searching inside an object...

[Minime80]

Okay, I was wondering if there was an easy way to make a function that searches an array of objects. Here's what I was thinking:

1. You pass the array of objects to the function along with the thing you're searching for and the field you want to find it in.
2. It displays all the data for all the objects that have what you were looking for

i.e. you have a player structure with 3 strings: name, favorite color, and position.
The function can search and display all the players in the array for "red" being in the favorite color section, or "center" being their position.

I've got most of it figured out, I just didn't know how to tell the function which field to search. Any clue?

[Minime80]

Is this even possible?

[fafnir]

I am assuming that your array contains all the same class and that you know what it is.

If this is the case then I think something this might be what you want (I haven't compiled, but it should illustrate what I mean):


//class to be used in the array
class Character {
private:
char **name;
char **colour
public:
Character(const char **charName, const char **charColour) {
strcpy(name, charName);
strcpy(colour, charColour);
}

char **getName() {return name;}
char **getColour() {return colour;}
//...
};

class CharacterArray {
private:
Character **array;
int length;

public
CharacterArray() : length(0) {}

//search takes a pointer to a Character
//method that returns a char** and takes
//no parameters, and a char** to compare
//the result to, and returns the Character
//object that matches.
Character *search(
char **(Character::*comparisonMethod)(),
const char **comparisonValue) {
for (int i = 0; i < length; i++) {
if (strcmp(array[i]->*comparisonMethod(),
comparisonValue) == 0)
return array[i];
}
}

//... methods to populate array etc
};

int main(void) {
CharacterArray characterArray;

//... array is initialised at some point
Character *fred = characterArray.search(
&Character::getName(), "fred");

Character *red = characterArray.search(
&Character::getColour(), "red");
}



The key is the declaration: char **(Character::*comparisonMethod)()

this is a pointer called comparisonMethod, that points to a method in a Character object that returns char** and takes no parameters. This allows you to pass any method from the Character class that matches this signiature.


HTH,
Fafnir

[This message has been edited by fafnir (edited 12 November 2000).]

[This message has been edited by fafnir (edited 12 November 2000).]

[Minime80]

Okay, I sort of get it. So how would I do it with a structure something like this:

struct player
{
char name[10];
char favorite_color[10];
char position[10];
};

[The_Stack]

/* main */

#include <string.h>
#include <stdlib.h>
#include <malloc.h>
#include <stdio.h>

#define ARRAY_LEN 10
#define NAME_LEN 10
#define FAV_COLOR_LEN 10
#define POSITION_LEN 10

struct player
{
char name[NAME_LEN];
char favorite_color[FAV_COLOR_LEN];
char position[POSITION_LEN];
};

struct player* array[ARRAY_LEN];

int init(struct player** p, int l)
{
int retval=0;
int i;

memset(array,0,sizeof(array));

for(i=0;i<l;++i)
{
struct player* newp=(struct player*) malloc(sizeof(struct player));
if (newp)
{
p[i]=newp;
}
else
{
retval=-1;
break;
}
}
return retval;
}

void reclaim(struct player** p,int l)
{
int i;
for(i=0;i<l;++i)
{
free(p[i]);
p[i]=0;
}
}

void do_printp(struct player* p)
{
printf("name=%s\n",p->name);
printf("favorite_color=%s\n",p->favorite_color);
printf("position=%s\n",p->position);
}

void printp(char* s, struct player** p, int l, int field)
{
int i;

for (i=0;i<l;++i)
{
switch (field)
{
case 1:
if (strcmp((const char*)s,((const char*)p[i]->name))==0) do_printp(p[i]);
break;
case 2:
if (strcmp((const char*)s,((const char*)p[i]->favorite_color))==0) do_printp(p[i]);
break;
case 3:
if (strcmp((const char*)s,((const char*)p[i]->position))==0) do_printp(p[i]);
break;
default:
{
}
};

}
}

int main()
{
int errcode=init(array,ARRAY_LEN);

switch (errcode)
{
case -1:
reclaim(array,ARRAY_LEN);
break;
case 0:
{

strcpy(array[1]->name,"joe");
strcpy(array[1]->favorite_color,"red");
strcpy(array[1]->position,"goalkeep");

strcpy(array[2]->name,"bob");
strcpy(array[2]->favorite_color,"red");
strcpy(array[2]->position,"forward");


printp("red",array,ARRAY_LEN,2);

reclaim(array,ARRAY_LEN);
}
break;
};

return errcode;
}


I probably left something out, oh well. http://www.linuxnewbie.org/ubb/smile.gif

[Minime80]

