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How to generate random number without repetition?

This is a discussion on How to generate random number without repetition? within the C Programming forums, part of the General Programming Boards category; Originally Posted by pheininger Old: Code: double arr[30][30]; New: Code: double arr[30][30]={0.0}; Old: Code: i= (int) rand()0; arr[i][j]= (double) rand()/RAND_MAX; ...

  1. #16
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    Quote Originally Posted by pheininger View Post
    Old:
    Code:
       double arr[30][30];
    New:
    Code:
       double arr[30][30]={0.0};


    Old:
    Code:
              i= (int) rand()0;
              arr[i][j]= (double) rand()/RAND_MAX;
              if (arr[i][j] > 0){
                 m[j]++;
    }
    New:
    Code:
              i= (int) rand()0;
              if (arr[i][j] == 0.0) {
                 arr[i][j]= (double) rand()/RAND_MAX;
                 if (arr[i][j] > 0.0) { 
                     m[j]++;
    Thank you so much sir!!!! It's work

  2. #17
    - - - - - - - - oogabooga's Avatar
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    I don't see how that answers your question about generating random numbers without repetition. But if you're happy, I'm happy.
    The cost of software maintenance increases with the square of the programmer's creativity. - Robert D. Bliss

  3. #18
    spurious conceit MK27's Avatar
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    Quote Originally Posted by oogabooga View Post
    I don't see how that answers your question about generating random numbers without repetition.
    It does if the OP is just trying to avoid writing to the same index randomly more than once, but does not care if the value assigned is a duplicate of one at some other index (which is pretty unlikely anyway considering the range is a double > 0.0 and <= 1.0). IMO opinion it would be better to create an array of possible indexes and shuffle that, then pop them off one at a time tho.

    Hopefully Suntang used %30 for the index and not the code posted .
    Last edited by MK27; 03-18-2012 at 12:55 PM.
    C programming resources:
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    Current ISO draft standard
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  4. #19
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    My understanding was that the OP wanted 4 random "reviewers" (numbers from 0 to 29, without repetition for any particular "object") to review each object, and that a "review" was a random number from 0.0 to 1.0. But I could have been reading into the situation.
    The cost of software maintenance increases with the square of the programmer's creativity. - Robert D. Bliss

  5. #20
    spurious conceit MK27's Avatar
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    Quote Originally Posted by oogabooga View Post
    My understanding was that the OP wanted 4 random "reviewers" (numbers from 0 to 29, without repetition for any particular "object") to review each object, and that a "review" was a random number from 0.0 to 1.0. But I could have been reading into the situation.
    That's what the new code will do, look:

    Code:
    i= (int) rand()0; // [sic]
    if (arr[i][j] == 0.0) {
       arr[i][j]= (double) rand()/RAND_MAX;
       if (arr[i][j] > 0.0) {
           m[j]++;
    presuming m[j] is the count for that column, it only gets incremented if arr[i][j] was 0.0 to start with and then set to >0.0. The problem before was that repeated values for i meant the same element might get set twice, and count each time. Now, there is a check to see if the element has already been set, and if so, there is no second set and no increment of the count.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  6. #21
    - - - - - - - - oogabooga's Avatar
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    OK. I see it now.
    The cost of software maintenance increases with the square of the programmer's creativity. - Robert D. Bliss

  7. #22
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    Hello, all sir!! I still have one more question to ask, when i increase the number of i and j to (1500) each. i got the error call " Class StackOverflow", then i decided to use calloc() function like below:
    Code:
    double** arr;
    int rows = 1500; 
     int columns = 1500; 
     int m[1500];
     int i,j,r;
     i=0;
     arr= (double **) malloc(rows*sizeof(double*));
     for(r=0; columns; r++)
    arr[r]=(double*)malloc(columns*sizeof(double));
     srand((unsigned)time(NULL));
     for(j=0; j<columns; j++){
     m[j]= 0;
     .......
     .......
      }
     free(arr);
    getch();
    }
    But at the result i got only some data not all. What's wrong in my code sir? Any idea?

    Thanks in advance!!!

  8. #23
    spurious conceit MK27's Avatar
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    Code:
    for(r=0; columns; r++)
    Is obviously an infinite loop, since if columns is 1500, the middle condition will always be true. So this is not your actual code, this is some version of it. Making it impossible to say "what's wrong", because you did not post the real code. Please do not do that in the future, it is a waste of time for everybody. If you can't post the actual code, write a compilable program that re-creates the problem, ie, one the can actually be compiled and run to demonstrate the error or problem.

