char 双指针混淆行为

Question

我正在 https://www.codingame.com/ 做练习练习一些 C 指针。

任务是将输入字符转换为 ASCII 艺术。例如字母A是:

 # 
# #
###
# #
# #

https://www.codingame.com/ide/puzzle/ascii-art

整个 ASCII 字母表作为不同行的单个输入提供(A 到 Z 加？):

 #  ##   ## ##  ### ###  ## # # ###  ## # # #   # # ###  #  ##   #  ##   ## ### # # # # # # # # # # ### ### 
# # # # #   # # #   #   #   # #  #    # # # #   ### # # # # # # # # # # #    #  # # # # # # # # # #   #   # 
### ##  #   # # ##  ##  # # ###  #    # ##  #   ### # # # # ##  # # ##   #   #  # # # # ###  #   #   #   ## 
# # # # #   # # #   #   # # # #  #  # # # # #   # # # # # # #    ## # #   #  #  # # # # ### # #  #  #       
# # ##   ## ##  ### #    ## # # ###  #  # # ### # # # #  #  #     # # # ##   #  ###  #  # # # #  #  ###  #  

我试图将字母表存储为双指针(alphabet_input)。

char ** alphabet_input;

int main()
{
    int length = 4;
    int height = 5;

    alphabet_input = (char **)malloc(sizeof(char *)*height);

    for (int i = 0; i < height; i++) {
        char ROW[1025];
        fgets(ROW, 1025, stdin);

        alphabet_input[i] = ROW;

        // print alphabet lines for the first time
        printf("%s", alphabet_input[i]);
    }

    // print alphabet lines for the second time
    printf("%s", alphabet_input[0]);
    printf("%s", alphabet_input[1]);
    printf("%s", alphabet_input[2]);
    printf("%s", alphabet_input[3]);
    printf("%s", alphabet_input[4]);

    return 0;
}

但是，当我尝试打印它时，我反复收到最后一行。这就是我得到的结果:

 #  ##   ## ##  ### ###  ## # # ###  ## # # #   # # ###  #  ##   #  ##   ## ### # # # # # # # # # # ### ### 
# # # # #   # # #   #   #   # #  #    # # # #   ### # # # # # # # # # # #    #  # # # # # # # # # #   #   # 
### ##  #   # # ##  ##  # # ###  #    # ##  #   ### # # # # ##  # # ##   #   #  # # # # ###  #   #   #   ## 
# # # # #   # # #   #   # # # #  #  # # # # #   # # # # # # #    ## # #   #  #  # # # # ### # #  #  #       
# # ##   ## ##  ### #    ## # # ###  #  # # ### # # # #  #  #     # # # ##   #  ###  #  # # # #  #  ###  #  
# # ##   ## ##  ### #    ## # # ###  #  # # ### # # # #  #  #     # # # ##   #  ###  #  # # # #  #  ###  #  
# # ##   ## ##  ### #    ## # # ###  #  # # ### # # # #  #  #     # # # ##   #  ###  #  # # # #  #  ###  #  
# # ##   ## ##  ### #    ## # # ###  #  # # ### # # # #  #  #     # # # ##   #  ###  #  # # # #  #  ###  #  
# # ##   ## ##  ### #    ## # # ###  #  # # ### # # # #  #  #     # # # ##   #  ###  #  # # # #  #  ###  #  
# # ##   ## ##  ### #    ## # # ###  #  # # ### # # # #  #  #     # # # ##   #  ###  #  # # # #  #  ###  #  

我希望将输入打印两次:

 #  ##   ## ##  ### ###  ## # # ###  ## # # #   # # ###  #  ##   #  ##   ## ### # # # # # # # # # # ### ### 
# # # # #   # # #   #   #   # #  #    # # # #   ### # # # # # # # # # # #    #  # # # # # # # # # #   #   # 
### ##  #   # # ##  ##  # # ###  #    # ##  #   ### # # # # ##  # # ##   #   #  # # # # ###  #   #   #   ## 
# # # # #   # # #   #   # # # #  #  # # # # #   # # # # # # #    ## # #   #  #  # # # # ### # #  #  #       
# # ##   ## ##  ### #    ## # # ###  #  # # ### # # # #  #  #     # # # ##   #  ###  #  # # # #  #  ###  #  
 #  ##   ## ##  ### ###  ## # # ###  ## # # #   # # ###  #  ##   #  ##   ## ### # # # # # # # # # # ### ### 
# # # # #   # # #   #   #   # #  #    # # # #   ### # # # # # # # # # # #    #  # # # # # # # # # #   #   # 
### ##  #   # # ##  ##  # # ###  #    # ##  #   ### # # # # ##  # # ##   #   #  # # # # ###  #   #   #   ## 
# # # # #   # # #   #   # # # #  #  # # # # #   # # # # # # #    ## # #   #  #  # # # # ### # #  #  #       
# # ##   ## ##  ### #    ## # # ###  #  # # ### # # # #  #  #     # # # ##   #  ###  #  # # # #  #  ###  #  

我应该如何操作alphabet_input的索引以正确地将每一行打印为字符串？

我真的很困惑，因为在 for 循环内打印似乎工作得很好。然而，在此之后索引似乎崩溃了。

Answer 1

for (int i = 0; i < H; i++) {
    char ROW[1025];
    fgets(ROW, 1025, stdin);
    alphabet_input[i] = ROW;
}

变量ROW位于 for 主体的范围内，因此它在每次 for 迭代时创建和销毁。您点alphabet_input[i]这个变量在迭代结束时立即被销毁，因此最终会得到悬空指针。

而不是指向alphabet[i]对于局部变量，您需要为其分配空间，然后将该行的内容复制到其中。

char ** alphabet_input;
alphabet_input = malloc(height * sizeof *alphabet_input);

for (int i = 0; i < height; i++) {
    char row[1025];
    fgets(row, 1025, stdin);

    size_t len = strlen(row);

    alphabet_input[i] = malloc(len + 1);
    strcpy(alphabet_input[i], row);
}

您可能需要根据您想要的长度来调整上面的代码(例如 1025 而不是 strlen，或者删除尾随的换行符等)

我想说的几点:

• 这就是我建议编写malloc的方式:

  pointer_var = <no cast> malloc(<num_elements> * sizeof *pointer_var);
  alphabet_input = malloc(height * sizeof *alphabet_input);
  

• sizeof(char)保证为 1按照标准，您可以跳过 sizeof(char)在malloc :

  alphabet_input[i] = malloc(len + 1);
  

• 避免所有大写变量。它们通常用于 C 中的宏。