Right... I think I should regroup and try to explain what I want to do a different way, or maybe just be more specific. I don't even know if it's possible, but after messing with some of the code here I got it down to 3 errors. Anyways, here's what I'm trying to say. I want to have an array of these player structures. Not another object holding this array though. I just want to declare it in main like this:

player players[5];

Then I want a function in the program (not in the structure, functions for returning the data are fine in the structure) where I can send it the array that I just created, somehow tell it what field to look in (without using if's if possible because I don't want 100 if's when there's 100 different data members in the structure), and also send it the string I'm looking for. Now, inside this search function there should just be one loop with one if statement containing something that accomplishes this:
strcmp(value, player[i].field)
where value is the string I'm looking for and field is the data member of the player that I want to find it in. After that, if they match it will print out that players' data (that's the easy part), and then keep going through the loop until it gets to the end of the array.

Well, I hope this clears things up a little. I have learned a ton from what you guys have posted already, it just wasn't exactly what I was asking, but it was helpful. Thanks for the help so far... keep it coming... I'm learning a LOT! http://www.linuxnewbie.org/ubb/smile.gif

[Minime80]

Oh, about those errors I'm getting. I made a function with a *comparisonMethod() and two of the errors are when I call the function (I called it twice in my program). It says that it can't execute the function without an object, but I don't want to execute the function until later when I'm in the loop and start comparing the results of the function it's pointing to in the structure to what I'm looking for. The other error is in the actual function itself, and it's saying this: "no matching function for call to 'player::comparisonMethod()'"
I'm not exactly sure what it's trying to tell me here. Anyways, that's what I've got so far.

[The_Stack]

Haven't you looked at my code!?

[Minime80]

Originally posted by The_Stack:
Haven't you looked at my code!?

Oh, sorry. I guess I didn't really look at it closely enough. Sorry. I think it's just because I never really did C. I just learned C++ and those malloc's and stuff threw me. Speaking of those, does it have to be dynamically created. It's just more involved than I need to get right now. I know that there's only going to be 5 players, so I just need to make that array of 5 players. I've got a function to get the info from the user. Well, here, I'll post what I've been trying to do. I know it doesn't work, but maybe one of you could change it a tad and show me what I need to do to get it to work. #include <iostream.h>
#include <string.h>

struct player
{
char name[10],color[10],position[10];
char *getName();
char *getColor();
char *getPosition();
};

char *player::getName() {return name;}
char *player::getColor() {return color;}
char *player::getPosition() {return position;}

void getinfo(player *);
void display(player *);
void search(player *, char **(player::*comparisonMethod)(), const char *);

int main()
{
player players[5];

getinfo(players);
display(players);
search(players, &player::getColor(), "red");
search(players, &player::getPosition(), "mid");

return 0;
}

void getinfo(player *person)
{
for (short i=0;i<5;++i)
{
cout << "\nEnter player's name: ";
cin.getline(person[i].name, 10);
cout << "Enter " << person[i].name << "'s favorite color: ";
cin.getline(person[i].color, 10);
cout << "Enter " << person[i].name << "'s position: ";
cin.getline(person[i].position, 10);
}
}

void display(player *person)
{
cout << "\nNAME " << "FAVORITE COLOR " << "POSITION ";
cout << "\n~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n";
for (short i=0;i<5;++i)
{
cout.width(17);
cout << person[i].name;
cout.width(21);
cout << person[i].color;
cout << person[i].position << endl;
}
}

void search(player *person, char **(player::*comparisonMethod)(), const char *value)
{
cout << "\n All players whose data contains " << value << ":";
cout << "\nNAME " << "FAVORITE COLOR " << "POSITION ";
cout << "\n~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n";
for (short i=0;i<5;++i)
{
if (!strcmp(value,person[i].comparisonMethod()))
{
cout.width(17);
cout << person[i].name;
cout.width(21);
cout << person[i].color;
cout << person[i].position << endl;
}
}
}

There ya have it. Probably not the best of work, but I've only been doing it about 5 months now. Sorry again that I didn't pay more attention to what you had done there Stack.


[This message has been edited by Minime80 (edited 14 November 2000).]