What does Tokens do and why they need to be created in C programming

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I am now reading a book (Programming Principles and Practice by Bjarne Stroustrup).

Tokens are introduced in this episode:

"A token is a string of letters that represents a unit, such as a number or an operator." 

That's how a C++ compiler handles its source code. 

In fact, "tokenizing" in some form or another is how most text analysis begins."

class Token {
public:
    char kind;
    double value;
};

I understand what they are, but he never explains it in depth, which makes it difficult for me.


Aug 1 in C++ by Nicholas
• 6,240 points
46 views

1 answer to this question.

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Tokenization is essential in determining what a programme does. 

What Bjarne is referring to in respect to C++ code is how tokenization rules alter the meaning of a programme. 

We need to know what the tokens are and how they are determined. 

Specifically, how can we recognise a single token when it comes among other characters, and how should tokens be delimited if  there is ambiguity?

Consider the prefix operators ++ and +, for example. Assume we have just one token + to deal with. 

What does the following excerpt mean?

int i = 1;
++i;

Is the above going to apply unary + on i twice with + only? Or will it only increase it once? Naturally, it's vague. 

We require an additional token, thus ++ is introduced as its own "word" in the language.

But there is now another (though minor) issue. 

What if the programmer just wants to use unary + twice without incrementing? 

Rules for token processing are required. 

So, if we discover that a white space is always used as a token separator, our programmer may write:

int i = 1;
+ +i;

A C++ implementation begins with a file full of characters, converts them to a series of tokens ("words" with meaning in the C++ language), and then tests to see if the tokens occur in a "sentence" with some valid meaning.

answered Aug 2 by Damon
• 4,960 points

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