neutral refactorings to silence Xcode 4.5.2 warnings

[algal]

These changes silence compiler warnings by fixing small bugs or clarifying the code to help the static analyzer.

Fixes:

• initialized some function local variable at their declaration
• corrected a NSLog format specifier

hints:

• use standard Obj-C idiom to initialize ivars
• rename local vars so they don't shadow vars from the enclosing scope
• enhance conditional to silence warning about div-by-zero