    Code:
    free(arr);
    That's a memory leak. You have to free each row in arr first, just as you allocated each row.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  9. #24
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    Quote Originally Posted by MK27 View Post
    Code:
    for(r=0; columns; r++)
    Is obviously an infinite loop, since if columns is 1500, the middle condition will always be true. So this is not your actual code, this is some version of it. Making it impossible to say "what's wrong", because you did not post the real code. Please do not do that in the future, it is a waste of time for everybody. If you can't post the actual code, write a compilable program that re-creates the problem, ie, one the can actually be compiled and run to demonstrate the error or problem.

    Code:
    free(arr);

    That's a memory leak. You have to free each row in arr first, just as you allocated each row.


    I'm so sorry sir, this is the hold code:

    Code:
    #include <stdio.h>
    #include <conio.h>
    #include <stdlib.h>
    #include <time.h>
    int main(void)
    {
     double** arr;
     int rows = 1500; 
            int columns = 1500; 
     int m[1500];
     int i,j,r;
     i=0;
     arr= (double **) malloc(rows*sizeof(double*));
     for(r=0; columns; r++)
            arr[r]=(double*)malloc(columns*sizeof(double));
     srand((unsigned)time(NULL));
     for(j=0; j<columns; j++){
      m[j]= 0;
      while(m[j] < 15){
       i= (int) rand()%1500;
       if (arr[i][j] == 0.0) {
        arr[i][j]= (double) rand()/RAND_MAX;
        if(arr[i][j] > 0){
         m[j]++;
         printf("arr[%d][%d] = %lf\t", i, j, arr[i][j]);
                printf("\n");
        }
       }
      }
     }
     free(arr);
    getch();
    }

  10. #25
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    Read this > FAQ > Casting malloc - Cprogramming.com

    Take MK's advice about free()ing your array. When you allocate, "arr" is an array of pointers. Assuming you allocate a "3x3" array of 8-byte values, it will look something like this:
    arr[0][0] (0x6060)
    arr[0] (0x5050) points to (0x6060) arr[0][1] (0x6068)
    arr[0][2] (0x6070)
    arr[1][0] (0x8080)
    arr (0x4040) points to (0x5050) arr[1] (0x5058) points to (0x8080) arr[1][1] (0x8088)
    arr[1][2] (0x8090)
    arr[2][0] (0x2020)
    arr[2] (0x5060) points to (0x2020) arr[2][1] (0x2028)
    arr[2][2] (0x2030)

    Only freeing arr will result in (the example above) 0x5050-0x5060 being freed, even though these memory addresses contain pointers to still-allocated memory​.

  11. #26
    spurious conceit MK27's Avatar
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    Main() should return an int.

    You shouldn't cast the return value of malloc unless you need to use this code with a C++ compiler.

    So, you do have an infinite loop in there >_<. Add this to the for loop at line 14 and run the code to see what happens:

    Code:
     for(r=0; columns; r++) {
    	arr[r]=(double*)malloc(columns*sizeof(double));
    	fprintf(stderr, "allocated row %d\n", r);
     }
    And remember what I wrote in post #23 about freeing all the pointers in **arr, not just **arr itself.
    Last edited by MK27; 03-19-2012 at 01:34 PM.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  12. #27
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    Quote Originally Posted by MK27 View Post
    Main() should return an int.

    You shouldn't cast the return value of malloc unless you need to use this code with a C++ compiler.

    So, you do have an infinite loop in there >_<. Add this to the for loop at line 14 and run the code to see what happens:

    Code:
     for(r=0; columns; r++) {
        arr[r]=(double*)malloc(columns*sizeof(double));
        fprintf(stderr, "allocated row %d\n", r);
     }
    And remember what I wrote in post #23 about freeing all the pointers in **arr, not just **arr itself.
    Now i had changed my code from "malloc" to "calloc" because if i use malloc i don't get any output

