i want to create queue with dynamic array, the problem is when i test the q.rear value in int main() (for inserting data later) it's not -1 anymore. Which part did i do wrong?
here's a piece of the code:
#include <stdio.h>
#include <conio2.h>
#include <stdlib.h>
typedef struct{
int *data;
int front,rear,max;
}tqueue;
tqueue create(int size)
{
tqueue q;
q.data=(int *)malloc(size*sizeof(int));
q.front=0;
q.rear=-1;
q.max=size;
return q;
}
int main()
{
tqueue q;
int size=4;
create(size);
printf("\n%d",q.rear);
getch();
return 0;
}
You didn't assign the result of create() to q. The following calls create() and discards the result, leaving q in its original state:
create(size);
To fix, replace the above line with:
q = create(size);
You have two different tqueues, both called q. One (the one you create in create) you initialize and one (the one you create in main) you don't.
You forgot to assign the return value:
q = create(size);
Related
So I am working on a project in C that requires that I pass pointers to a struct into functions. The project is structured as follows:
struct structName {
unsigned short thing2;
char thing1[];
};
void function_1(struct structName *s) {
strcpy(s->thing1, "Hello");
printf("Function 1\n%s\n\n", s->thing1); // prints correctly
}
void function_2(struct structName *s) {
// can read thing2's value correctly
// thing1 comes out as a series of arbitrary characters
// I'm guessing it's an address being cast to a string or something?
printf("Function 2\n%s\n\n", s->thing1); // prints arbitrary characters ('É·/¨')
}
int main() {
struct structName s;
function_1(&s);
printf("Main\n%s\n\n", s.thing1);
function_2(&s);
printf("Main 2\n%s\n\n", s.thing1);
}
This code outputs the following:
Function 1
Hello
Main
Hello
Function 2
É·/¨
Main 2
É·/¨
Obviously, the program has more than just what I've written here; this is just a simplified version; so if there's anything I should check that might be causing this let me know. In all honesty I reckon it's probably just a stupid rookie error I'm making somewhere.
[EDIT: Seems like s.thing1 is being mutated in some way in the call to function_2(), since the odd value is replicated in main() - I should point out that in my program the printf()s are located right before the function call and in the first line of the function, so there's no chance that it's being written to by anything I'm doing. I've updated the example code above to show this.]
Thanks in advance!
The structure contains a flexible member at its end, if you declare a static object with this type, the length of this member will be zero, so strcpy(s->thing1, "Hello"); will have undefined behavior.
You are supposed to allocate instances of this type of structure with enough extra space to handle whatever data you wish to store into the flexible array.
Here is an example:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct pstring {
size_t length;
char data[];
} pstring;
pstring *allocate_pstring(const char *s) {
size_t length = strlen(s);
pstring *p = malloc(sizeof(*p) + length + 1);
if (p != NULL) {
p->length = length;
strcpy(p->data, s);
}
return p;
}
void free_pstring(pstring *p) {
free(p);
}
int main() {
pstring *p = allocate_pstring("Hello");
printf("Main\n%.*s\n\n", (int)p->length, p->data);
free_pstring(p);
return 0;
}
I have a project in which I have to make a basic database in C. My problem lies that keep passing a variable n from function to function and at one point the value changes.
I have made a test program which illustrates the same issue I'm having:
#include <stdio.h>
#include <stdlib.h>
void functionTWO(int *n)
{
printf("\n%d",*n);
}
functionONE(int *n)
{
int i=0;
i++;
i++;
*n = i;
printf("\n%d", *n);
functionTWO(&n);
}
int main()
{
int n=0;
printf("%d",n);
functionONE(&n);
return 0;
}
The n in the second function is displayed as a very very high number, e.g.:
0
2
2752268
Now, I know this probably is intended, but could you guys kindly explain why this happens this way?
In your functionONE(), n is already a pointer. You don't need to pass the address of the pointer if you intend to change that value-at-the-address.
