Methods 3.1. Review.Review.Review (cont.).2. Class-wide vs. local variables.What are the local variables? Local to what?“Local” is a relative term.Why local variables?Uses of local variables.Class-wide variables.Slide 11Note the differences:3. Why C# bans global variables.A Global variable in C++Global variables.What’s wrong with global variables?Why bother with parameters?What are side-effectsSide effects (cont.).Solution of C#4. Nested blocks.Nesting (cont.).Slide 23Nesting and local variables.Nested blocks (cont.).Slide 26What if we do use the same name for a class-wide and a local variable?Example:5. Scope of identifiers.Scope of identifiers (cont.).Slide 31Slide 32Slide 33Methods 3.1. Review.2. Class-wide vs. local variables.3. Why C# bans global variables.4. Nested blocks.5. Scope of identifiers.1. Review.Recall that creating a function requires both (a) designing the interface = the visible, communication mechanism and (b) designing the implementation = the “invisible” code hidden inside the function.A. The interface = the WHAT; includes function name (the task) and the parameters (communication). It specifies what is to be done and what gets sent back and forth.Review.Communication requires matching the actual parameters (in the function call) with the formal parameters (in the function heading). How are parameters matched?NOT by name—the name can be the same or different.By: 1) data type; 2) number; 3) position.Review (cont.).B. The implementation = the HOW; behind the scenes, what machinery (code) actually carries out the task. This includes all the code inside the function’s scope brackets { }.The implementation may include “local variables”….2. Class-wide vs. local variables.Definition.A local variable is defined inside a block of code, i.e. within scope brackets { }, and is only accessible there.Memory for local variables is allocated in a region called the stack on a demand basis: it does not exist before the block is entered or after the block is exited.What are the local variables? Local to what?class Bundle {static double Find_Big (double X, double Y) {  double Big;  Big = X; if (Y > X) Big = Y; return Big; } // end Find_Bigstatic void Main (string[] arg)  { double Num1, Num2, Answer; Answer = Find_Big (Num1, Num2); } // end Main()} // end Bundle“Local” is a relative term.Local to Find_Big: 1) the formal parameters X and Y. 2) BigLocal to Main(): Num1, Num2, AnswerFind_Big CANNOT access Num1, Num2, Answer. Why?Main() CANNOT access X, Y or Big. Why?Why local variables?We typically have variables local to Main(), but we may also put local variables inside our own Methods (or in other blocks of code). WHY? What are they for?Uses of local variables.Local variables are a good solution for indexes and intermediary values that are only needed within a method (or other block).They are not a good solution if the data values are required after the method / block is exited, because?Class-wide variables.If data needs to survive after a method call, or it makes sense for several methods to share data, C# allows class-wide variables.These are data members. All methods in the class can access these variables directly without passing them as parameters. They are “common knowledge,” to everyone in the class.Class-wide variables.class Bundle { double Big; // Big is a class-wide variable.static void Find_Big (double X, double Y) { Big = X; if (Y > X) Big = Y;} // end Find_Bigstatic void Main (string[] arg) { double Num1, Num2; Find_Big (Num1, Num2); Console.Write (“The answer is ” + Big); // legal!} // end Main()} // end BundleNote the differences:Both Find_Big and Main() can access the class-wide variable Big directly, without passing it as a parameter.Does that mean anyone can access Big?No. Only class members can see Big.Can a local variable have the same name as a class-wide variable?Yes, but it is not a good idea. More later.3. Why C# bans global variables.Some languages allow global variables, e.g. C and C++.Global variables are defined outside of any class at “file scope.”They can therefore be accessed by methods in any class.Using global variables, on can avoid parameter passing altogether.Here is an example in C++:A Global variable in C++#include <iostream.h>int Global_Num; // a global variable.int main () { Global_Num = 5; // accesses global variable}void Another_Function () { Global_Num = 7; // does it again}Global variables.Notice that if a variable is defined globally, it can be accessed anywhere, *without* passing it as a parameter.But then why pass parameters? If we don’t, too much danger of data corruption. WHY?What’s wrong with global variables?Global variables lead to poor interface design. They make it unclear to another programmer what is supposed to pass in and out of a function. Listing parameters forces us to think precisely about the communication between functions.Why bother with parameters?Using parameters is like “going through the proper channels. It avoids doing an end-run / going around the side.If we don’t do this, we allow “side effects.”What are side-effectsDefinition: A side effect is any communication between 2 modules (functions) that is not specified by the interface.Side effects (cont.).Analogies:1. Restaurant analogy. See diagram. Suppose the cook brings out food with no input from the customer, or the customer goes straight into the kitchen for food!2. Theology: claiming Jesus is one of many ways to access the Father, yet there is only *one* mediator between God and man (1 Tim. 2: 5).Solution of C#C# is a more constrained language than C++.In addition to rigorous type-checking, checking for initialization of variables and code-path checking, the designers of C# chose to remove temptation:C# bans global variables.4. Nested blocks.Consider Russian dolls:a miniature doll fits inside a slightly larger duplicate which fits inside another, until we reach the outermost doll. The smaller dolls are nested inside the larger dolls.Nesting (cont.).Likewise, in a computer program, one block can be nested inside another.static void Main () { //outer blockwhile (Count