    Code:
    Code:
    #include <stdio.h>
    #include <conio.h>
    #include <stdlib.h>
    #include <time.h>
    int main(void)
    {
     double** arr;
     int rows = 1500; 
            int columns = 1500; 
     int m[1500];
     int i,j,r;
     i=0;
     arr= (double **) calloc(rows,sizeof(double*));
     for(r=0; columns; r++){
            arr[r]=(double*)calloc(columns,sizeof(double));
     fprintf(stderr, "allocated row %d\n", r);
     }
     // FREEING MEMORY        
     for (r = 0; r < columns; i++)    {        
      free(arr[r]);    
     }  
     srand((unsigned)time(NULL));
     for(j=0; j<columns; j++){
      m[j]= 0;
      while(m[j] < 15){
       i= (int) rand()%1500;
       if (arr[i][j] == 0.0) {
        arr[i][j]= (double) rand()/RAND_MAX;
        if(arr[i][j] > 0){
         m[j]++;
         printf("arr[%d][%d] = %lf\t", i, j, arr[i][j]);
                printf("\n");
        }
       }
      }
     }
     free(arr);
    getch();
    }
    I got the following error:


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    And the output :
    allocated row 11299
    allocated row 11300
    allocated row 11301
    allocated row 11302
    allocated row 11303
    allocated row 11304
    allocated row 11305
    allocated row 11306
    allocated row 11307
    allocated row 11308
    allocated row 11309
    allocated row 11310
    allocated row 11311
    allocated row 11312
    allocated row 11313
    allocated row 11314
    allocated row 11315
    allocated row 11316
    allocated row 11317
    allocated row 11318
    allocated row 11319
    allocated row 11320
    allocated row 11321
    allocated row 11322
    allocated row 11323
    allocated row 11324
    allocated row 11325
    allocated row 11326
    allocated row 11327
    allocated row 11328
    allocated row 11329
    allocated row 11330
    allocated row 11331
    allocated row 11332
    allocated row 11333
    allocated row 11334
    allocated row 11335
    allocated row 11336
    allocated row 11337
    allocated row 11338
    allocated row 11339
    allocated row 11340
    allocated row 11341
    allocated row 11342
    allocated row 11343
    allocated row 11344
    allocated row 11345
    allocated row 11346
    allocated row 11347
    allocated row 11348
    allocated row 11349
    allocated row 11350
    allocated row 11351
    allocated row 11352
    allocated row 11353
    allocated row 11354
    allocated row 11355
    allocated row 11356
    allocated row 11357
    allocated row 11358
    allocated row 11359
    allocated row 11360
    allocated row 11361
    allocated row 11362
    allocated row 11363
    allocated row 11364
    allocated row 11365
    allocated row 11366
    allocated row 11367
    allocated row 11368
    allocated row 11369
    allocated row 11370
    allocated row 11371
    allocated row 11372
    allocated row 11373
    allocated row 11374
    allocated row 11375
    allocated row 11376
    allocated row 11377
    allocated row 11378
    allocated row 11379
    allocated row 11380
    allocated row 11381
    allocated row 11382
    allocated row 11383
    allocated row 11384
    allocated row 11385
    allocated row 11386
    allocated row 11387
    allocated row 11388
    allocated row 11389
    allocated row 11390
    allocated row 11391
    allocated row 11392
    allocated row 11393
    allocated row 11394
    allocated row 11395
    allocated row 11396
    allocated row 11397
    allocated row 11398
    allocated row 11399
    allocated row 11400
    allocated row 11401
    allocated row 11402
    allocated row 11403
    allocated row 11404
    allocated row 11405
    allocated row 11406
    allocated row 11407
    allocated row 11408
    allocated row 11409
    allocated row 11410
    allocated row 11411
    allocated row 11412
    allocated row 11413
    allocated row 11414
    allocated row 11415
    allocated row 11416
    allocated row 11417
    allocated row 11418
    allocated row 11419
    allocated row 11420
    allocated row 11421
    allocated row 11422
    allocated row 11423
    allocated row 11424
    allocated row 11425
    allocated row 11426
    allocated row 11427
    allocated row 11428
    allocated row 11429
    allocated row 11430
    allocated row 11431
    allocated row 11432
    allocated row 11433
    allocated row 11434
    allocated row 11435
    allocated row 11436
    allocated row 11437
    allocated row 11438
    allocated row 11439
    allocated row 11440
    allocated row 11441
    allocated row 11442
    allocated row 11443
    allocated row 11444
    allocated row 11445
    allocated row 11446
    allocated row 11447
    allocated row 11448
    allocated row 11449
    allocated row 11450
    allocated row 11451
    allocated row 11452
    allocated row 11453
    allocated row 11454
    allocated row 11455
    allocated row 11456
    allocated row 11457
    allocated row 11458
    allocated row 11459
    allocated row 11460
    allocated row 11461
    allocated row 11462
    allocated row 11463
    allocated row 11464
    allocated row 11465
    allocated row 11466
    allocated row 11467
    allocated row 11468
    allocated row 11469
    allocated row 11470
    allocated row 11471
    allocated row 11472
    allocated row 11473
    allocated row 11474
    allocated row 11475
    allocated row 11476
    allocated row 11477
    allocated row 11478
    allocated row 11479
    allocated row 11480
    allocated row 11481
    allocated row 11482
    allocated row 11483
    allocated row 11484
    allocated row 11485
    allocated row 11486
    allocated row 11487
    allocated row 11488
    allocated row 11489
    allocated row 11490
    allocated row 11491
    allocated row 11492
    allocated row 11493
    allocated row 11494
    allocated row 11495
    allocated row 11496
    allocated row 11497
    allocated row 11498
    allocated row 11499
    allocated row 11500
    allocated row 11501
    allocated row 11502
    allocated row 11503
    allocated row 11504
    allocated row 11505
    allocated row 11506
    allocated row 11507
    allocated row 11508
    allocated row 11509
    allocated row 11510
    allocated row 11511
    allocated row 11512
    allocated row 11513
    allocated row 11514
    allocated row 11515
    allocated row 11516
    allocated row 11517
    allocated row 11518
    allocated row 11519
    allocated row 11520
    allocated row 11521
    allocated row 11522
    allocated row 11523
    allocated row 11524
    allocated row 11525
    allocated row 11526
    allocated row 11527
    allocated row 11528
    allocated row 11529
    allocated row 11530
    allocated row 11531
    allocated row 11532
    allocated row 11533
    allocated row 11534
    allocated row 11535
    allocated row 11536
    allocated row 11537
    allocated row 11538
    allocated row 11539
    allocated row 11540
    allocated row 11541
    allocated row 11542
    allocated row 11543
    allocated row 11544
    allocated row 11545
    allocated row 11546
    allocated row 11547
    allocated row 11548
    allocated row 11549
    allocated row 11550
    allocated row 11551
    allocated row 11552
    allocated row 11553
    allocated row 11554
    allocated row 11555
    allocated row 11556
    allocated row 11557
    allocated row 11558
    allocated row 11559
    allocated row 11560
    allocated row 11561
    allocated row 11562
    allocated row 11563
    allocated row 11564
    allocated row 11565
    allocated row 11566
    allocated row 11567
    allocated row 11568
    allocated row 11569
    allocated row 11570
    allocated row 11571
    allocated row 11572
    allocated row 11573
    allocated row 11574
    allocated row 11575
    allocated row 11576
    allocated row 11577
    allocated row 11578
    allocated row 11579
    allocated row 11580
    allocated row 11581
    allocated row 11582
    allocated row 11583
    allocated row 11584
    allocated row 11585
    allocated row 11586
    allocated row 11587
    allocated row 11588
    allocated row 11589
    allocated row 11590
    allocated row 11591
    allocated row 11592
    allocated row 11593
    allocated row 11594
    allocated row 11595
    allocated row 11596
    allocated row 11597

