Understand Scope of an Identifier Know the Storage Classes of variables and functions Related Chapter ABC 5 10 5 11 18 2 1 Problem Definition Write a function swap that has two integer input arguments This function should swap the values of the variables 2 Refine Generalize Decompose the problem definition i e identify sub problems I O etc Input two integers Output no value is returned to the calling function but the values of the called variables should be swapped 3 Develop Algorithm temp a a b b temp 18 3 C Function to swap values include stdio h void swap int int prototype for swap void main void function header int x 1 int y 2 swap x y printf x i y i n x y prints x 1 y 2 void swap int a int b int temp a a b b temp return x 1 y 2 1000 Address Main Memory before call Main Memory while executing swap but before the return statement x 1 y 2 a 2 b 1 1000 3000 Address Why swap doesn t work The swap function will successfully swap the values of the variables a and b But since non arrays are passed by value the swap of the values for a and b will not have any affect on the variables x and y It is important to note that even if we had used the variable names x and y rather than a and b the swap function would still not work See the next slide Modified function to swap values include stdio h void swap int int void main void int x 1 int y 2 swap x y printf x i y i n x y prints x 1 y 2 void swap int x int y we now use x and y int temp x x y y temp return 1 y 2 x 1 y 2 x 2 y 1 1000 3000 Address x 1000 Address Why the modified swap doesn t work Main Memory while executing swap Main Memory before call but before the return statement The C compiler will keep track of the variables x and y declared in main and the variables x and y declared in swap The x in main is a different variable than the x in swap Similarly for the y variable The reason that the x variable in main is different than the x variable in swap is that two declarations of x occur in different blocks That is the scope of the x in main is different than the scope of x in swap One solution to the problem is to use pointer variables We will discuss pointer variables in a future lecture An identifier is a name that is composed of a sequence of letters digits and the underscore character Variable names are identifiers and so are function names and symbolic constant names The scope of an identifier is the portion of the program in which the identifier is visible or accessible Both variables and functions have two attributes type and storage class The scope of a variable or function is related to its storage class 18 8 Local Variables are declared within a block and cannot be referenced by outside the block i e each function is assigned its own local storage areas Global Variables declared outside any block and are known to all blocks in the same file By default global variables are static storage class In the examples on the following slides drawing a picture of memory is helpful in understanding how scoping works 18 9 int globalVar 1 global variable int myFunction int function prototypes void main void local variable result in main int result result myFunction globalVar call myFunction printf i result prints 2 printf i globalVar prints 2 printf i x compile error x not known in main int myFunction int x Local variable x x printf i x prints value 2 printf i globalVar prints value 1 globalVar return x 18 10 globalVar 1 result 1000 Address Main Memory before call to Myfunction globalVar result x 1 1000 2 3000 Address Main Memory while executing MyFunction but before globalVar 18 11 globalVar result x 1000 2 3000 2 Address Main Memory while executing MyFunction but before return x globalVar 2 result 2 1000 Address Main Memory after call to Myfunction 18 12 int globalVar 1 int myFunction int global variable function prototypes void main void int result result myFunction globalVar call myFunction printf i result prints 2 int myFunction int x Local variables x globalVar int globalVar new local variable printf i n globalVar prints 18 13 return x 1 int myFunction int function prototypes void main void local variables x result in main int result x 2 result myFunction x call myFunction printf i result prints 3 printf i x prints 2 WHY int myFunction int x Local variable x x x 1 printf i n x prints 3 return x 18 14 include stdio h int x int y x is a global variable y is a global variable void swap int int prototype for swap void main void x 1 x and y are not declared here y 2 swap x y printf x i y i n x y prints x 1 y 2 void swap int x int y int temp x x y y temp return A function can be called by any other function provided that either the function definition or its prototype is in the same file as the calling function and precedes the function call If no prototype is specified for a function its header serves as the function prototype Note Functions cannot be defined within each other 18 16 Determines the storage duration and scope of identifiers and also linkage between files auto creates storage for variables when the block that declares them is entered and deletes the storage when the block is exited Local variables have auto storage by default e g typing auto float a b c is equivalent to typing float a b c 18 17 static creates and initializes storage for variables when the program begins execution Storage continues to exist until execution terminates If an initial value is not explicitly stated a static variable is initialized to 0 We can retain values of local variables by declaring them to be static In the following example the static variable sum is initialized to 1 static int sum 1 The initialization takes place only once If the above declaration appears in a user defined function the first time the function is called the variable sum is initialized to 1 The next time the function containing the above declaration is executed sum is not reset to 1 18 18 extern used to reference identifiers in another file Function names are extern by default e g extern int foreignVar 18 19 1 Problem Definition Write a function sum that keeps a running total of the sum of every value passed to sum To test this function modify the previous program of Lecture 15 3 that compute the average of values in a file 2 Refine …