I'm new in C programming. I'm developing a priority queue in C'99 with the heap data structure.
I'm using heapifyDown() in combination with swapValues() to sort the heap array for extracting the first element (min-heap) pqueue_extractMin() function. My structure looks like this:
typedef struct ProrityQueue_s PriorityQueue;
typedef struct PriorityQueue_Entry_s *PriorityQueue_Entry;
struct ProrityQueue_s {
int size, last;
char error;
PriorityQueue_Entry *entries;
};
struct PriorityQueue_Entry_s {
char *value;
float priority;
};
For information – Full code for information on gist: https://gist.github.com/it4need/ddf9014bfda9fe6a64bb01a7417422bc
Questions:
Insertion into the Priority queue ("minheap") looks good. Everything is fine. But when I'm extract more than one element at once, I will get this error: "Process finished with exit code 11".
Is this line allowed to copy the whole contents of the last element to the first of the heap? priorityqueue->entries[0] = priorityqueue->entries[priorityqueue->last];
swapValues(priorityqueue, currentPositionIndex, smallestChild); Can I swap values of whole Structure elements? -> implementation (bottom).
HeapifyDown():
void heapifyDown(PriorityQueue *priorityqueue)
{
int currentPositionIndex = 0;
while(currentPositionIndex < priorityqueue->last)
{
int smallestChild = currentPositionIndex;
int leftChildIndex = (2 * currentPositionIndex) + 1;
int rightChildIndex = (2 * currentPositionIndex) + 2;
smallestChild = (priorityqueue->entries[leftChildIndex]->priority < priorityqueue->entries[smallestChild]->priority && priorityqueue->last > leftChildIndex)
? leftChildIndex : smallestChild;
smallestChild = (priorityqueue->entries[rightChildIndex]->priority < priorityqueue->entries[smallestChild]->priority && priorityqueue->last > rightChildIndex)
? rightChildIndex : smallestChild;
if(smallestChild == currentPositionIndex)
{
break;
}
swapValues(priorityqueue, currentPositionIndex, smallestChild); // #todo: Why does this line break the function on two function calls by negative values
currentPositionIndex = smallestChild;
}
}
SwapValues():
void swapValues(PriorityQueue *priorityqueue, int firstIndex, int secondIndex)
{
// #todo: Does this work properly?
PriorityQueue_Entry tmp_entry = priorityqueue->entries[firstIndex];
priorityqueue->entries[firstIndex] = priorityqueue->entries[secondIndex];
priorityqueue->entries[secondIndex] = tmp_entry;
}
extractMin():
char *pqueue_extractMin(PriorityQueue *priorityqueue)
{
if(isEmpty(priorityqueue))
{
priorityqueue->error = ERROR_PRIORITY_QUEUE_EMPTY;
}
priorityqueue->last--;
char *tmp = priorityqueue->entries[0]->value;
priorityqueue->entries[0] = priorityqueue->entries[priorityqueue->last]; // #todo: Is this allowed?
heapifyDown(priorityqueue); // #todo: Why does this line break the extractMin() function on two function calls -> check swapValues() in heapifyDown()
return tmp;
}
Full code for information on gist: https://gist.github.com/it4need/ddf9014bfda9fe6a64bb01a7417422bc
Related
My code doesn't adding second node to tree. It gives me SIGSEGV fault when i'm adding the second node.I think its about strcmp function but when i'm trying to understand how it works properly at the very bottom of main func it returns -1 so i've wrote it like this.And most of my variables named Turkish so here are the translations of them to make you understand more easily
dugum=node,kok=root;sol=left;sag=right;anne=mother
// C program to demonstrate insert operation in binary search tree
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct tree {
char *harf;
char *morskodu;
struct tree *left;
struct tree *right;
} agac;
agac *kok = NULL;
void ekle(char *harf, char *morskodu) {
if (kok == NULL) {
kok = (agac *)malloc(sizeof(agac));
kok->harf = harf;
kok->morskodu = morskodu;
kok->left = NULL;
kok->right= NULL;
} else {
agac *yeni = (agac *)malloc(sizeof(agac));
yeni->harf = harf;
yeni->morskodu = morskodu;
yeni->left = NULL;
yeni->right = NULL;
agac *dugum = kok, *anne;
while (dugum != NULL) {
anne = dugum;
if (harf <= dugum->harf)
dugum = dugum->left;
else
dugum = dugum->right;
}
if (harf <= dugum->harf)
anne->left = yeni;
else
anne->right = yeni;
}
}
void dolas(agac *dugum) {
if (dugum != NULL) {
printf(" %s ", dugum->harf);
dolas(dugum->left);
dolas(dugum->right);
}
}
void main() {
ekle("a", "-");
ekle("b", "-.");
dolas(kok);
int x = strcmp("A", "B");
printf("%d", x);
}
You try to dereference a NULL pointer.
while (dugum != NULL) {
anne = dugum;
if (harf <= dugum->harf)
dugum = dugum->sol;
else
dugum = dugum->sag;
}
This loop ends when dugum is NULL.