  13. #28
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    Use a little logic and think about what you wrote. What good does freeing the memory do before you can actually use it? Free everything at the end, where you freed the main array.

    Did you not read my other post? Quit casting malloc! And if it gives you errors, compile as C (not c++).

    Also note that the error might be because every time you run calloc() in the loop, you're allocating 12,000 bytes. Run this 1500 times, and you'll be allocating 18,000,000 bytes. Your OS probably doesn't allow a single program to take up this much memory.

  14. #29
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    You are getting good advice above.

    Making the first malloc() into a calloc() is not necessary since immediately you initialize all of the elements of this dimension in the first for loop when you make the other calls malloc() or calloc().

    The second call needs to be either a calloc() so that the individual review values are 0.0 or you need a separate loop to initialize all the reviews to 0.0 using a separate loop. I am assuming that the floating point on your implementation is one where 0.0 is all bits zero. I have not seen one where that is not true. But I am not going to take the time to check the standard to see if that is required of floating point values.

  15. #30
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    Quote Originally Posted by pheininger View Post
    The second call needs to be either a calloc() so that the individual review values are 0.0 or you need a separate loop to initialize all the reviews to 0.0 using a separate loop. I am assuming that the floating point on your implementation is one where 0.0 is all bits zero. I have not seen one where that is not true. But I am not going to take the time to check the standard to see if that is required of floating point values.
    Use a separate loop. calloc only gives all bits zero, and there is no guarantee that 0.0 is represented as all bits zero.
    pheininger likes this.

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