Unix basics

Sieve Program
C and Java Side-by-side

Differences between C and Java: This page presents the algorithm that uses the "Sieve of Eratosthenes" to calculate all prime numbers less than 2000.

For a great poem about sieves, see The Jumblies, by Edgar Lear.

Here are two implementations side-by-side, with C on the left and Java on the right. Differences between the programs are highlighted in red.

C Sieve Program	Equivalent Java Sieve Program
/--------- File: sieve.h -------------------/ void makesieve(int p[], int len); void printsieve(const int p[], int len); void printprimes(const int p[], int len); /--------- File: sieve.c -------------------/ #include <stdio.h> #include "sieve.h" static void initializesieve(int p[], int len) { int i; p[0] = 0; p[1] = 0; for (i = 2; i < len; i++) p[i] = 1; } static void one(int p[], int len, int prime) { int i; for (i = 2prime; i < len; i = i + prime) p[i] = 0; } void makesieve(int p[], int len) { int i; initializesieve(p, len); for (i = 2; ii < len; i++) if (p[i] == 1) one(p, len, i); } void printsieve(const int p[], int len) { int i; printf("Sieve follows:"); for (i = 0; i < len; i++) { if (i%100 == 0) printf("\n"); printf("%d", p[i]); } printf("\n"); } void printprimes(const int p[], int len) { int i; int count = 0; printf("Primes follow:"); for (i = 0; i < len; i++) { if (p[i] == 1) { if (count%10 == 0) printf("\n"); printf("%d\t",i); count++; } } printf("\n"); } /--------- File: sievemain.c ---------------/ #include <stdio.h> #include "sieve.h" #define SIZE 2000 int main() { int list[SIZE]; int listsize = SIZE; makesieve(list, listsize); printsieve(list, listsize); printprimes(list, listsize); return 0; }	/--------- File: Sieve.java ----------------/ public class Sieve { private static void initializeSieve(int[] p, int len) { p[0] = 0; p[1] = 0; for (int i = 2; i < len; i++) p[i] = 1; } private static void one(int[] p, int len, int prime) { for (int i = 2prime; i < len; i = i + prime) p[i] = 0; } public static void makeSieve(int[] p, int len) { initializeSieve(p, len); for (int i = 2; ii < len; i++) if (p[i] == 1) one(p, len, i); } public static void printSieve(int[] p, int len) { System.out.print("Sieve follows:"); for (int i = 0; i < len; i++) { if (i%100 == 0) System.out.println(); System.out.print(p[i]); } System.out.println(); } public static void printPrimes(int[] p, int len) { int count = 0; System.out.print("Primes follow:"); for (int i = 0; i < len; i++) { if (p[i] == 1) { if (count%10 == 0) System.out.println(); System.out.print(i + "\t"); count++; } } System.out.println(); } } /--------- File: SieveMain.java ------------/ public class SieveMain { final static int SIZE = 2000; static int[] list = new int[SIZE]; public static void main(String[] args) { int listSize = SIZE; Sieve.makeSieve(list, listSize); Sieve.printSieve(list, listSize); Sieve.printPrimes(list, listSize); } }
Common Output
Sieve follows: 0011010100010100010100010000010100000100010100010000010000010100000100010100000100010000010000000100 0101000101000100000000000001000100000101000000000101000001000001000100000100000101000000000101000101 0000000000010000000000010001010001000001010000000001000001000001000001010000010001010000000001000000 0000000100010100010000000000000100000100000000010100010000010000000100000100000100010000010000000100 0100000001000000000101000000000101000001000100000100000001000101000100000000000100000001000100000001 0001000001000000000001010000000000000000010000010000000001000001000001010000010000000001000001000001 0100000100000100010100000000000100000000010100010000010000010100000000000100010000010000000100000000 0100000001000000000100000001000001000001000100000001000001000100000001000100000000000001000000000100 0000000001010000000001010001010000000001000000000000010001010001000000000000010001010001000000000000 0000000100010000000100000000010000000100010000010000010000000000000100010000010000010000000100000100 0000000001000100000101000000000101000001000000000101000000000101000001000000000000000001000101000100 0001000001000000010000010000010000000000000000000001010000000001000000010000000001000001000001000000 0100000000000100010000010000010100000100000000000100000000010000000000000000010100010000010100000100 0101000100000000000101000001000000000000000000000000000000000100000100000100000001000000000000000001 0000000001000000000000010001010001000001000000010001010000010000000000010000000001010001010001000001 0000000000010000000000010000000100000000000100000100010000010000000100010000000100010000000000000100 0100000101000100000101000001000000000100000000000000000001000001000101000000000000000000000001000101 0000000001000000000001010000000001000000010000010000010000010000000000000000010000010001010000000000 0100000000010000000000010000000100000000000000010000000000000100000100010100010100000000010000000000 0100000100000100000000000000000101000000000000000101000000000000000000000100000100000001000001000101 Primes follow: 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199 211 223 227 229 233 239 241 251 257 263 269 271 277 281 283 293 307 311 313 317 331 337 347 349 353 359 367 373 379 383 389 397 401 409 419 421 431 433 439 443 449 457 461 463 467 479 487 491 499 503 509 521 523 541 547 557 563 569 571 577 587 593 599 601 607 613 617 619 631 641 643 647 653 659 661 673 677 683 691 701 709 719 727 733 739 743 751 757 761 769 773 787 797 809 811 821 823 827 829 839 853 857 859 863 877 881 883 887 907 911 919 929 937 941 947 953 967 971 977 983 991 997 1009 1013 1019 1021 1031 1033 1039 1049 1051 1061 1063 1069 1087 1091 1093 1097 1103 1109 1117 1123 1129 1151 1153 1163 1171 1181 1187 1193 1201 1213 1217 1223 1229 1231 1237 1249 1259 1277 1279 1283 1289 1291 1297 1301 1303 1307 1319 1321 1327 1361 1367 1373 1381 1399 1409 1423 1427 1429 1433 1439 1447 1451 1453 1459 1471 1481 1483 1487 1489 1493 1499 1511 1523 1531 1543 1549 1553 1559 1567 1571 1579 1583 1597 1601 1607 1609 1613 1619 1621 1627 1637 1657 1663 1667 1669 1693 1697 1699 1709 1721 1723 1733 1741 1747 1753 1759 1777 1783 1787 1789 1801 1811 1823 1831 1847 1861 1867 1871 1873 1877 1879 1889 1901 1907 1913 1931 1933 1949 1951 1973 1979 1987 1993 1997 1999

