比較C中鏈表中的值時出現的問題

Question

我正在用C語言編寫一個“簡單”程序，我們在其中創建一個鏈表，該鏈表的結構類似於電影，用於存儲影片，標題，制作年份，評分（1-5）和指向該影片的指針。下一個節點。 我們不允許在此結構中添加任何內容或定義函數。

最重要的是，（出於某種原因）我們需要在main（）的主體中編寫整個鏈接列表，以便為該問題添加一定程度的意大利面條。 無論如何，在此程序中，我們應該讓用戶輸入“ U”進行更新，或者輸入“ S”進行電影搜索。 更新完成您所期望的，然后輸入標題，年份，等級。 由此，我們應該將節點插入到鏈表的末尾 。

我們的搜索應循環訪問此鏈接列表，如果找到匹配項，則應打印出電影，標題和年份。

盡管代碼的更新部分有效，但似乎無法使搜索生效。 我正在使用一個名為temp的電影結構，該結構從頭開始並遍歷列表以嘗試查找電影。 在通過printf運行了一些測試之后，我發現temp不管是什么，無論我輸入什么電影，都只是一個空節點。

我假設這與我調用malloc的方式有關？ 還是與未正確分配節點有關？ 老實說，我不確定，很遺憾，實驗室的TA也不知道D怎么了。

這是我的代碼：

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

struct movie_node {
        char title[250];
        int year;
        unsigned char rating;
        struct movie_node *next;
};

typedef struct movie_node movie;

int main() {

        // variables
        int loop = 1;
        movie *head = NULL; // represents first
        movie *current; // represents the last node
        movie *temp; // used for traversing the linked list
        head = malloc(sizeof(movie));
        int amountOfMovies = 0; // increment after each addition
        char choice; // either 'u' (update) or 's' (search)
        // ask user for input
        while(loop) {

                printf("%d Movie(s) in the database. Update or search (U/S): ", amountOfMovies);
                scanf(" %c", &choice);
                /* CHOICE 1, UPDATE */
                if(choice == 'U') {

                        // case 1, head is null, we must create a new node
                        if(head == NULL) {
                                // get input
                                printf("Name of the movie: ");
                                scanf(" %[^\n]%*c", head->title);
                                printf("Year: ");
          scanf("%d", &head->year);
                                printf("Rating: ");
                                scanf("%hhu", &head->rating);
                                head->next = NULL;
                                head = current; // set head to point to current
                        } else {
                                current = head;
                                // need to find where current is
                                while(current != NULL) {
                                        current = current->next;
                                } // end while
                                // current is now at the null position, indicating empty node
                                current = malloc(sizeof(movie)); // allocate mem
                                // get user input
                                printf("Name of the movie: ");
                                scanf(" %[^\n]%*c", current->title);
                                printf("Year: ");
                                scanf("%d", &current->year);
                                printf("Rating: ");
                                scanf("%hhu", &current->rating);
                                current->next = NULL;
                        } // end else
                        // output movie
                        printf("Movie \"%s\" is added to the database.\n", current->title);
                        amountOfMovies++; // increment amount of movies in database
                } else if(choice == 'S') {
                /* CHOICE 2, SEARCH */
                        // temp string
                        char tempTitle[250];
                        // flag to let us know if we found something
                        bool found = false;
                        // temp linked list to traverse
                        temp = head;
                        temp = malloc(sizeof(movie));
                        // ask user for input
                        printf("Name of movie: ");
                        scanf(" %[^\n]%*c", tempTitle);
                        printf("NAME OF MOVIE IN HEAD: %s\n", temp->title);             // test, take out later
                        while(temp != NULL) {
                                printf("NAME OF CURRENT MOVIE TO COMPARE TO: %s\n", temp->title); // test
                                if(strcmp(temp->title, tempTitle) == 0) {
                                        // match
                                        printf("Year: %d\n", temp->year);
                                        printf("Rating: %hhu\n", temp->rating);
                                        found = true;
                                        break;
                                } else { // no match so far
                                        temp = temp->next;
                                        printf("HAVEN'T FOUND MATCH, NEXT TITLE TO CHECK IS: %s\n", temp->title); // test print
                                        found = false;
                                }  // end else
                        } // end while
                        if(found == false) { // no match confirmed
                                printf("Movie \"%s\" does not exist in the database.\n", tempTitle);
                        }
                } else { // choice is invalid
                        loop = 0; // exit
                } // end if-else

        } // end while
        // free all the nodes

        return 0;
}

注意：我還沒有實現的唯一事情就是釋放內存。.我不確定百分百應該如何實現。

任何幫助是極大的贊賞..

Answer 1

問題出在您的malloc()調用上。 首先，您要做的是：

movie *head = NULL;
// ...
head = malloc(sizeof(movie));

這意味着head不再為null，您將無法以所需的方式插入第一部電影-將malloc()移動到其他位置。

其次，執行以下幾行代碼：

current = head; // <- this is OK
// ...
current = malloc(sizeof(movie)); // allocate mem <- this is NOT OK, for the same reason as before

同樣，您可以像這樣讀取電影的標題： scanf("%249s", head->title) 。

如果您知道該怎么走，請告訴我。

除了代碼中的問題外，還有另一個問題： 實驗室的TA也不知道哪里出了問題 。

Answer 2

樣品修復

movie *new_node(void){//aggregate the creation of a new node
    movie *node = malloc(sizeof(*node));//error check omit
    printf("Name of the movie: ");
    scanf(" %249[^\n]%*c", node->title);
    printf("Year: ");
    scanf("%d", &node->year);
    printf("Rating: ");
    scanf("%hhu", &node->rating);
    node->next = NULL;
    return node; 
}

int main() {
    int loop = 1;
    movie *head = NULL; 
    movie *current; 
    movie *temp; 
    //head = malloc(sizeof(movie));//"if(head == NULL) {" don't work 
    int amountOfMovies = 0; 
    char choice; 

    while(loop) {
        printf("%d Movie(s) in the database. Update or search (U/S): ", amountOfMovies);
        scanf(" %c", &choice);

        if(choice == 'U') {
            if(head == NULL) {
                current = head = new_node();//need set to current
            } else {
                current = head;
                while(current->next != NULL) {//"current != NULL" can't link
                    current = current->next;
                }
                current = current->next = new_node(); 
            } 
            printf("Movie \"%s\" is added to the database.\n", current->title);
            amountOfMovies++; 
        } else if(choice == 'S') {
            char tempTitle[250];
            bool found = false;//need <stdbool.h>
            temp = head;
            //temp = malloc(sizeof(movie));//Unnecessary
            printf("Name of movie: ");
            scanf(" %249[^\n]%*c", tempTitle);
            //printf("NAME OF MOVIE IN HEAD: %s\n", temp->title);            
            while(temp != NULL) {
                //printf("NAME OF CURRENT MOVIE TO COMPARE TO: %s\n", temp->title); 
                if(strcmp(temp->title, tempTitle) == 0) {
                    printf("Year: %d\n", temp->year);
                    printf("Rating: %hhu\n", temp->rating);
                    found = true;
                    break;
                } else { 
                    temp = temp->next;
                    printf("HAVEN'T FOUND MATCH, NEXT TITLE TO CHECK IS: %s\n", temp->title); 
                    //found = false;//Unnecessary
                }  
            } 
            if(found == false) { 
                printf("Movie \"%s\" does not exist in the database.\n", tempTitle);
            }
        } else { 
            loop = 0; 
        } 
    } 
    // free all the nodes
    return 0;
}