As an advice, always check the data types and enable the compiler warnings to help you.
functionONE()
your are passing a address of address. So it's printing address of n.
Now you want a correct output then n as double pointer **n.
void functionTWO(int **n)
{
printf("\n%d",**n);
}
First of all, in C int & n is not the reference of n, it is it's address.Now, what you've done is created an int - sent it's address to functionONE - did something to the value in that address and then sent the the argument's address to functionTWO, instead of sending the argument itself, becuase it's already a pointer. Do this instead:
#include <stdio.h>
#include <stdlib.h>
void functionTWO(int *n)
{
printf("\n%d",*n);
}
functionONE(int *n)
{
int i=0;
i++;
i++;
*n = i;
printf("\n%d", *n);
functionTWO(n); // <-- send the pointer, not it's address
}
int main()
{
int n=0;
printf("%d",n);
functionONE(&n);
return 0;
}
In functionONE, n is int *, just a pointer, when you do functionTWO(&n);, you get the address of n, so it output a big value.
The value being printed in functionTWO function is the address of the pointer variable n declared in functionONE.In that case you can either modify functionTWO like this
void functionTWO(int **n)
{
printf("\n%d",**n);
}
or
Just pass the value of the pointer variable like this
functionTWO(n);
I have learned how to use functions and structs and pointers. I want to combined them all into one. But the code that I write doesn't seem to work. The compiler tells me the test is an undeclared identifier. Here is the code:
#include <stdio.h>
#include <stdlib.h>
struct character
{
int *power;
};
void test (use_power)
int main ()
{
test (use_power)
printf("%d\n",*power);
return 0;
}
void test ()
{
int use_power = 25;
struct character a;
a.power = &use_power;
}
Your code has many mistakes it can't even compile
Multiple missing semicolons.
Implicit declaration of test() here
test (use_power)
with a missing semicolon too.
power is not declared in main().
This line
void test use_power()
does not make sense and is invalid, and also has no semicolon.
The a instance in test() defined at the end is local to test() and as such will be deallocated when test() returns. The use_power int, has exactly the same problem and trying to extract it's address from the function is useless because you can't access it after the function has returned.
I have no idea what you were trying to do, but this might be?
#include <stdio.h>
#include <stdlib.h>
struct character {
int *power;
};
/* Decalre the function here, before calling it
* or perhaps move the definition here
*/
void test(struct character *pointer);
/* ^ please */
int
main(void) /* int main() is not really a valid signature */
{
struct character instance;
test(&instance);
if (instance.power == NULL)
return -1;
printf("%d\n", *instance.power);
free(instance.power);
return 0;
}
void
test(struct character *pointer)
{
pointer->power = malloc(sizeof(*pointer->power));
if (pointer->power != NULL)
*pointer->power = 25;
}
Your code seems to be wrong. Your definition for test contains no arguments as
void test ()
{
int use_power = 25;
struct character a;
a.power = &use_power;
}
but your prototype contains one argument
void test (use_power)
which is wrongly put. First there are no semicolons; at the end of your prototype declaration, secondly by looking at your code, use_power is a variable and not a datatype so it cannot be present solely in a function declaration.
You will get an argument mismatch error.
You have used the line in main()
printf("%d\n",*power);
which is absolutely wrong. you cannot access any member of a structure without a structure variable.
And again, you have not mentioned the; after your call to the incorrect test()before this line
As you have not put your question so properly, I must figure out what you wish to achieve. I bet you want to hold the address of a integer in the pointer member of a structure and then print its value.
Below is a code snippet which will work as you desire.