Directly after you try to access dugum->harf:
if (harf <= dugum->harf)
This leads to undefined behavior.
Also note that this comparisons compare the pointers to string literals, and is therefore also undefined behavior. To compare two C strings you should use strcmp.
Im having trouble initialising structures (well doing everything actually, but structures first). The struct is first made in a header as follows
typedef enum cell
{
BLANK, RED, CYAN
} Cell;
#define NAMELEN 20
typedef struct player
{
char name[NAMELEN + NULL_SPACE];
Cell token;
unsigned score;
} Player;
void initFirstPlayer(Player * player);
void initSecondPlayer(Player * player, Cell token);
#endif
=======================================================================
and I tried to initialise it here
void initFirstPlayer(Player * player)
{
int randNo = rand() % 2;
if (randNo == 0) {
token = RED;
}
else() {
token = CYAN;
}
player ; p1 = {
"placeholder",
token,
0,
}
}
void initSecondPlayer(Player * player, Cell token)
{ }
What is the correct way to initialise this player struct?
I suspect this should work for you. Use a generic initPlayer function. Use that to allocate memory for the player and set the initial values. Be sure to also include a freePlayer function where you free() the player when you're done.
#include <stdlib.h>
#include <string.h>
Player* initPlayer()
{
Player* player = malloc(sizeof(Player));
int randNo = rand() % 2;
if (randNo == 0) {
player->token = RED;
}
else {
player->token = CYAN;
}
const char* initName = "placeholder";
strcpy(player->name, initName);
player->score = 0;
return player;
}
void freePlayer(Player* p)
{
free(p);
}
The way you'd use this would be like so:
int main()
{
Player* p1 = initPlayer();
Player* p2 = initPlayer();
play(p1, p2);
freePlayer(p1);
freePlayer(p2);
}
Assuming you have at least C99 support, so that compound literals and designated initializers are available to you, then you can use:
void initFirstPlayer(Player *player)
{
*player = (Player){ .token = rand() % 2 ? CYAN : RED,
.score = 0,
.name = "placeholder"
};
}
This does a structure assignment to the variable whose address is passed to the function. It compresses it all into one statement; you can split it out into several if you wish. This is an occasion where the ternary ? : operator is useful. You might prefer (rand() % 2) with the extra parentheses; I'd probably add them as often as I'd omit them.
The compound literal comes from (typename){ ...initializer for typename... }.
The designated initializers are the .member = value notations.
If you're stuck with C90 support, you have to work harder, perhaps creating a local variable with the correct information and then doing the structure assignment.
void initFirstPlayer(Player *player)
{
Player p1 = { "placeholder", rand() % 2 ? CYAN : RED, 0 };
*player = p1;
}
Now the onus is on you to list the initializers in the correct sequence.
Another way is to receive the player you want to inicialize as parameter:
void initPlayer(Player* player)
{
int randNo = rand() % 2;
if (randNo == 0) {
player->token = RED;
}
else {
player->token = CYAN;
}
const char* initName = "placeholder";
strcpy(player->name, initName);
player->score = 0;
}
int main() {
Player p1;
initPlayer(&p1);
}
You can have an array of players or allocate dinamically with malloc.
I have tried to create a CD struct like :
typedef struct
{
char* nameCD;
int yearOfpublication;
song* listOfSongs;
int priceCD;
int numberOfSongs;
} CD;
and I have a song struct :
typedef struct
{
char* nameSong;
char* nameSinger;
int lenghtOfSong;
} song;
void SongInput(song *psong, CD *pcd)
{
pcd->numberOfSongs++;
pcd->listOfSongs = (song*)malloc(pmcd->numberOfSongs*sizeof(song));
// here all the code that update one song..
but what should I write to update the next song?
how do I change it into an array which update the number of the songs and how can I save all the songs?
I tried this :
printf("Enter lenght Of Song:");
scanf("%d", &psong->lenghtOfSong);
but I don't understand the pointers..
and how to update the next song?
