读取和解析之间的区别

Question

我在某处读到用C编程语言编写的scanf 解析了数据（ http://sekrit.de/webdocs/c/beginners-guide-away-from-scanf.html）-有人可以解释一下这是什么意思（我尝试使用Google搜索但无法正确获取）。

Answer 1

"Parse data" means that you take some raw data and put a specific meaning into the data.

"Read data" is a less specific term but in this context is can refer to "getting raw data into your program from some device". Here "some device" can be many things - it can be a terminal (stdin), a disk, a network connection and so on. A program can read raw data without knowing what the data means. The program will just get a sequence of numbers (bytes) without knowing what these numbers mean. In pseudo-code:

 // READ DATA
 unsigned char array[some-size];
 size_t index = 0;
 while(input-device-has-data)
 {
     array[index] = get-input-from-device();
     ++index;
 }

After this loop that reads data, the array may contain:

array[0]: 74
array[1]: 79
array[2]: 69
array[3]: 32
array[4]: 52
array[5]: 50
array[6]: 0

A sequence of numbers that doesn't really seem mean anything. So the next step is to setup some rules for the data to make them meaningful. The first step is the encoding, ie what does the number 74 mean? One such encoding is the ascii table that defines how numbers are translated into characters. Using the ascii table the numbers above become:

array[0]: J
array[1]: O
array[2]: E
array[3]: space
array[4]: 4
array[5]: 2
array[6]: NUL

Then you can setup rules for the data you want your program to receive. In this simple example the rule would be something like: name age

So in your program you will like to "convert the raw data" into two variables. One variable that holds the name and another variable that holds the age. In pseudo-code:

string name = get_name(array);
int age = get_age(array);

That is "to parse" data, ie take some raw data (a sequence of numbers) an put specific meaning (semantic) into the data while following some rules for the data (syntax).

And that is exactly what the scanf-family functions can do for you. In this case like:

// Parsing
char name[20];
int age;
int result = sscanf(array, "%19s %d", name, &age);

Here sscanf parses the raw data contained in the array and tries to map the raw data into a word (aka name - max 19 characters) followed by a space followed by a number (aka age). This is the rules that the format specifier sets, ie the string "%19s %d" .

If sscanf can parse the data according to those rules, it will return the value 2 to telle that data was parsed into 2 variables and the variable name will hold the first word and the variable age will hold the number. That's what parsing is about.

Answer 2

You read it wrong, the scanf() function scans information from stdin and converts the information read according to what type modifier characters (eg %s, %d, %c) are specified. Parsing is a much more sophisticated technique that involves tokenizing the input, validating it against a set rules in a context free grammar, and building an abstract syntax tree to verify if the input is part of a language.