循环和结构数组时打印的值不正确

Posted 2023-02-18

【中文标题】循环和结构数组时打印的值不正确

【英文标题】：Incorrect values printed when looping through and array of structs

【发布时间】：2021-12-31 03:23:55

【问题描述】：

我正在读取文件并将每一行的值保存到结构中。我要保存的值是一个字符、一个字符串和两个整数。然后将每个结构保存到结构数组中。我想打印出这些值中的每一个以确保它们被正确保存，并且当我在 for 循环中从数组中访问每个结构时，除了字符串之外的所有内容都正确打印出来。我不明白为什么会这样。为了检查，我在将字符串添加到给出预期输出的结构后立即打印出字符串。只有当我尝试在我的 for 循环中访问这些字符串元素时，我才会得到不正确的输出。这是我的代码：

    char wordFirstLetter;
    struct BankAccount arrAccounts[20];
    struct TransactionRequest arrTransactions[20];
    int numAccounts = 0;
    int numTransactions = 0;
    int currAccountIndex = 0;
    int currClient = 1;

    while(fgets(line, sizeof(line),fp))
    
        // We will be getting the words on each line using strtok()
        //Gets the first word in the line
        char *word = strtok(line, " ");
        wordFirstLetter = word[0];

        // Checks if the first letter of the line is 'a'
        // If it is, then we know we are setting up the account
        if(wordFirstLetter == 'a')
        
            //not related
        
        // Otherwise we are dealing with a client
        else
        
            while(word != NULL)
            
                if(word[0] == 'c')
                
                    // Move on to the next word if we see that we are
                    // dealing with a client
                    word = strtok(NULL, " ");
                
                else
                
                    // Create a structure to represent the current request
                    struct TransactionRequest transaction;

                    // Append the client number of the client doing the transaction
                    transaction.client = currClient;
                    

                    // Append the type of transaction and move to next word
                    char *transType = word;
                    transaction.requestType = transType;
                    printf("This is the value of word: %s\n", word);
                    printf("%s\n", transaction.requestType);
                    word = strtok(NULL, " ");
                    

                    // Append the account number that will be altered and move to next word
                    transaction.account = word[1] - '0';
                    word = strtok(NULL, " ");
                    

                    // Append the amount for the transaction and move to next word
                    int transAmount = atoi(word);
                    transaction.amount = transAmount;
                    word = strtok(NULL, " ");
                    

                    // Append this transaction to an array containing all transactions in the file
                    arrTransactions[numTransactions] = transaction;
                    numTransactions++;

                
            
            // Each line represents a client, so when we are done with the
            // line we move to the next client
            currClient++;
        
    

    for(int i = 0; i < numTransactions; i++)
    
        struct TransactionRequest transaction = arrTransactions[i];
        printf("This is the client number: %d\n", transaction.client);
        printf("This is the request type: %s\n", transaction.requestType);
        printf("This is the account to be accessed: %d\n", transaction.account);
        printf("This is the amount: %d\n", transaction.amount);
        printf("\n");
    

这是我正在使用的结构：

struct TransactionRequest

    // ID if the client doing the transaction
    int client;
    // Type of transaction the client is doing
    char *requestType;
    // Account number of the account the client is dealing with
    int account;
    // Amount of money that is boing moved around
    int amount;
;

打印出这些字符串元素时，我是否在 for 循环中做错了什么？如果不是，为什么会出现这个问题。

【问题讨论】：

有一个可以运行的示例文件会有所帮助吗？

@Neil 如何上传我的文件？只有 2 个文件。

@MarcoAiello，您可以直接在问题中发布您文件的最小 sn-p，比如说，三五行，足以重现问题！除此之外，“不正确的值”是什么意思？可以举个例子吗？

【参考方案1】：

word 在while 循环之后超出范围，因此您的结构将包含指向无效数据的指针。即使您在 while 循环之前声明了 word，TransactionRequest 的所有实例都将包含 word 的最新值，这可能不是预期的行为。如果您知道requestType 不会超过某个长度，那么您可以在结构中使用固定长度的字符数组。这样，无论您的结构实例是在堆栈还是堆上分配，requestType 数据对于每个实例都是唯一的，并将与整个结构一起分配/释放。

#define REQUEST_TYPE_MAX_LENGTH 256
struct TransactionRequest

    // ID if the client doing the transaction
    int client;
    // Type of transaction the client is doing
    char requestType[REQUEST_TYPE_MAX_LENGTH];
    // Account number of the account the client is dealing with
    int account;
    // Amount of money that is boing moved around
    int amount;
;

尽管如此，您的代码仍需要重新设计。例如，您不能使用赋值将数据从word 变量传输到TranscationRequest 结构的requestType 字段。您将需要使用诸如 strncat 之类的函数和手动 null 终止。

虽然不是严格相关，但我还想指出，TransactionRequest 中的帐号只能取 0 到 9（含）之间的值，除非您的原始文件包含一些奇怪的 ASCII 字符。这是故意的吗？