}
void CDInput(CD *pcd)
{
int numberOfSongs = 0;
//here all the other update of cd.
//songs!!!!!!!!!!!!!!!!!!!!!!!!!!!!
pcd->numberOfSongs = 0;
pcd->listOfSongs = (song*)malloc(numberOfSongs*sizeof(song));
}
Do I need to write anything else?
void CDInput(CD *pcd)
{
int i;
//...
printf("Enter number Of Song:");
scanf("%d", &pcd->numberOfSongs);
pcd->listOfSongs = (song*)malloc(pcd->numberOfSongs*sizeof(song));
for(i = 0; i < pcd->numberOfSongs; ++i){
SongInput(&pcd->listOfSongs[i]);
}
//...
}
It depends on if you want to write the structure once completely or you really want to add one item.
For the first case, please refer to BLUEPIXY's answer, for the second one, thigs are slightly more complicated.
bool add_song(song *psong, CD *pcd)
{
song* newone = realloc(pcd->listOfSongs, (pmcd->numberOfSongs+1)*sizeof(song));
if (!newone) {
// return and complain; the old remains intact.
return false; // indicating failure.
}
// now we can assign back.
pcd->listOfSongs = newone;
newone[pcd->numberOfSongs++] = *psong;
return true; // indicating success.
}
After hours of thinking and tinkering I almost gave up but decided to turn to the community for their help. I'm new to C and I just learned bubble sort. For example the following code sorts by name, what I would like to implement is a sub sort where it also sorts by person ID, how would I do that or change the following code to do just that? (This is structure problem).
struct human {
char name;
char id;
}
function sorting(struct human person)
{
struct human temp
int i, unsorted;
do{
unsorted = 0;
for(i = o; i<count-1; i++)
{
if(strcmp(person[i].name, person.name) > 0)
{
temp = person[i];
person[i] = person[i+1];
person[i+1] = temp;
unsorted = 1;
}
}while(unsorted);
}
First, it would help to break out your comparison function into its own function:
int compare_people(struct human *person1, struct human *person2)
{
return strcmp(person1->name, person2->name);
}
Then, you can more easily change the logic to compare ID if the name is equal:
int compare_people(struct human *person1, struct human *person2)
{
int d = strcmp(person1->name, person2->name);
if (d == 0) {
return person2->id - person1->id;
} else {
return d;
}
}
I define a structure in a header file like so:
typedef struct {
void *data;
} point;
I want to keep other people from accessing *data directly, so I thought I'd declare the structure in the .c file instead and use something like extern typedef struct point; in the header file. That doesn't work however.
What's the best way to achieve this?
In your (public) header file:
typedef struct point point;
In your .c file:
struct point
{
void *data;
};
Note that users of your code will no longer be able to create a point on the stack, as the compiler doesn't know how big it is. You may have to provide a point_create() function which allocates memory and returns its address to the caller.
Use C++
Since jokes seem not be allowed here is the pure C version.
As another commenter pointed out if you really want to protect your internals from users of your Api you have seen and used plenty of such Apis. This Apis are e.g. the Windows or Linux user mode Apis. There you create kernel objects to which you never shall have access to. The Apis to deal with kernel objects use a synthetic construct called handle which is not simply a pointer to your own object but instead it is an index to an array where the kernel has stored the relevant meta data for your object.
You can use the same idea for your Apis as well.
Here for example is a C-Style public Api:
// Public.h
#include <stdlib.h>
typedef enum
{
None = 0,
PointType = 1
} Types;
typedef int Handle;
Handle CreateType(Types type);
int DeleteType(Handle object);
void IncrementX(Handle point);
void PrintPoint(Handle point);
As you can see you have generic methods which create and delete your objects which are defined here in an enum. Your methods which use the object will then need to lookup the integer handle to get the meta data object where the real data is stored.
This design is not very efficient if the objects you manage are small since for every object a second object is need which stores the object type, handle value and the pointer to the real data.
