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Defining variables in a C program

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• Syntax to define a variable in C
  • Syntax to define a variable in C (same as Java):

    data-type variableName [= initialValue] ;

    Examples:

    int x; float y = 4.0;

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• Where can you define variables in C ?
  • Places in the C program where you define variables:

    A variable can be defined inside a function Example: int main(int argc, char *argv[] ) { int x; // Variables can be defined float y = 4.0; // inside a function ... } Variables defined inside a function are local variables Local variables in C behaves exactly the same as local variables in Java: Local variables can only be accessed (= used) by statements inside the defining function A variable can be defined outside every function Example: int x; // Variables can be defined float y = 4.0; // outside EVERY function int main(int argc, char *argv[] ) { ... ... } Variables defined outside every function are global variables: Global variables can be accessed (= used) by statements inside any function in the C program However: You may need to make some provisions ("hints") in the program to "let the C compiler know" that a variable is a global variable

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  • Example:

    int x = 1; // Global variable void f( ) { int y = 4; // Local variable printf("x = %d, y = %d\n", x, y ); } int main(int argc, char *argv[] ) { int z = 2; // Local variable printf("x = %d, z = %d\n", x, z ); x = 777; // Update the global variable f( ); }

    Output:

    x = 1, z = 2 x = 777, y = 4

    The global variable x can be accessed in main( ) and f( )

    The local variable y can only be accessed in f( )

    The local variable z can only be accessed in main( )

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set1/variables1.c

    How to run the program:

    To compile:   gcc variables1.c          To run: a.out

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• Important properties of global and local variables
  • Now that you know how to define:

    Global variables          and            Local variables

    you still need to learn how these variables behave

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  • I think the best way to illustrate/teach the C features is relate to same (or similar) features that you have already learned

    Relating to Java:

    Global variables in C is the same as class variables in Java Local variables in C is the same as local variables in Java

    This C program:

    int x = 1; // Global variable void f( ) { int y = 4; // Local variable printf("x = %d, y = %d\n", x, y ); } int main(int argc, char *argv[] ) { int z = 2; // Local variable printf("x = %d, z = %d\n", x, z ); x = 777; // Update the global variable f( ); }

    is equivalent to the following Java program:

    public class variables1 { static int x = 1; // Global variable = class variable // (Only 1 copy of this variable) static void f( ) { int y = 4; // Local variable System.out.printf("x = %d, y = %d\n", x, y ); } static void main(String argv[] ) { int z = 2; // Local variable System.out.printf("x = %d, z = %d\n", x, z ); x = 777; // Update the class (~= global) variable f( ); } }

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  • Therefore, I can use what you have learned in Java to state these properties:

    There is one copy of each global variable Each invocation of a function will create a new copy of all local variables

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• (Simple) Rule for selecting global vs local variable
  • Suppose you need to create a variable to store information

    Question:

    How do you decide whether it should be a global variable or a local variable ???

    Answer:

    If the information stored in the variable is only used by one function: This variable is a local variable in that function If the information stored in the variable is used by multiple function: This variable is a glocal variable that will be accessible (= shared !!!) by all function It is your job to manage the (shared) global variables in C !!! I.e.: make sure you don't abuse the global variables in a way that it is not meant to be used