C中的值返回null，不知道爲什麼

Question

爲什麼print語句返回null？

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

struct Node 
{ 
    char abbreviation; 
    double number; 
    struct Node *next; 
}; 

void insert(char abbreviation, double number, struct Node *head) { 
    struct Node *current = head; 
    while(current->next != NULL) 
    { 
     current = current->next; 
    } 

    struct Node *ptr = (struct Node*)malloc(sizeof(struct Node)); 
    ptr->abbreviation = abbreviation; 
    ptr->number = number; 
    current->next = ptr; 

    return; 
} 

int main(int argc, char* argv[]) { 
    struct Node *head = (struct Node*)malloc(sizeof(struct Node)); 
    insert('n',456,head); 
    printf("%s\n", head->abbreviation); 
} 

Answer 1

abbreviation是一個字符，而不是一個字符串。你想要printf("%c\n", head->abbreviation);。

Answer 2

您正在創建頭戴式> next指向一個節點，然後插入值出現。您從不在頭節點中設置任何值。

Answer 3

爲了增加UncleO's answer，在你main()調用insert()之前，設置head->abbreviation和head->number到所需的值並初始化head->next爲NULL。

Answer 4

請嘗試此代碼。

#include <stdio.h>> 
#include <stdlib.h> 
struct Node 
{ 
    char abbreviation; 
    double number; 
    struct Node *next; 
}; 
void insert(char abbreviation, double number, struct Node *head) { 
struct Node *current = head; 
while(current->next != NULL) 
{ 
    current = current->next; 
} 

struct Node *ptr = (struct Node*)malloc(sizeof(struct Node)); 
ptr->abbreviation = abbreviation; 
ptr->number = number; 
if(head->next==NULL) 
{ 
    head=current=ptr; 
} 
else 
{ 
    current->next = ptr; 
} 
return; 
} 

int main(int argc, char* argv[]) 
{ 
struct Node *head = (struct Node*)malloc(sizeof(struct Node)); 
insert('n',456,head); 
printf("%s\n", head->abbreviation); 
} 

Answer 5

看到的變化在這裏：

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

struct Node 
{ 
char abbreviation; 
double number; 
struct Node *next; 
}; 

void insert(char abbreviation, double number, struct Node *head) { 
struct Node *current = head; 
while(current->next != NULL) 
{ 
current = current->next; 
} 

struct Node *ptr = (struct Node*)malloc(sizeof(struct Node)); 
ptr->abbreviation = abbreviation; 
ptr->number = number; 
current->next = ptr; 

return; 
} 

int main(int argc, char* argv[]) { 
struct Node *head = (struct Node*)malloc(sizeof(struct Node)); 
insert('n',456,head); 
insert('m',453,head); 
printf("%c\n", head->next->abbreviation); 
printf("%c\n", head->next->next->abbreviation); 
}