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Scope of Local and Parameter variables in C

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• Reminder: Program Scope and scoping rules
  • Program scope: (See: cs170 material on scoping )

    A program scope is delimited by a pair of (matching) braces { .... }

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  • There are 2 special scope configurations formed by multiple pairs of braces:

    Non-overlapping scopes: Nested scopes:

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  • Scoping rules for the 2 special scope configurations:

    A variable defined inside one disjoint scope is not accessible inside the other disjoint scope Illustrated: A variable defined inside the outer scope is accessible inside the inner scope Illustrated:

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• The scope of Local variables in C
  • Scope of Local variables in C:

    Starts at the location where the Local variable is defined Ends at the end of the enclosing scope Except: Inside an inner scope where you define another local variable with same name as an outer local variable !!!

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  • Example:

    Explanation:

    There are 2 different local variables defined inside the main() function: double r is defined inside the outer scope int r is defined inside the inner scope Note: The inner local variable defintion is illegal in Java; but perfectly OK in C ! The scope of the inner variable definition (int r) is the cyan colored area in the following figure: The scope of the outer variable definition (double r) is the green colored area: Consequently, the name r refers to different local variables in different locations inside the function:

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  • Example:

    #include <stdio.h> int main( int argc, char* argv[] ) { /* Outer scope */ double r = 3.14; /* (1) */ printf( "1. r = %lf\n", r ); /* Access r(1) */ { /* Inner scope (a new scope !) */ printf( "2. r = %lf\n", r ); /* Access r(1) */ double r = 4.444; /* (2) */ printf( "3. r = %lf\n", r ); /* Access r(2) */ } printf( "4. r = %lf\n", r ); /* Access r(1) */ }

    Result:

    1. r = 3.140000 2. r = 3.140000 3. r = 4.444000 4. r = 3.140000

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set1/scoping1.c

    How to run the program:

    To compile:   gcc scoping1.c          To run: ./a.out

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• Note on scoping rules in Java
  • Java does not allow a variable with the same name to be defined inside an inner scope

    So this example is illegal in Java:

    public class scoping1 { public static void main( String[] args ) { double r = 3.14; { /* Inner scope (a new scope !) */ int r = 4; /* Illegal in Java */ } } }

    Gosling (designer of Java) thinks that variables with the same name inside one function is too confusing....

    Here's a true story: I know a professor who teaching a compiler construction course who was confused by Java's scoping rule....

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set1/scoping1.java

    How to run the program:

    To compile:   javac scoping1.java          Look for this error message: scoping1.java:12: error: variable r is already defined in method main(String[]) int r = 4; /* Illegal in Java */ ^ 1 error

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• The scope of Parameter variables in C
  • Scope of Parameter variables in C:

    Starts at the beginning of the function (in which the Local variable is defined Ends at the end of the function Except: Inside an inner scope where you define another local variable with same name as an outer local variable !!!

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  • Example:

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  • Example in C:

    void func( double r ) { { /* Start of a new scope */ /* NOTE: without this inner scope, you will get error: 'r' redeclared !!! */ printf( "1. r = %lf\n", r ); /* Access parameter r */ double r = 4.444; /* (1) */ printf( "2. r = %lf\n", r ); /* Access r(1) */ } } int main( int argc, char* argv[] ) { func( 3.14 ); // Pass 3.14 into parameter variable r }

    Result:

    1. r = 3.140000 2. r = 4.444000

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set1/scoping2.c

    How to run the program:

    To compile:   gcc scoping2.c          To run: ./a.out

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• Final note: no need to declare parameter/local variables
  • Fact:

    There is no way/need to declare a local variable or a parameter variable

    Reason:

    A local/parameter variable is only accessible inside the function where it is defined Since the local/parameter variable is not accessible in other functions, there is no need to declare them in other places....