Thread: Data Structures:

I made comments as to what I think is happening. I get the concept but still not grasping it totally.

Code:

 #include <stdio.h>
#include <stdlib.h>
#define MAXCHARS 21

struct music {
    char name[MAXCHARS];
    char artist[MAXCHARS];
    int number_of_songs;
    struct music *next;
};

void display(struct music *root)
{
    struct music *temp=root;
    while(temp != NULL)
    {
        printf("%s ",temp -> name);
        temp = temp -> next;
    }
    printf("\n");
    
}

int main(int argc, const char * argv[])
{
    int i = 0, j=0;
    struct music *root;
    root = NULL;                 /* initialize the top-of-stack pointer */
    
    char name[20];
    char artist[20];
    int number_of_songs = 0; 
    
    printf("Enter the following:\n");
    
    while (j < 3) {
        printf("\nName of the Band:");
        
        scanf("%s",&name[20]);
        printf("\nArtist:");
       
        scanf("%s",&artist[20]);
        printf("\nNumber of songs:\n ");
        
        scanf("%d",&number_of_songs);
        
    
        if (root == NULL) {                      // if == to end of line //
            root = malloc(sizeof(struct music)); // makes space //
            int size = strlen(name);             // finds length of name but makes a error //
            
            for (i=0;i<size;i++)
                root->name[i] = name[i];  // loop is empty//
            root->next = NULL;
        }
        else                                // loop is not empty //
        {
            struct music *temp = root; {       // making a node //
                struct music *prev = NULL; {  // until node is == to end of line //
                    
                    while (temp != NULL) {   // while the line is not ended //
                        prev = temp;         
                        temp = temp->next;
                    }
                    temp = malloc(sizeof(struct music));  // makes memory for temp //
                    temp -> name[i]=name[i];  // loop and pointer
                    temp -> next=NULL;        //points to the top //
                    prev -> next=temp;       // points to the new node //

                };
                j++;            // not working //
            };
        
            display(root);  
        }
         
    
        return 0;
    }
    
}

]

I wrote the whole thing again. It's better but now working. Any more advice?

Code:

 #include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXCHARS 21


struct music {
    char name[MAXCHARS];
    char artist[MAXCHARS];
    int number_of_songs;
    struct music *next;
};

void input()
{
    char name[MAXCHARS];
    char artist[MAXCHARS];
    int number_of_songs = 0;
    
    printf("Enter 3 Band Names, Artists and number of songs they have");
    gets(name);
    gets(artist);
    scanf("%d",&number_of_songs);

}

void output(char *name)
{
    struct music *root;
    
    root=(struct music *)malloc(sizeof(struct music));
    if (root == (struct music *) NULL) {
        printf("Did not create enough memory");
        exit(1);
    }
    
    struct music *temp = root;
    struct music *prev = NULL;
    
    while (temp != NULL) {
        prev = temp;
        temp = temp->next;
    }
    temp = malloc(sizeof(struct music));
    strcpy(temp->name,name);
    temp -> next=NULL;
    prev -> next=temp;
    
}
void display(struct music *root)
{
    struct music *temp=root;
    while(temp != NULL)
    {
        printf("%s ",temp -> name);
        temp = temp -> next;
    }
    printf("\n");
    
}


int main(int argc, const char * argv[])
{
    
    input();
    
    output();
    
    display();
    
    
    
    return 0;
}

gets() is the only thing I can think of without having to use strlen( ) which keeps failing. I was using strlen during a for loop which I later found out I didn't have to use.

OK!! Please disregard my last comment. gets is the only thing I can think of to use without having to use a loop and strlen(). I have this example on my prior codes above.

You should never use gets(). This is a very dangerous function that leads to many many problems. You should consider using fgets() instead.

Jim

Fixed, thanks!

Code:

 printf("Enter 3 Band Names, Artists and number of songs they have");
    fgets(name,MAXCHARS,stdin);
    fgets(artist,MAXCHARS,stdin);
    scanf("%d",&number_of_songs);

Why is this failing in main.

Code:

 temp=(structmusic*)(malloc(sizeof(structmusic));

Code:

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

struct music {
    char *name;
    char *artist;
    int number_of_songs;
    struct music *next;
};
char* receiveInput(){
    char *s = (char*) malloc( 20 );
    scanf("%s",s);
    return s;
}
void get_inputs(struct music *ptr)
{
    printf("%s","Enter the name:");
    ptr->name=receiveInput();
    printf("%s","Enter the artist:");
    ptr->artist=receiveInput();
    printf("%s","Enter the number of songs:");
    scanf("%d",&ptr->number_of_songs);
}
void add(struct music *root,struct music *ptr)
{
    if (root==NULL)
    {
        root=(struct music*)malloc(sizeof(struct music));
        root->name=ptr->name;
        root->artist=ptr->artist;
        root->number_of_songs=ptr->number_of_songs;
        root->next=NULL;
    }
    else
    {
        struct music * temp=root;
        struct music * prev=NULL;
        while(temp)
        {
            prev=temp;
            temp=temp->next;
        }
        temp=(struct music*)malloc(sizeof(struct music));
        temp->name=ptr->name;
        temp->artist=ptr->artist;
        temp->number_of_songs=ptr->number_of_songs;
        temp->next=NULL;
        prev->next=temp;
    }
}

void output(struct music * root)
{
    struct music *temp=root;
    while(temp)
    {
        printf("%s",temp->name);
        printf("\n");
        temp=temp->next;
    }
}
int main()
{
    struct music * root=NULL;
    int i=0;
    while(i<3)
    {
        struct music * temp;
        temp=(struct music*)(malloc(sizeof(struct music));
                             
                      get_inputs(temp);
                      add(root,temp);
                      output(root);
    }
                             
    }

If by failing you mean failing to compile.... what does your compiler say? GCC said:

Code:

test.c:70:57: error: expected ‘)’ before ‘;’ token

Think that's pretty clear. You have 4 open brackets and only 3 close brackets on that line. Compare it to the similar lines that compile correctly and fix it.


You have a general problem with your add() call. I haven't read the whole thread so not sure if someones mentioned it already. You're passing a pointer to a struct. You can change the fields in that struct in the function, but you can't change the value of the pointer. That is, you calls to malloc to modify root and temp will set the value of the root and temp inside add(), but those changes will be lost as soon as the function returns, then they'll have the same values they did before the call.
So nothing will actually get added!

If you only change one pointer value, a common trick is to return the modified pointer. No good if you're modifying two values though. For this, you should pass a pointer to the pointer.

Code:

void add(struct music **root,struct music **ptr)
{
    if (*root==NULL)
...
...
   add(&root, &temp);

Hope that makes sense. Just remember that everything in C is passed by value. There's no way to modify an argument inside a function. You must always pass a pointer to the data to by modified. Arrays confuse the issue slightly but for this code, it's straightforward pointer-need.

Thank you. I keep making simple mistakes like this.

Are you referring to passing by reference?

Indeed I am.

I tried the way to mentioned but it didn't work. Now another issue is my while loop is never ending.

Code:

 void display(struct music * root)
{
    struct music *temp=root;
    while(temp)
    {
        printf("%s",temp->name);
        printf("\n");
        temp=temp->next;
    }
}
int main()
{
    struct music * root=NULL;
    int i=0;
    
    while(i<3)
    {
        struct music * temp;
        temp=(struct music*)(malloc(sizeof(struct music)));
                             
                      get_inputs(temp);
                      add(root,temp);
                      display(root);
    }
    i++;
    }