Syntax and semantics of C#.1. Review of the program life-cycle.2. Syntax vs. Semantics.3. Syntax.Slide 5Syntax (cont.).Slide 7Slide 8Slide 94. Semantics.Semantics (cont.).Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 195. Lab.Syntax and semantics of C#.1. Review.2. Syntax vs. semantics.3. Syntax.4. Semantics.5. Lab.1. Review of the program life-cycle.A. The Design Phase.Creation of an algorithm.B. The Implementation Phase.Creation of a program.C. The Maintenance Phase.Use and modification of a program.2. Syntax vs. Semantics.Any language has 2 dimensions, a syntax and a semantics.Syntax refers to….The form or “shape” of the language.Semantics refers to…The content or “meaning” of the language.3. Syntax.Syntax includes: (A) grammar, (B) spelling, (C) punctuation.A. Grammar.In English: “To go boldly,” not “To boldly go.”In C#: any input statement uses an input method (such as Console.ReadLine()); an assignment statement begins with a variable (or “lvalue”).3. Syntax.For example,Answer = Num + 5; // correctNum + 5 = Answer; // incorrect—WHY?Syntax (cont.).B. Spelling.Most reserved words (except Main() and certain built-in constants) must be lower case and spelled correctly.Variable names can be made up by the programmer, but must begin with either a letter or an underscore, followed by up to 127 letters, numbers or underscores.Syntax (cont.).B. Spelling.Although various possibilities are legal, it is helpful to distinguish our own variable names from reserved words (typically lower case) and constants (typically upper case), by using mixed case.Syntax (cont.).C. Punctuation.Before each item in a Console.Write statement, there must be a concatenation operator e.g.Console.Write (Num1 + “ plus ” + Num2  + “ = ” + Answer);Main is a method (function), so it requires argument and scope brackets Main() { }Variables in a list are separated by commas.Statements must be terminated by ;Syntax (cont.).Why does syntax matter?The compiler will only translate source code into object code if the source code has perfect syntax.Hence, one cannot run a program with syntax errors.4. Semantics.Concerns content/meaning.We can distinguish (a) the reference of terms e.g. What does “zebra” refer to? from (b) the meaning of whole statements, e.g. “There are zebras jumping on your pick-up truck.”Semantics (cont.).In a programming language, the terms e.g. variable names / function names refer to items in the computer’s memory, while statements are imperatives whose meaning is specified by what they make the computer do.Semantics (cont.).Reference of terms:Terms may refer to built-in functions and operations (e.g. sqrt(), *) or to user-defined variables and functions.A variable name like “Sum” or “Total” is really a symbolic address. The compiler translates it into a memory location where a number can be stored.Semantics (cont.).A good analogy is given by a mailbox.See diagram.1. The variable name is like: the name on the mailbox.2. The variable (memory location) is like: the mailbox itself (re-usable).3. The value of the variable is like: the mail in the mailbox.Semantics (cont.).A variable declaration causes memory to be reserved. This is crucial because we cannot access memory if we have not reserved it.float Num1;Num1 = 7; // OKNum2 = Num1 + 5; // The name ‘Num2’ does not exist in the //current contextSemantics (cont.).Constants? E.g. for Pi.const float PI = 3.14159;const int WEEKS=52;const char YES=‘Y’;Constant names also refer to memory. Obvious difference from variables?Semantics (cont.).Meaning of whole statements.Imperative semantics: what does it make the machine do?1. “non-executable”: e.g. variable declarations---reserve memory but do not perform an operation.2. “executable”: perform some operation on variables e.g. input / assign / output.Semantics (cont.).E.g. of executable statements:// input:Num1 = Convert.ToInt32 (Console.ReadLine());Num2 = Convert.ToInt32 (Console.ReadLine());//assignment:Num 3 = Num1 * Num2;// output:Console.Write (Num1 + “ times ” + Num2 + “= ” + Num3);Semantics (cont.).Be careful: the assignment operator (=) looks like identity, but isn’t the same.Total = Total + 5; // doesn’t say Total is equal to Total + 5 (impossible), but…?An assignment statement is a command, like “Let there be light!” “Ensign, engage!”Semantics (cont.).So:Num = 9; // means Make Num equal to 9.Total = Total + 5; // means make the new value of total equal the old value plus 5.To keep things straight, C# uses a different symbol for identity, = =E.g. if (Num_Visits == 9) Console.Write(“Oh, no, not you again!”);5. Lab.We will use the Syntax_Errors.cs program under CSC250