'a'=='b'。这是一个很好的方法吗？

Question

What happens if I compare two characters in this way:

if ('a' == 'b')
    doSomething();

I'm really curious to know what the language (and the compiler) does when it finds a comparison like this. And, of course, if it is a correct way to do something, or if I have to use something like strcmp() .

EDIT Wait wait.
Since someone haven't understood what I really mean, I decided to explain in another way.

char x, y;
cout << "Put a character: ";
cin >> x;
cout << "Put another character: ";
cin >> y;

if (x == y)
    doSomething();

Of course, in the if brackets you can replace == with any other comparison operator.
What really I want to know is: how the character are considered in C/C++? When the compiler compares two characters, how does it know that 'a' is different than 'b'? It refers to the ASCII table?

Answer 1

您可以通过比较运算符==绝对安全地比较基本类型

Answer 2

In C and C++, single character constants (and char variables) are integer values (in the mathematical sense, not in the sense of int values). The compiler compares them as integers when you use == . You can also use the other integer comparison operators ( < , <= , etc.) You can also add and subtract them. (For instance, a common idiom to change a digit character into its numerical value is c - '0' .)

Answer 3

For single chars , this form is correct. If both operands are known at compile time as in your example, then the condition can (and almost certainly will) be evaluated at compile time and not result in any code.

Note that a char ( 'a' ) is different from a single-character string ( "a" ). For the latter, comparison has a different meaning: it would compare the pointers rather than the characters.

Answer 4

Your processor would subtract both operands and if it's zero, zero condition bit is set, your values were the same.

For example: on arm machines you have the nzcv (negative, zero, carry, overflow) bits which tell you what happened.

Answer 5

Nothing will happen as a doesn't equal b .

If you question is just about is that the correct way, then the answer is yes.

Answer 6

First 'a' and 'b' are not strings, they are characters. The nuance is important because of its implications.

You can compare characters to characters just fine the same way you can compare integers to integers and floats with floats. It's usually not done because the outcome will always be the same. ie 'a' == 'b' will always be false.

If you're comparing strings, however, you'll have to use something like strcmp() .

Answer 7

Compiler simply inserts an instruction for comparing two bytes for equality - a very efficient operation. Of course in your case 'a'=='b' is equivalent to a constant false .

Answer 8

The compiler will compare the numeric ASCII codes. So, 'a' is never equal to 'b'. But, 'a' < 'b' evaluates to true, since 'a' appears before 'b' in the ASCII table.

Answer 9

Of course, you want to use variables like

char myChr = 'a' ;
if( myChr == 'b' ) puts( "It's b" ) ;

Now you can start to think about "Yoda conditions" , where you would do

if( 'b' == myChr )  puts( "It's a b" ) ;

so that in case you accidently typed one equals sign in the 2nd example:

if( 'b' = myChr ) puts( "It's a b" ) ;

that would raise a compiler error