C language character classification and conversion

Before the order

C Just learned conditional judgment in language if When the sentence is , There's just one line of code like this

if( ch >= 'a' && ch <= 'z' )        //   Judge whether the character is lowercase 

I thought it was ordinary , Until 《C With the pointer 》 See the standard library “ctype.h” A sentence in the introduction ：

So I began to think about the robustness of this judgment sentence , Random decision learning “ctype.h” library .

The library is roughly divided into two parts , The first part is the character classification function , The second type is the character conversion function .

Character classification function

Use the above function , The type of return value is integer . If true, return a value greater than 0 Number of numbers , Otherwise return to 0.

char ch = 'a';
printf("d\n", isupper( ch ));            //   Decide if it's capital  
printf("%d\n", islower(ch));         //   Whether it is lowercase 

When the judgment is true , The return value is a value greater than 0 The random number .

Character conversion function

According to the meaning, we can see , The first is to convert to lowercase , The second is to convert to uppercase .

char ch = (char) tolower('A'); 
printf("ch \c", ch);

Be careful , tolower And toupper The return value of is integer , Therefore, you need to force the type to be converted to character type , Otherwise, the system will forcibly convert the character type variable into an integer variable .