为什么我不能将NULL值赋给这个指针数组元素？(Why can I not assign NULL value to this pointer array element?)

 我仍然对C语言编程不熟悉。如果这种问题已经被问到，我很抱歉，我真的不知道该怎么搜索。 
 
 作为练习，我正在编写一本字典。 用户应该能够在练习的这个阶段添加，打印和删除单词。 单词存储在一个名为'dict'的字符指针数组中。 函数'removeWord'应该确保被删除的单词被替换为数组'dict'中的最后一个单词。 所以最后一个单词应该被复制到必须删除的元素上，最后一个元素应该被删除（即赋值为NULL）。 当要删除的单词是数组'dict'中的最后一个单词时，它也应该被删除。 
 
 当数组'dict'中的任何单词除去阵列中的最后一个单词时，该程序将停止运行。 当除了最后一个单词之外的任何单词被删除时，我想将NULL分配给最后一个单元。 
 
 我希望我的问题很清楚。 我非常感谢您提供的任何帮助。 
 
int numberOfWordsInDict(char **dict)
{
    int i,cnt=0;
    for(i=0;i<10;i++)
    {
        if(dict[i]!=NULL)
        {
            cnt++;
        }
    }
    return cnt;
}

void addWord(char **dict, char *word)
{
    int i=0;
    if(numberOfWordsInDict(dict)==10)
    {
        printf("Dictionary is already full!\n");
    }
    int k=numberOfWordsInDict(dict);
    dict[k]=(char*)malloc((strlen(word)+1)*sizeof(char));

    strcpy(dict[k],word);
}


void printDict(char **dict)
{
    int i=0;
    printf("Dictionary:\n");
    if(numberOfWordsInDict(dict)==0)
    {
        printf("The dictionary is empty.\n");

    }else
    {
        for(i=0;i<10;i++)
        {
        printf("- %s\n", dict[i]);
        }
    }
}

void removeWord(char **dict, char *word)
{
    int i,j=0;
    int swapped=0;
    j=numberOfWordsInDict(dict);
    for(i=0;i<j;i++)
    {
        if(strcmp(dict[i],word)==0 && swapped==0)
        {
        swapped=1;
        //strcpy(dict[i],dict[j-1]);
        dict[j-1] = NULL;
        }
    }

}
 
 在字典[j-1]发生错误。 
 
int main()
{
    char wordToBeAdded[36]={};
    char wordToBeRemoved[36]={};
    char *dict[10]={};
    char operation;

    while(1)
    {
        printf("Command (a/p/r/q): ");
        scanf(" %c", &operation);

        switch(operation)
        {
            case 'a':
                printf("Add a word: ");
                scanf(" %s", &wordToBeAdded);
                addWord(dict,wordToBeAdded);
                break;

            case 'p':
                printDict(dict);
                break;

            case 'r':
                printf("Remove a word: ");
                scanf(" %s", &wordToBeRemoved);
                removeWord(dict,wordToBeRemoved);
                break;

            case 'q':
                return 0;
        }
    }
}

I am still new to programming in C. I am sorry if this kind of question has already been asked, I did not really know what to search for exactly. 
 
As an exercise I am programming a dictionary. The user should be able to add, print, and remove words in this stage of the exercise. Words are stored in a character pointer array called 'dict'. The function 'removeWord' should make sure the word to be removed is replaced by the last word in the array 'dict'. So the last word should be copied to the element that has to be removed and subsequently the last element should be removed (i.e. assigned the value NULL to). When the word to be removed is the last word in the array 'dict' it should also be removed.
 
The program stops running when any word in the array 'dict' is removed except for the last one in the array. When any word except for the last one is removed I want to assign the value NULL to the last element. 
 
I hope my question is clear. I really appreciate any help you can provide.
 
int numberOfWordsInDict(char **dict)
{
    int i,cnt=0;
    for(i=0;i<10;i++)
    {
        if(dict[i]!=NULL)
        {
            cnt++;
        }
    }
    return cnt;
}

void addWord(char **dict, char *word)
{
    int i=0;
    if(numberOfWordsInDict(dict)==10)
    {
        printf("Dictionary is already full!\n");
    }
    int k=numberOfWordsInDict(dict);
    dict[k]=(char*)malloc((strlen(word)+1)*sizeof(char));

    strcpy(dict[k],word);
}


void printDict(char **dict)
{
    int i=0;
    printf("Dictionary:\n");
    if(numberOfWordsInDict(dict)==0)
    {
        printf("The dictionary is empty.\n");

    }else
    {
        for(i=0;i<10;i++)
        {
        printf("- %s\n", dict[i]);
        }
    }
}

void removeWord(char **dict, char *word)
{
    int i,j=0;
    int swapped=0;
    j=numberOfWordsInDict(dict);
    for(i=0;i<j;i++)
    {
        if(strcmp(dict[i],word)==0 && swapped==0)
        {
        swapped=1;
        //strcpy(dict[i],dict[j-1]);
        dict[j-1] = NULL;
        }
    }

}
 
at dict[j-1] the error occurs.
 
int main()
{
    char wordToBeAdded[36]={};
    char wordToBeRemoved[36]={};
    char *dict[10]={};
    char operation;

    while(1)
    {
        printf("Command (a/p/r/q): ");
        scanf(" %c", &operation);

        switch(operation)
        {
            case 'a':
                printf("Add a word: ");
                scanf(" %s", &wordToBeAdded);
                addWord(dict,wordToBeAdded);
                break;

            case 'p':
                printDict(dict);
                break;

            case 'r':
                printf("Remove a word: ");
                scanf(" %s", &wordToBeRemoved);
                removeWord(dict,wordToBeRemoved);
                break;

            case 'q':
                return 0;
        }
    }
}

原文：https://stackoverflow.com/questions/45618585