But you get much stronger safety guarantees such as
Type safety
Invalid handles are easy to find
Double free is impossible since you can manage the free state in the meta object
A typical usage of your Api might look like this:
Handle h = CreateType(PointType);
IncrementX(h);
IncrementX(h);
PrintPoint(h);
DeleteType(h);
And there is the super secret implementation in private.cpp where the Handle lookup array and some helper methods exist:
// Private.C
#include "stdafx.h"
#include <stdlib.h>
#include <Windows.h> // for ZeroMemory
#include "Public.h"
typedef struct
{
LPVOID pData;
Types type;
Handle handle;
} HandleInfo;
typedef struct
{
int x;
int y;
} Point;
HandleInfo *pAllocated;
int HandleBuffer = 0xffff;
unsigned char bInit = 0;
HandleInfo *GetFreeHandle()
{
int i;
if( !bInit )
{
pAllocated = (HandleInfo *) malloc(sizeof(HandleInfo)*HandleBuffer);
bInit = 1;
ZeroMemory(pAllocated, sizeof(HandleInfo)*HandleBuffer);
}
for(i=0; i<HandleBuffer; i++)
{
HandleInfo *pInfo = (pAllocated+i);
if( 0 == pInfo->handle )
{
pInfo->handle = i+1;
return pInfo;
}
}
return NULL;
}
HandleInfo * GetHandleInfo(Handle h)
{
if( h <= 0 || h >= HandleBuffer-1)
{
return NULL;
}
return (pAllocated+h-1);
}
Handle CreateType(Types typeId)
{
HandleInfo *pInfo;
pInfo = GetFreeHandle();
if( NULL == pInfo )
{
return -1;
}
pInfo->type = typeId;
switch(typeId)
{
case PointType:
pInfo->pData = malloc(sizeof(Point));
ZeroMemory(pInfo->pData, sizeof(Point));
break;
}
return pInfo->handle;
}
int DeleteType(Handle object)
{
HandleInfo *pInfo = GetHandleInfo(object);
if( NULL == pInfo )
{
return -1;
}
if( pInfo->handle != 0 )
{
free(pInfo->pData);
pInfo->pData = NULL;
pInfo->handle = 0;
return 1;
}
else
{
return 0; // Handle was already closed
}
}
void *GetObjectOfCorrectType(Handle object, Types type)
{
HandleInfo *p = GetHandleInfo(object);
if( p == NULL )
{
return NULL;
}
if( p->type != type)
{
return NULL; // handle has wrong object type
}
return p->pData;
}
void IncrementX(Handle point)
{
Point *pPoint = (Point *) GetObjectOfCorrectType(point, PointType);
if( pPoint == NULL )
{
return;
}
pPoint->x++;
}
void PrintPoint(Handle point)
{
Point *pPoint = (Point *) GetObjectOfCorrectType(point, PointType);
if( pPoint == NULL )
{
return;
}
printf("Point has x: %d y: %d", pPoint->x, pPoint->y);
}
Yours,
Alois Kraus
This is the pointer to implementation or pimpl idiom. See http://en.wikibooks.org/wiki/C++_Programming/Idioms#Pointer_To_Implementation_.28pImpl.29 for a tutorial for C++, but the idea should work in C as well.
typedef struct {
/* private members; don't access directly */
void *data;
} point;
You can have separate public header and private header files. Some libraries have conventions for this:
Xt (X11) -> header.h and headerP.h, e.g: X11/Vendor.h vs X11/VendorP.h
Qt -> header.h vs private/header_p.h, e.g: qapplication.h vs private/qapplication_p.h
If you do not want to use the declaration method (because you want the library user to access other members of your struct, for example) it is convention to prepend private member with an underscore, like this:
typedef struct {
void * _data;
} point;
Of course people could still access _data if they would really want to (just like people can access private data in C++ by adding a #define private public before their includes) but that is their own responsibility; at least you have indicated that they shouldn't do that if they want your library to behave as it should.
I use this approach in order to let client alloc the module instance in his STACK.
struct module_private {
int data;
}
typedef uint8_t module_t [sizeof (struct module_private) ];
Client will be able to see private struct content, but not access it without doing a cast that he shouldn't.
Use the following workaround:
#include <stdio.h>
#define C_PRIVATE(T) struct T##private {
#define C_PRIVATE_END } private;
#define C_PRIV(x) ((x).private)
#define C_PRIV_REF(x) (&(x)->private)
struct T {
int a;
C_PRIVATE(T)
int x;
C_PRIVATE_END
};
int main()
{
struct T t;
struct T *tref = &t;
t.a = 1;
C_PRIV(t).x = 2;
printf("t.a = %d\nt.x = %d\n", t.a, C_PRIV(t).x);
tref->a = 3;
C_PRIV_REF(tref)->x = 4;
printf("tref->a = %d\ntref->x = %d\n", tref->a, C_PRIV_REF(tref)->x);
return 0;
}
Result is:
t.a = 1
t.x = 2
tref->a = 3
tref->x = 4