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Recursion

Recursion. Review: Stacks. Classical semantics (behavior) Elements can only be inserted and removed from the front of the collection This is known as Last-In, First-Out (LIFO) Random-access is not defined. Stack – behavioral methods.

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Recursion

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  1. Recursion CS-2851Dr. Mark L. Hornick

  2. Review: Stacks • Classical semantics (behavior) • Elements can only be inserted and removed from the front of the collection • This is known as Last-In, First-Out (LIFO) • Random-access is not defined CS-2851Dr. Mark L. Hornick

  3. Stack – behavioral methods • The naming for the structure and the methods is an analogy for how the data elements within the structure are accessed • Principal methods that define behavior: • push() – place an element on (top of) the stack • pop() – return and remove an element from the (top of the) stack • peek() – return the top element of the stack • Without removing (popping) it CS-2851Dr. Mark L. Hornick

  4. Program execution and the Stack • Whenever a method is called, information related to the method call is stored • within a data structure that exhibits behavior of a Stack • This information is referred to as an activation record CS-2851Dr. Mark L. Hornick

  5. The program execution Stack • Stack frame/activation record • Each activation record contains: • A variable that contains the return address in the calling method • For each parameter in the called method, a variable that contains a copy of the corresponding argument • For each local variable declared in the called method, a variable that contains a copy of that declared variable. • For a method A calling a method B • The activation record for A is pushed before the call to B • Popped when the method B completes execution CS-2851Dr. Mark L. Hornick

  6. CS-2851Dr. Mark L. Hornick

  7. Stack settings for JVM Each thread in the JVM uses a stack for storing activation records The –Xss<nk> JVM setting • sets the maximum stack size that can be used by Java code in each thread to n kilobytes. • The default units for n are bytes. n must be > 1k bytes. • The default stack size is 400 kilobytes ("-Xss400k") • "k" for kilobytes or "m" for megabytes CS-2851Dr. Mark L. Hornick

  8. Heap settings for JVM Each thread in the JVM uses the heap for creating new objects The –Xms<n>m JVM setting • sets the initial heap size that is used by Java code in each thread to m megabytes. • The default units for n are bytes. • The initial heap size is 128MB ( same as "-Xms128m“) The –Xmx<n>m JVM setting • sets the maximumheap size • The default max heap size is 128MB ( same as "-Xmx128m“) CS-2851Dr. Mark L. Hornick

  9. Factorial of x What algorithm would you use to compute x! ?

  10. Recursive definition of x! Is there a way to make use of recursion?

  11. Another Recursion Example:Graphics Fill Algorithm The “Bucket” tool of MS Paint • Drag the bucket to an interior point of a graphical shape • Paint interior outward toward boundary • Stop when all interior pixels are filled CS-2851Dr. Mark L. Hornick

  12. Boundary Fill Patterns 4-connected 8-connected CS-2851Dr. Mark L. Hornick

  13. Boundary Fill Algorithm details Return if current position is: • Boundary color • Fill color • Otherwise/else • Set fill color • Recursively try neighbors • North, East, South, West (arbitrary) • Each neighbor recursively performs algorithm until “return” • 8-connected also tries NE, NW, SW, SE CS-2851Dr. Mark L. Hornick

  14. Boundary fill exercise CS-2851Dr. Mark L. Hornick

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