#include <stdio.h>
#include <stdlib.h>
struct character
{
int *power;
};
struct character a; //define a structure variable
void test ();
int main ()
{
test ();
printf("%d\n",*(a.power)); // print the member of structure variable a
return 0;
}
void test ()
{
int use_power = 25;
a.power = &use_power;
}
example
#include <stdio.h>
struct character {
int *power;
};
void test(struct character *var);
int main (void){
struct character use_power;
int power = 5;
use_power.power = &power;
test(&use_power);
printf("%d\n", power);
return 0;
}
void test(struct character *var){
int use_power = *var->power;
*var->power = use_power * use_power;
}
Today I'm trying to implement a queue, but one that works with structures within arrays (I've always hated the C 'Array of Structs' terminology, as I am not trying to make that). However, when I try and do a basic initialization, I run into the compiler error as follows.
"Request for member '**' in something not a structure or union. "
Here's my code, not much as of now.
//------------------------Preprocessor Instructions. ------------------------------------
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#define MAX 128
#define BUFFER 120
//-------------------------Global Stuff -------------------------------------------------
int head=-1;
int tail=-1; //Starting head and tail at -1.
struct Entry{
int Data;
int Hops;
};
struct Entry Queue[MAX]; //Queue made up of structs.
int visited[MAX];
//------------------------Function Definitions. -----------------------------------------
int QueuePush(struct Entry *q, int num);
int QueuePop(struct Entry *q);
int IsEmpty(struct Entry *q);
//------------------------Main. ---------------------------------------------------------
int main(void)
{
int i;
while(i<MAX){
Queue.Data[i]=0;
Queue.Hops[i]=0;
i++;
}
for(i=0;i<=10;i++){
printf("Queue Data[%d] = %d \n", i, Queue[i].Data);
printf("Queue Hops = %d \n", Queue[i].Hops);
}
}
Am I making some scary, large error in the way I'm defining the array? Or is the issue syntactical? Thanks in advance.
You should write
Queue[i].Data = 0;
Queue[i].Hops = 0;
not
Queue.Data[i] = 0;
Queue.Hops[i] = 0;
In you initialization while loop, you have written Queue.Data[i] instead of Queue[i].Data.
And another thing:
int i;
while(i<MAX){
Queue.Data[i]=0;
Queue.Hops[i]=0;
i++;
}
Here i contains random number and this loop will not work, so you should write int i = 0; or use for instead of while.
I created a structure and wanted to assign the values to a Function Pointer of another structure. The sample code I wrote is like below. Please see what else I've missed.
#include <stdio.h>
#include <string.h>
struct PClass{
void *Funt;
}gpclass;
struct StrFu stringfunc;
struct StrFu{
int a ;
char c;
};
Initialise(){
}
main()
{
stringfunc.a = 5;
stringfunc.c = 'd';
gpclass.Funt = malloc(sizeof(struct StrFu));
gpclass.Funt = &stringfunc;
memcpy(gpclass.Funt,&stringfunc,sizeof(struct StrFu));
printf("%u %u",gpclass.Funt->a,gpclass.Funt->c);
}
There are several problems:
A function pointer is not the same as void *, in fact you cannot rely on being able to convert between them.
You shouldn't cast the return value of malloc() in C.
You shouldn't call malloc(), then overwrite the returned pointer.
You don't need to use malloc() to store a single pointer, just use a pointer.
You shouldn't use memcpy() to copy structures, just use assignment.
There are two valid main() prototypes: int main(void) and int main(int argc, char *argv[]), and you're not using either.
there is lots of problem in your code , I try to correct it ,hope it will help
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
struct PClass{
void *Funt;
}gpclass;
struct StrFu{
int a ;
char c;
};
struct StrFu stringfunc;
int main()
{
stringfunc.a = 5;
stringfunc.c = 'd';
gpclass.Funt = malloc(sizeof(struct StrFu));
gpclass.Funt = &stringfunc;
memcpy(gpclass.Funt,&stringfunc,sizeof(struct StrFu));
printf("%d %c",((struct StrFu*)gpclass.Funt)->a,((struct StrFu*)gpclass.Funt)->c);
return 0;
}
it outputs
5 d