C Sieve Program

Equivalent Java Sieve Program


/*--------- File: sieve.h -------------------*/
void makesieve(int p[], int len);
void printsieve(const int p[], int len);
void printprimes(const int p[], int len);
/*--------- File: sieve.c -------------------*/
#include <stdio.h>
#include "sieve.h"

static void initializesieve(int p[], int len) {
   int i;
   p[0] = 0;
   p[1] = 0;

   for (i = 2; i < len; i++)
      p[i] = 1;
}

static void one(int p[], int len, int prime) {
   int i;
   for (i = 2*prime; i < len; i = i + prime)
      p[i] = 0;
}

void makesieve(int p[], int len) {
   int i;
   initializesieve(p, len);
   for (i = 2; i*i < len; i++)
      if (p[i] == 1)
           one(p, len, i);
}

void printsieve(const int p[], int len) {
   int i;
   printf("Sieve follows:");
   for (i = 0; i < len; i++) {
      if (i%100 == 0)
         printf("\n");
      printf("%d", p[i]);
   }
   printf("\n");
}

void printprimes(const int p[], int len) {
   int i;
   int count = 0;
   printf("Primes follow:");
   for (i = 0; i < len; i++) {
      if (p[i] == 1) {
         if (count%10 == 0)
             printf("\n");
         printf("%d\t",i);
         count++;
      }
   }
   printf("\n");
}

/*--------- File: sievemain.c ---------------*/
#include <stdio.h>
#include "sieve.h"
#define SIZE 2000

int main() {
   int list[SIZE];
   int listsize = SIZE;

   makesieve(list, listsize);
   printsieve(list, listsize);
   printprimes(list, listsize);
   return 0;
}






