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Array bound check

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• Array bound check: a notable difference between arrays in C and Java
  • Fact:

    The more information you maintain on a thing, the more services/capabilities you can provide with that thing.

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  • Information maintained on arrays:

    Java	C
    Location (address) of the array in memory      Number of elements in the array	Location (address) of the array in memory

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  • Consequence:

    Using the length (= number of elements) information, the Java compiler can detect if the array index is out of bound (= too large) Because C does not keep any information on the length (= number of elements) of an array, the C compiler cannot detect if an array index is out of bound !!!

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  • Example: Java detects array index out of bound error

    public class array2 { public static void main( String[] args ) { double[] a = new double[10]; int i; for ( i = 0; i < 10; i++ ) a[i] = i; for ( i = 0; i < 30; i++ ) System.out.println( "a[" + i + "] = " + a[i] ); } }

    Result:

    a[0] = 0.0 a[1] = 1.0 a[2] = 2.0 a[3] = 3.0 a[4] = 4.0 a[5] = 5.0 a[6] = 6.0 a[7] = 7.0 a[8] = 8.0 a[9] = 9.0 Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 10 at array2.main(array2.java:14)

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac array2.java To run:          java array2

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  • Example: a C program does not check on array index out of bound error (insuffience information)

    int main( int argc, char* argv[] ) { double a[10]; int i; for ( i = 0; i < 10; i++ ) a[i] = i; for ( i = 0; i < 30; i++ ) printf( "a[%d] = %lf\n", i, a[i] ); }

    Sample output:

    a[0] = 0.000000 a[1] = 1.000000 a[2] = 2.000000 a[3] = 3.000000 a[4] = 4.000000 a[5] = 5.000000 a[6] = 6.000000 a[7] = 7.000000 a[8] = 8.000000 a[9] = 9.000000 a[10] = 0.000000 a[11] = 0.000000 a[12] = 0.000000 ... a[22] = -2243494719485605301116825811.... ...

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   gcc array2.c To run:          ./a.out

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• Postscript: run time overhead
  • The reason that Java can detect an index out of bound error is because:

    Each time a Java program uses an array variable, the Java run time system performs an index out of bound check

    These index out of bound checks takes up CPU cycles (uses the CPU)

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  • That is why:

    C programs are more efficient than Java program (i.e., faster) because C programs not not perform array bound checks at run time

    We say that:

    C programs incur less run time overhead....

    (BTW, C is a "system language" and one of its goal is speed !!!)