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# Sorting algorithm (Easy but long)

Posted on 2001-06-03
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// I have this class:

class Person
{
protected:
char *first;
char *last;
char *city;
char *state;
long int zip;
long int phone;

public:
Person();
Person(char *First,char *Last,char *Address,char *City,char *State, long int zip, long int phone);

// I have array of Person objects that need to be sorted by last,first name.  Assume the array has 50 or less items and should be sorted using a simple Quick sort or Merge sort whichever is more efficient."NO BUBBLE SORT I can do that".  For an A grade I need code that will work and an explanation of how it works.  Please make the code as simple as possible.
0
Question by:DwarfBaby
• 7
• 4
• 2
• +1

LVL 5

Accepted Solution

djbusychild earned 200 total points
you could use qsort or use STL's template classes' own sort functions.. to use qsort do something like this..
pretty self-explanatory... I think.. It's for example purposes only.

#include <stdlib.h>
#include <string.h>
#include <iostream.h>

class Person
{
protected:
char *first;
char *last;
char *city;
char *state;
long int zip;
long int phone;

public:
void setMembers(char *First,char *Last)
{
/* not checking arr bonds */
first=new char[strlen(First)];
last=new char[strlen(Last)];
strcpy(first,First);
strcpy(last,Last);
}
char* getLastName(){return last;};
char* getFirstName(){return first;};
};

/*
<0 elem1<elem2
0 elem1==elem2
>0 elem1>elem2
*/
int compareByLastName(const void *elem1, const void *elem2)
{
Person* person1=(Person*)elem1;
Person* person2=(Person*)elem2;

return strcmp(person1->getLastName(),person2->getLastName());
}
int compareByFirstName(const void *elem1, const void *elem2)
{
Person* person1=(Person*)elem1;
Person* person2=(Person*)elem2;

return strcmp(person1->getFirstName(),person2->getFirstName());
}
int main(int argc, char* argv[]) {

Person people[5];
people[0].setMembers("a","e");
people[1].setMembers("b","d");
people[2].setMembers("c","c");
people[3].setMembers("d","b");
people[4].setMembers("e","a");

cout << endl<< "sort by last name"<<endl;

qsort(people,5,sizeof(Person),compareByLastName);

int i;
for (i=0;i<5;i++) {
cout << "Last Name:" << people[i].getLastName() << " Firt Name:"<<people[i].getFirstName()<<endl;
}
cout << endl<<"sort by first name"<<endl;

qsort(people,5,sizeof(Person),compareByFirstName);
for (i=0;i<5;i++) {
cout << "Last Name:" << people[i].getLastName() << " Firt Name:"<<people[i].getFirstName()<<endl;
}
return 0;
}
0

LVL 5

Expert Comment

you should do your own tidying it up, encapsulating, err checking, etc, etc... this is purely a quick hack example. =)
0

LVL 5

Expert Comment

qsort is a builtin function in stdlib that does qsort for you.

the man page entry can be found here

http://www.opengroup.org/onlinepubs/007908799/xsh/qsort.html

you're basically telling it how big the data is, how many of them there are, and what function shold be used to compare the two.
0

LVL 5

Expert Comment

I really should be allocating memory for strlen(last)+1 there.. (for the NULL terminating char), and if you're a C++ person you wouldn't use the C style casting that I used either.
0

Author Comment

Ok, I understand your code but I'm trying to learn how the quick sort works.  I need the code for a quick sort not the code for how to use the quick sort.  Sorry, I was not very clear about what I wanted.
0

LVL 5

Expert Comment

oh, well here's an example with full explanation (There are gazilliong qsort implementations on the web)

http://ciips.ee.uwa.edu.au/~morris/Year2/PLDS210/qsort1a.html
0

LVL 5

Expert Comment

0

Author Comment

I need to get something to eat and then I am going to try to work a quick sort into my code.  I will let you know if I need more explanation in a couple hours.  Thanks
0

Expert Comment

this question sounds like homework
0

LVL 5

Expert Comment

hmm... come to think of it, it does. idyott... ;)
0

Author Comment

Thanks for the help.
0

LVL 22

Expert Comment

DwarfBaby, if this is homework then the solution above will not be satisfactory.  You would be expected to write your own sort procedure, not use one that is part of the standard library.  (Is like ordering take-out food for a cooking class in home-ec.)

And if that is the case, then we can't _give_ you the answer.  You need to write it yourself.  But we coudl help you.
0

Author Comment

The question was not really homework.  I say not really because all the instructions called for either Selection sort or an insertion sort, which I know how to do.  I just thought I'd throw in a quicksort too for some browny points.  Although I did finally get the quick sort to work I barley understand how it works.  If anyone knows of a good website that can explain it better I will set up some extra points for you.  Thanks
0

Expert Comment

you can use "the c programming language"
in the index look 4 qosrt and u will c how it is written.

very generally, it uses divides the array in2 2 parts and sorts each of them recursively
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