/*--------- File: Sieve.java ----------------*/
public class Sieve {


   private static void initializeSieve(int[] p, int len) {
      
      p[0] = 0;
      p[1] = 0;

      for (int i = 2; i < len; i++)
         p[i] = 1;
   }

   private static void one(int[] p, int len, int prime) {

      for (int i = 2*prime; i < len; i = i + prime)
         p[i] = 0;
   }

   public static void makeSieve(int[] p, int len) {

      initializeSieve(p, len);
      for (int i = 2; i*i < len; i++)
         if (p[i] == 1)
            one(p, len, i);
   }

   public static void printSieve(int[] p, int len) {

      System.out.print("Sieve follows:");
      for (int i = 0; i < len; i++) {
         if (i%100 == 0)
            System.out.println();
         System.out.print(p[i]);
      }
      System.out.println();
   }

   public static void printPrimes(int[] p, int len) {
      int count = 0;

      System.out.print("Primes follow:");
      for (int i = 0; i < len; i++) {
         if (p[i] == 1) {
            if (count%10 == 0)
                System.out.println();
            System.out.print(i + "\t");
            count++;
         }
      }
      System.out.println();
   }
}
/*--------- File: SieveMain.java ------------*/
public class SieveMain {

   final static int SIZE = 2000;
   static int[] list = new int[SIZE];

   public static void main(String[] args) {
      int listSize = SIZE;

      Sieve.makeSieve(list, listSize);
      Sieve.printSieve(list, listSize);
      Sieve.printPrimes(list, listSize);
   }
}

Common Output

Sieve follows:
0011010100010100010100010000010100000100010100010000010000010100000100010100000100010000010000000100
0101000101000100000000000001000100000101000000000101000001000001000100000100000101000000000101000101
0000000000010000000000010001010001000001010000000001000001000001000001010000010001010000000001000000
0000000100010100010000000000000100000100000000010100010000010000000100000100000100010000010000000100
0100000001000000000101000000000101000001000100000100000001000101000100000000000100000001000100000001
0001000001000000000001010000000000000000010000010000000001000001000001010000010000000001000001000001
0100000100000100010100000000000100000000010100010000010000010100000000000100010000010000000100000000
0100000001000000000100000001000001000001000100000001000001000100000001000100000000000001000000000100
0000000001010000000001010001010000000001000000000000010001010001000000000000010001010001000000000000
0000000100010000000100000000010000000100010000010000010000000000000100010000010000010000000100000100
0000000001000100000101000000000101000001000000000101000000000101000001000000000000000001000101000100
0001000001000000010000010000010000000000000000000001010000000001000000010000000001000001000001000000
0100000000000100010000010000010100000100000000000100000000010000000000000000010100010000010100000100
0101000100000000000101000001000000000000000000000000000000000100000100000100000001000000000000000001
0000000001000000000000010001010001000001000000010001010000010000000000010000000001010001010001000001
0000000000010000000000010000000100000000000100000100010000010000000100010000000100010000000000000100
0100000101000100000101000001000000000100000000000000000001000001000101000000000000000000000001000101
0000000001000000000001010000000001000000010000010000010000010000000000000000010000010001010000000000
0100000000010000000000010000000100000000000000010000000000000100000100010100010100000000010000000000
0100000100000100000000000000000101000000000000000101000000000000000000000100000100000001000001000101
Primes follow:
2       3       5       7       11      13      17      19      23      29
31      37      41      43      47      53      59      61      67      71
73      79      83      89      97      101     103     107     109     113
127     131     137     139     149     151     157     163     167     173
179     181     191     193     197     199     211     223     227     229
233     239     241     251     257     263     269     271     277     281
283     293     307     311     313     317     331     337     347     349
353     359     367     373     379     383     389     397     401     409
419     421     431     433     439     443     449     457     461     463
467     479     487     491     499     503     509     521     523     541
547     557     563     569     571     577     587     593     599     601
607     613     617     619     631     641     643     647     653     659
661     673     677     683     691     701     709     719     727     733
739     743     751     757     761     769     773     787     797     809
811     821     823     827     829     839     853     857     859     863
877     881     883     887     907     911     919     929     937     941
947     953     967     971     977     983     991     997     1009    1013
1019    1021    1031    1033    1039    1049    1051    1061    1063    1069
1087    1091    1093    1097    1103    1109    1117    1123    1129    1151
1153    1163    1171    1181    1187    1193    1201    1213    1217    1223
1229    1231    1237    1249    1259    1277    1279    1283    1289    1291
1297    1301    1303    1307    1319    1321    1327    1361    1367    1373
1381    1399    1409    1423    1427    1429    1433    1439    1447    1451
1453    1459    1471    1481    1483    1487    1489    1493    1499    1511
1523    1531    1543    1549    1553    1559    1567    1571    1579    1583
1597    1601    1607    1609    1613    1619    1621    1627    1637    1657
1663    1667    1669    1693    1697    1699    1709    1721    1723    1733
1741    1747    1753    1759    1777    1783    1787    1789    1801    1811
1823    1831    1847    1861    1867    1871    1873    1877    1879    1889
1901    1907    1913    1931    1933    1949    1951    1973    1979    1987
1993    1997    1999

