Sorting problem

[buxBonehead]

Hi there

I'm having some trouble with a C program for an assignment. Its a program which is designed to read exam data from a text file, store them and then output a certain exam in order of marks. Heres the code so far:-

#include <stdio.h>

int results[40][5];

void bubblesort (int students, int test)
{int j, k, tmp1, tmp2;

for (k=0;k < students - 1; ++k)
  for(j=students-1;j>k;--j)
    if(results[j-1][test] < results[j][test]);
      {tmp1 = results[j-1][test];
        results[j-1][test]=results[j][test];
        results[j][test]=tmp1;
       tmp2 = results[j-1][0];
        results[j-1][0]=results[j][0];
        results[j][0]=tmp2;
}
}


int main (void)

{

int junkcontinue, x = 0, count = 0, choice = 3;
FILE *my_in, *my_out;



    printf("\nWelcome. This program is specifically designed to read and assess");
    printf("\nexam data retrieved from the 'datafile.txt' file in your U: drive.");
    printf("\n\nIf you are confident such a file exists in your U: drive, type");
    printf("\n'y' and press Enter.\n\n");
    
    scanf("%d", &junkcontinue);
    
    if ((my_in = fopen ("C:\\datafile.txt","rt")) == NULL)
    {
        printf("\nDatafile.txt is not found.\n");
		system("pause");
        return(1);
    }
    
    if ((my_out = fopen ("C://dataout.txt","wt")) == NULL)
    {
        printf("\nError: Dataout.txt is not found.\n");
		system("pause");
        return(1);
    }  
    
    printf("\nFile found. Reading file...\n\n");

    while  (fscanf(my_in, "%d,%d,%d,%d,%d", &results[x][0], &results[x][1], &results[x][2], &results[x][3], &results[x][4])!= EOF)
    {
        printf("*");
        x += 1;
        count++;
    }  
    
    int testnumber;
    
    printf("\n\nFiles Imported to array.\n\n");


    printf("Please type which test you would like to output (1,2,3,4) and press Enter:");
    fflush(stdin);
    scanf("%d", &testnumber);
    
    printf("\nYou have selected test %d.", testnumber);
    printf("\nOutputting results to dataout.txt...");
    
    printf("\n\nThe count is %d.\n\n", count);
    printf("\n\nThe testnumber is %d.\n\n", testnumber);
    
    printf("\n\t%d\t%d\t%d\t%d\t%d", results[0][0], results[0][1], results[0][2], results[0][3], results[0][4]);
    printf("\n\t%d\t%d\t%d\t%d\t%d", results[1][0], results[1][1], results[1][2], results[1][3], results[1][4]);
    printf("\n\t%d\t%d\t%d\t%d\t%d", results[2][0], results[2][1], results[2][2], results[2][3], results[2][4]);
    printf("\n\t%d\t%d\t%d\t%d\t%dn\n", results[3][0], results[3][1], results[3][2], results[3][3], results[3][4]);
    
    
    
    bubblesort(count-1, testnumber);

    printf("\n\t%d\t%d\t%d\t%d\t%d", results[0][0], results[0][1], results[0][2], results[0][3], results[0][4]);
    printf("\n\t%d\t%d\t%d\t%d\t%d", results[1][0], results[1][1], results[1][2], results[1][3], results[1][4]);
    printf("\n\t%d\t%d\t%d\t%d\t%d", results[2][0], results[2][1], results[2][2], results[2][3], results[2][4]);
    printf("\n\t%d\t%d\t%d\t%d\t%dn\n", results[3][0], results[3][1], results[3][2], results[3][3], results[3][4]);

    
    while (x < count)
    {
        fprintf(my_out, "%d,%d\n", results[x][0], results[x][3]);
        x += 1;
    }
    
    printf("\n\nDataout.txt file has finished writing.");

    fclose(my_in);
    fclose(my_out);

    system("pause");

return (0);
}

The testnumber is 3.


        105683  23      65      0       100
        105475  96      72      35      61
        105286  23      45      15      86
        104762  23      17      5       56n

        105475  23      65      35      100
        105683  96      72      0       61
        105286  23      45      15      86
        104762  23      17      5       56n

The above shows the data in the array before the bubble sort, and after the bubble sort. Row 3 should be in order 35/15/5/0 but as you can see it appears only the top 2 values have been swapped. I know the code is messy (I'm v.new to this) but can anyone spot the problem? I just cant figure this out.

