C中的fscanf csv。未分配值

Question

我试图创建一个文件来读取包含 CSV 的文本文件。然而出于某种原因，有些 Fscanfs 不返回值，但有些却返回。

FILE* reader;
char OptionOne[30]="";
char OptionTwo[30]="";
char OptionThree[30]="";
char OptionFour[30]="";
char answer[1]="";
char CorrectAnswer[1];
char Question[51];

reader = fopen("C:\\Users\\Finley\\Desktop\\Programming\\QuizMachine\\QuizMachine\\fscanf_questions.txt", "r");

if (reader == NULL)
{
    printf("Unable to Open File: %s\n", FileName);
}

fscanf(reader, "%[^,],", Question);
fscanf(reader, "%[^,],", OptionOne);
fscanf(reader, "%[^,],", OptionTwo);
fscanf(reader, "%[^,],", OptionThree);
fscanf(reader, "%[^,],", OptionFour);
fscanf(reader, "%[^,],", answer);

为什么第一个 fscanf 返回一个值，但所有其他 fscanf 都没有返回一个值。我看不到任何东西，它们看起来都一样，格式相同 文件格式是这样的：

你用什么函数来打开文件？,fscanf,fclose,fopen,main,3

以下哪个不是变量类型？,int,float,char,string,4

Answer 1

1. 需要使用足够大的缓冲区。答案，作为一个字符串至少需要2个char.@Daniel

2. fscanf() 应限制输入以防止缓冲区损坏。宽度说明符通常比缓冲区的大小小 1。

   char Question[51];   
   fscanf(reader, "%50[^,],", Question);
   

3. 应该检查fscanf()的结果。

   if (fscanf(reader, "%50[^,],%29[^,],%29[^,],%29[^,],%29[^,],%1s", 
       Question, OptionOne, OptionTwo, OptionThree, OptionFour, answer) != 6) {
     Handle_Bad_input();
   }
   

4. 如果输入没有后续','，则不要使用"%[^,]" 作为最后一个值，否则fscanf() 将读取下一行。

5. 建议在每个"[^,]" 前加一个空格以跳过前导空格。

   if (fscanf(reader, " %50[^,], %29[^,], %29[^,], %29[^,], %29[^,],%1s", 
   

6. 更好的是：使用fgets() 读取行，然后扫描缓冲区。

   char buf[200];
   if (fgets(buf, sizeof buf, reader) == NULL) Handle_IOErrorOrEOF();
   if (sscanf(buffer, "%50[^,],%29[^,],%29[^,],%29[^,],%29[^,],%1s", 
       Question, OptionOne, OptionTwo, OptionThree, OptionFour, answer) != 6) {
     Handle_Bad_input();
   }
   

Answer 2

answer 的缓冲区太小，所以 fscanf 写入太多会导致损坏。

您只给了它一个字符的大小，但fscanf 会在其中写入 两个 字符：您想要的字符 3 和空字符 \0。

将缓冲区大小增加到 2 即可解决问题。

Answer 3

this line: fscanf(reader, "%[^,],", Question); (and those like it) 
have a couple of problems. 
1) should always check the returned value from a input I/O statement
   to assure the conversion was successful 
2) the format string is missing the type conversion, in this case 's'
this line: fscanf(reader, "%[^,],", answer); will fail if the line does not end in a ','


"Why does the first fscanf return a value but all the others not return a value." 
ans: 1) because the ' ' after the comma is not consumed
     2) use a leading ' ' in the format strings to consume white space
        this will also consume an trailing '\n' on each line
     3) a type specifier is NOT optional in a format string, use 's'
     3a) I would have read in the 'answer' as a char using '%c' for a format conversion
         and only created a char, not a char array.
     4) to avoid buffer overruns, 
        include the 'size-1' optional part of each format  conversion,
        just in case some string length is = or greater than the 
        receiving buffer

"I cannot see anything and they all seem the same with the same formating
File formatting is like this:"
ans: where is the file formatting info?

this line: printf("Unable to Open File: %s\n", FileName); 
the variable FileName is not set. 
suggest setting char * FileName = C:\\Users\\Finley\\Desktop\\Programming\\QuizMachine\\QuizMachine\\fscanf_questions.txt‌​" 
and using 'FileName' in the fopen() statement. 

this line: printf("Unable to Open File: %s\n", FileName); 
after this line, should insert 'exit(EXIT_FAILURE);
rather than continue in the program, trying to use that NULL file descriptor pointer 

this line: char answer[1]=""; 
will be a problem as the input is being read in as strings, 
so even for a single char string, there still needs to be
room for the trailing '\0'

this line: fscanf(reader, "%[^,],", answer);
1) fails if each line of the input file, including the last line,
   does not end in ','