Commentary about the programs: It looks like quite a bit of red, but many of these differences are superficial and repetitious, so it isn't as bad as it seems.

Names: Variable and function names in both languages can be almost anything, but the two languages have customary ways of naming objects. In C, only lower-case is used for all but named constants (all upper-case). The variables are typically run together, as with printprimes, although C sometimes uses an underscore: print_primes. Java, on the other hand, starts names with an lower-case letter, followed by lower-case, but with an upper-case letter starting each subsequent part. Thus Java uses printPrimes. Both languages stick in digits also. Classes in Java start with an upper-case letter. All of this is just by convention and not required.
Structure of the Java program: The structure of the Java on the right (which you are more familiar with) is actually much more complex than that of C on the left. Each Java file is a named class that contains functions. Here it is simple, with a single static main function in one class, and with 5 static functions in the other class. In Java "static" means that the function can be used without creating an instance of the class in which it resides. (No matter how many instances are created, there is only one copy of each static function.) When the compiler compiles the file with the main function, it needs access to the other file to find out about these other functions also.
Structure of the C program: As mentioned above, this structure is less sophisticated. There are three files, one similar to the Java with the main function in it. One similar to the Java with the 5 additional functions in it. In both these files, the functions are just defined there, without any enclosing framework of a class. The third file is a file with three prototypes of functions. When this file is included into the source file for the main function, it tells the compiler what the functions that the main function calls are like (what they return and what their parameters are). In this way, the file with the main function can be compiled completely independently of the file with the function definitions. Two of the 5 functions are not used by the main function (they are "helper" functions for the other 3), and so they do not need a prototype in the file of prototypes.
Hidden functions: In the Java source file Sieve.java, the functions initializeSieve and one are declared private, meaning that they are buried in the class and are not accessible from the outside -- they can only be called by functions within the class.
In a similar way, in the C source file sieve.c, the functions initializesieve and one are declared static. In this case this also means that the functions cannot be accessed from outside the file. The keyword static has a different meaning when applied to variables, and it is basically insane for it to mean "private" when applied to functions, but that is indeed the case. When programming in C, if a function in a file has the "static" attribute, you should imagine that it says "private" instead.
Arrays as formal parameters: In Java a 1-dimensional array p is declared as a formal parameter as int[] p, while in C it must be declared as int p[]. (In Java you can also use the C form, but that is not recommended.)
Declaring the loop variable: In both Java and C++, you can declare the loop variable as part of the for loop: for (int i = 2; i < len; i++). The scope of i is limited to the for loop. This is not allowed in C.
Printing: C uses a special "print formatted" function printf which is powerful and handy, but not type-safe. This function is now in the latest version of Java (1.5.0) as System.out.printf.
Array allocation: Java only has dynamic array allocation, as in int[] list = new int[SIZE];. C and C++ allow dynamic array allocation, but the more normal method is to allocate on the run-time stack, using just: int list[SIZE];. The array list could have been declared outside the main function in the C program, as it was in the Java program.
Array lengths: In Java, an array is a full-fledged object, and in particular, each array knows its own length, as in list.length. In C (and C++) arrays are sort-of like objects, but they don't know their lengths. Thus in C you have to pass the length of the array as a separate parameter along with the array itself. The Java program below did this also, but it was not necessary.
Passing an array whose values will not be changed: In the C code for the functions printsieve and printprimes, the array formal parameter has the extra modifier const. This means that the array won't be changed inside the function, and the compiler can diagnose an error if you try to change an array value -- potentially valuable help. Java doesn't have this feature.

Sieve Program C and Java Side-by-side

Sieve Program
C and Java Side-by-side