1. The lifespan of a variable
Scope
The lifespan of a variable

2. Scope Not every variable is available everywhere Principle: Avoids:
Name collisions (15 things all called x)
Needless complexity
Principle:
Make variables available only where needed

3. Scope Scope : where a variable is available
Scope defined by the block { } a variable is in
x available anywhere in this block
disappears at line 16

4. Scope Scope : where a variable is available
Scope defined by the block a variable is in
y only available in the if statement block
disappears at line 12
Line 14 error!!

5. Scope
Can use variable in surrounding scope(s)

6. Scope
Variables in different scopes can have same name
BAD IDEA

7. Scope Problem
Might need value from calculated in a narrower scope:

8. Bad Scope Solution Bad idea: do all other work inside if/else
Repeating code is a sign of design failure!!

9. Better Scope Solution Declare variable in scope you want it
Modify it in if/else

10. Design Principle 1 General Principle:
Don't declare things until you need them Use maximally narrow scope
Yuck
Too much info up front

11. Maximally Narrow Scope
Better
Variables appear when needed

12. Design Principle 2 Don't repeat same work in multiple places Yuck
Same work in two places

13. Do Not Repeat Work If it always happens, keep it out of the if Better
Factor our common code to before or after if