[Dino]

You are using testnumber as the index (relative to 1) for which set of scores to test. You pass that as test to your bubblesort, and use it as an index, but it is still relative to 1, not zero, and, it's not used as the correct index either.

There are various other problems with your code as well I choose to not comment on.

[buxBonehead]

Thanks for the quick reply, but I'm afraid I have no idea what you are suggesting I do. The 'index' you refer to is in the function header I presume? How do I make the index relative to 0 as appose to 1? And forget the rest of the code, its a... work in progress.

[Dino]

You start out with a user entered value:

printf("Please type which test you would like to output (1,2,3,4) and press Enter:");
fflush(stdin);   <-- P.S.  no such thing 
scanf("%d", &testnumber);
...
bubblesort(count-1, testnumber);  // testnumber is 1, 2, 3 or 4

Then in bubblesort, you use testnumber (now test) as an index into the results array:

void bubblesort (int students, int test)
{int j, k, tmp1, tmp2;

for (k=0;k < students - 1; ++k)
  for(j=students-1;j>k;--j)
// test (previously known as testnumber) is still 1 through 4, and it's supposed to 
// be 0 through 3, and the first index, not the second
    if(results[j-1][test] < results[j][test]);
      {tmp1 = results[j-1][test];
        results[j-1][test]=results[j][test];
        results[j][test]=tmp1;
       tmp2 = results[j-1][0];
        results[j-1][0]=results[j][0];
        results[j][0]=tmp2;
}
}

On another note, you subtract 1 from count here...

bubblesort(count-1, testnumber);

and then do it again (above in green).

[swoopy]

>    if(results[j-1][test] < results[j][test]);

Another problem. Notice you've also got a semicolon at the end of this if(). This means that the if() statement is basically ignored.

Also as Todd explained:

bubblesort(count-1, testnumber);

Why count-1 here? Just pass in the number of students, and let the bubblesort() function do the offset, probably in the inner for-loop, not the outer one.

[swoopy]

>    while (x < count)
>    {
>        fprintf(my_out, "%d,%d\n", results[x][0], results[x][3]);
>        x += 1;
>    }

x would need to be reset back to 0, because you already incremented it in a prior loop. For-loops work nicely for this purpose:

    for (x=0; x<count; x++)
    {
        fprintf(my_out, "%d,%d\n", results[x][0], results[x][3]);
    }

[buxBonehead]

Thanks guys, it was the semicolon ignoring the if function. I've actually fixed it by just rebuilding it with an insertion sort instead and it worked.

[buxBonehead]

Another problem now...I'm trying to sort the test data from my arrays into another array based on the test# and size. For example test1 0-9 has 1 value, 10-19 has 0 values etc..

For simplicity I'm building it only for test 1 at the moment which has results 23,96,23,23. Heres the code:-

int grouping[10][4];  
int a = 0, b = 9, c = 0, d = 0, e = 0;    
  
   
while (e < 10)
{
      while (d < count)
      {
      if (a <= results[d][1] && results[d][1] <= b)
         {
         c += 1;
         }
      d += 1;
      }
grouping[e][0] = c;
a += 10;
b += 10;
e += 1;
}    

    
    printf("\n%d\n", grouping[0][0]);
    printf("\n%d\n", grouping[1][0]);
    printf("\n%d\n", grouping[2][0]);
    printf("\n%d\n", grouping[3][0]);
    printf("\n%d\n", grouping[4][0]);
    printf("\n%d\n", grouping[5][0]);

This just returns:

0
0
0
0
0
0

any ideas?

[vart]

step through your code in debugger and see by yourself what is happening

[buxBonehead]

I've been messing around with the debugger in Dev-C++ but I still cant figure out whats wrong, it seems like it should work

[Adak]

Your code doesn't include a variable type declaration for "count", so that can't be right.

Since this sorting code is seriously flawed, why not start with your insertion sorting code that works, and just make the alterations so it sorts according to the keys you want it to sort on, in this case?

[buxBonehead]

Count is declared earlier on in the code so its not that I think.

Since this sorting code is seriously flawed, why not start with your insertion sorting code that works, and just make the alterations so it sorts according to the keys you want it to sort on, in this case?

