如何将单字符转换为双字符

Question

I try to create a program that can evaluate simple math expression like "4+4". The expression is given from the user.

The program saves it in a char* and then searches for binary operation (+,-,*,:) and does the operation.

The problem is that I can't figure out how to convert the single char into a double value.

I know there is the atof function but I want to convert single char .

There is a way to do that without creating a char* ?

Answer 1

A char usually represents a character. However, a single char is simply an integer in range of at least [-127,+127] (signed version) or at least [0,255] (unsigned version).

If you obtained a character looking as a digit, the value stored in it is an ASCII number representing it. Digits start at code 48 (for zero) and go up incrementally till code 57 (for nine). Thus, if you take the code and subtract 48, you get the integer value. From there, converting it to double is a matter of casting.

Thus:

char digit = ...
double value = double(digit - 48);

or even better, for convenience:

char digit = ...
double value = double(digit - '0'); //'0' has a built-in value 48

Answer 2

There is a way to do that without creating a char* ???

Sure. You can extract the digit number from a single char as follows:

char c = '4';
double d = c - '0'; 
        // ^^^^^^^ this expression results in a numeric value that can be converted
        //         to double

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This uses the circumstance that certain character tables like ASCII or EBCDIC encode the digits in a continuous set of values starting at '0' .