expression usable as a <code>bool</code>, tested in order from the top. |
| <code>else</code> | Optional. Without it, no path runs when every condition is false. |
| `case <constant>` | A compile-time constant. Ranges and floating-point values are not allowed. |
| <code>break</code> | Ends the <code>switch</code>. Omitting it is legal and falls through deliberately. |

Only the first true branch of an <code>if</code> chain runs, so order the conditions from exceptional to normal.