Problem With Swapping 2d Char Arrays, Bubble Sort

[mahdiahmadirad]

Hi dears!
I wrote a simple bubble sort algorithm. it works properly when we compare full arrays but i want to sort a 2d array according to a specific part of array. it has some problem to swapping this array. please help me.

my scenario:
assume that we have a big 2d char array for example students[20][30] for 20 persons an 30 character for each person. first 15 chars contains first name and the rest is last name.
no i want to sort this array according to last name.

my Idea:
i defined char mapped[number][16]={""} and mapped 2nd 15 chars of student array. so mapped[i]=student[i] but the result is NULL. I don't now why!!!
when showing student there is nothing to display on screen.
someone help!

here is new bubble sort function:

void BubbleSort (void)
{
   char mapped[number][21]={""};
      for(i=0;i<number;i++)
        for(j=0;j<15;j++)
            mapped[i][j]=student[i][j+15];

    cout<<endl<<"mapped List:"<<endl;
    for(i=0;i<number;i++)
        cout<<mapped[i]<<endl;
                //now bubble sorting
                bool done = false;
                while (!done)
                {
                    done = true;
                    for (int n=0; n<number-1; n++)
                    if (strcmp(mapped[n], mapped[n+1]) > 0)
                    {
                       char temp[length+1];
                               strcpy(temp,student[n]);
                       strcpy(student[n], student[n+1]);
                       strcpy(student[n+1], temp);
                       done = false;
                     }
                }
    cout<<endl<<"Sorted Student List:"<<endl;
    for(i=0;i<number;i++)
        cout<<student[i]<<endl; //nothing displayed!!!
}

and definiton of student array, if u wanna know:

#define number 20
#define length 31

char student[number][length];

[newb16]

but the result is NULL.

result of what?

Anyway, you haven't allocated memory for 'mapped'. Then you don't swap mapped when you swap 'students', swap them as well or make some compare() function that compares two strings accordingto your layout.

[Banfa]

instead of

char students[20][30];

why don't you use

struct student_t {
    char first[15];
    char last[15];
} students[20];

Then it is easy to isolate the first and last names.

NOTE the structure uses the same amount of space as your array (assuming no padding) and provides space for names up to 14 characters in length.

[mahdiahmadirad]

id don't want use structures. it is a beginners program for practicing.

[mahdiahmadirad]

result of what?

result of output.
so, if i want to allocate memory for mapped, how should i define and declare it. i know how to allocate memory for 1d arrays:

char* mapped = new char[15];

what about 2d arrays. is this true?

char* mapped = new char[number][15];

[Banfa]

Firstly I would say a structure with 2 arrays in it is easier to manage than using a single array of char to hold 2 strings and therefore better suited for a beginner.

Secondly since you are using C++ you should be using std::string not arrays of char, you may as well learn the right way of doing things from the outset.

The allocation you would need would be

    char (*mapped)[15] = new char[20][15];

note, the parenthesise are important as they change the declaration from an array of pointers to a pointer to an array.

And finally in your original algorithm you use mapped to decided if entries in student need swapping but you never change the order of entries in mapped so the condition in the algorithm is always feed a list that still needs sorting.

[mahdiahmadirad]

thank u dear Banfa!
I learned how to allocate 2D arrays. very usefull. but I found out my mistake in another way and now swapping the original array fixed. I should swap mapped array and student array concerted and till i just swap student it keeps making the same comparisons over and over and over....
so the corrected code:

while (!done)
{
	done = true;
	for (int n=0; n<number-1 ; n++)
		if (strcmp(mapped[n], mapped[n+1]) > 0)
		{
		        char temp1[length+1],temp2[length+1];
    			
                        strcpy(temp1,student[n]);
                        strcpy(temp2, mapped[n]);

			strcpy(student[n], student[n+1]);
                        strcpy(mapped[n], mapped[n+1]);

			strcpy(student[n+1], temp1);
                        strcpy(mapped[n+1], temp2);
	
         		done = false;
		}
}

thank u very much!

[mahdiahmadirad]

great

thank u dear Banfa!
I learned how to allocate 2D arrays. very usefull. but I found out my mistake in another way and now swapping the original array fixed. I should swap mapped array and student array concerted and till i just swap student it keeps making the same comparisons over and over and over....
so the corrected code:

while (!done)
{
	done = true;
	for (int n=0; n<number-1 ; n++)
		if (strcmp(mapped[n], mapped[n+1]) > 0)
		{
		        char temp1[length+1],temp2[length+1];
    			
                        strcpy(temp1,student[n]);
                        strcpy(temp2, mapped[n]);

			strcpy(student[n], student[n+1]);
                        strcpy(mapped[n], mapped[n+1]);

			strcpy(student[n+1], temp1);
                        strcpy(mapped[n+1], temp2);
	
         		done = false;
		}
}

thank u very much!