I dont know what you mean here...I'm trying to group all the test results from test 1 by size into an array - 0-9 in cell [0][0] for example. You say this code is seriously flawed but how? I'm new to C and to me this seems like it should work, so whats wrong?

[swoopy]

>      if (a <= results[d][1] && results[d][1] <= b)

With the numbers you have in the array (23,96,23,23), how is this condition ever going to be true?

[Adak]

Count is declared earlier on in the code so its not that I think.

I dont know what you mean here...I'm trying to group all the test results from test 1 by size into an array - 0-9 in cell [0][0] for example. You say this code is seriously flawed but how? I'm new to C and to me this seems like it should work, so whats wrong?

I'm sure you're right about "count". My point is that when you don't have it in the code you post, we don't know WTD is going on, and left to guess.

I understand what you're trying to do, but you should understand that your sorting code is absolutely the worst stew of misguided thinking for a sort function, that I've ever seen.

You can't throw it away, fast enough.

You can't just "write a sorting function" that "seems like it should work".

First, you learn how a sort algorithm works, preferably a simple one, what it should look like, and play with that basic algorithm a bit - try this, try that, try something else - but now you know what the basic algorithm should look like, and come at the problem from a basic foundation of knowledge.

You don't have that foundation, yet. Looking at the code, without your text, I'd have no idea that you were even trying to sort.

I'll edit this now that I believe you're actually serious, with some code.

OK, this is your previous post and code:

another problem now...I'm trying to sort the test data from my arrays into 
another array based on the test# and size. 

For example test1 0-9 has 1 value, 10-19 has 0 values etc..

For simplicity I'm building it only for test 1 at the moment which has results 23,96,23,23. Heres the code:-

Code:

int grouping[10][4];  
int a = 0, b = 9, c = 0, d = 0, e = 0;    
  
   
while (e < 10)
{
      while (d < count)
      {
      if (a <= results[d][1] && results[d][1] <= b)
         {
         c += 1;
         }
      d += 1;
      }
grouping[e][0] = c;
a += 10;
b += 10;
e += 1;
}    

    
    printf("\n%d\n", grouping[0][0]);
    printf("\n%d\n", grouping[1][0]);
    printf("\n%d\n", grouping[2][0]);
    printf("\n%d\n", grouping[3][0]);
    printf("\n%d\n", grouping[4][0]);
    printf("\n%d\n", grouping[5][0]);


This just returns:

Code:

0
0
0
0
0
0


any ideas?

So int test1[] = {23, 96, 23, 23}, and you want the sorted data output to results[], right?

#include <stdio.h>

int main(void)   {
   int i, j, temp, gar;
   int test[] = { 23, 96, 0, 22 };  //number change to test the sort better.
   int results[4] = { 0 };

   for(i = 0; i < 3; i++)   {
      for(j = i + 1; j < 4; j++)   {
         if(test[i] > test[j])   {
            temp = test[i];
            test[i] = test[j];  
            test[j] = temp;    
         }
      }
   }
   for(i = 0; i < 4; i++)
      printf("%d, ", test[i]);
   gar = getchar(); gar++;
   return 0;


//888888888888888888888  Doesn't work 8888888888888888888888888888
for(i = 0; i < 3; i++)   {
   for(j = i + 1; j < 4; j++)   {
      if(test[i] > test[j]   {
         temp = test[i];
//         test[i] = test[j];  //remark out this line if you want test to remain unsorted
//         test[j] = temp;   //    "        "       "    "    "   "      "      "     "      "           "
      
          //above portion sorts test, in place. this puts the data into results[], instead of sorting in place.
          results[i] = test[j];
          results[j] = temp;
      }
      else
          results[i] = test[i];


      }
   }
}
//888888888888888888888888888 Doesn't work 8888888888888888

Second half of above code doesn't work because results[] never gets re-sorted, after the first pass. All the comparisons are
made with test[], and none are made with results[].

I would just sort test[] in place, and then write it's contents to results[]. Or, you could set up an index array, sort the index, by the
content of test[], and then write out the contents of test[], using that new, sorted, index. This is slightly more advanced, but a slick thing to know.

The latter sort by using an index, would leave test[] unsorted, but results would be sorted.

Hope that helps.