使用fscanf从包含整数数组的文件中读取

Question

I'm trying to read a JSON-format file that contains an integer array(eg [ 0, 1, 2, 3, 4 ]) I am wondering why fscanf skips the brackets in the file and go straight to the numbers when I use

   // the type of value is integer
  FILE* fp=fopen(file,"r");
  fscanf( fp,"%d",&value);

I'm still new to file I/O and I have no idea why this happens. I thought whenever I call fscanf, the file pointer would move 1 position forward.

Answer 1

You should check return values. The fscanf in your example should return 0 since the first non-white-space character encountered is [ which can not start a number so that parsing fails there. The assumed return value of 0 indicates that no succcessful conversion took place. The value of value will probably stay unchanged (I couldn't find a specific statement for that in the man page). The reading position in the file will be before the [ so that subsequent attempts to read an int from the file will fail as well.

How to read the json array:

Note that I do not handle errors in the examples below. You must do that though...

scanf format strings can can contain conversion specifications which contain a list of allowed or forbidden characters. This can be used to read "away" anything which is not a number: char buf[some large enough value]; fscanf(" %[^0-9]", buf); char buf[some large enough value]; fscanf(" %[^0-9]", buf); . The reading position is now before the first number.

Then you'll have a loop doing two things:

The number ahead of us can be read trivially with fscanf("%d", &value); . This will also skip possible whitespace before the number in later iterations. Now we must deal with the comma: fscanf(" %[,]", buf); ("read a comma which is optionally preceded by white space"). Now you can read the next number.

The last number will not be followed by a comma. The attempt to read a comma will therefore fail (ie return 0); this can be used as end-of-array indicator.

If more arrays or other stuff may follow you must read away the remaining whitespace and closing square bracket so that you leave the file position after the array for others.