Arduino - Character Functions

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All data is entered into computers as characters, which includes letters, digits and various special symbols. In this section, we discuss the capabilities of C++ for examining and manipulating individual characters.

The character-handling library includes several functions that perform useful tests and manipulations of character data. Each function receives a character, represented as an int, or EOF as an argument. Characters are often manipulated as integers.

Remember that EOF normally has the value 1 and that some hardware architectures do not allow negative values to be stored in char variables. Therefore, the character-handling functions manipulate characters as integers.

The following table summarizes the functions of the character-handling library. When using functions from the character-handling library, include the <cctype> header.

S.No. Prototype & Description
1

int isdigit( int c )

Returns 1 if c is a digit and 0 otherwise.
2

int isalpha( int c )

Returns 1 if c is a letter and 0 otherwise.
3

int isalnum( int c )

Returns 1 if c is a digit or a letter and 0 otherwise.
4

int isxdigit( int c )

Returns 1 if c is a hexadecimal digit character and 0 otherwise.

(See Appendix D, Number Systems, for a detailed explanation of binary, octal, decimal and hexadecimal numbers.)
5

int islower( int c )

Returns 1 if c is a lowercase letter and 0 otherwise.
6

int isupper( int c )

Returns 1 if c is an uppercase letter; 0 otherwise.
7

int isspace( int c )

Returns 1 if c is a white-space characternewline ('\n'), space

(' '), form feed ('\f'), carriage return ('\r'), horizontal tab ('\t'), or vertical tab ('\v')and 0 otherwise.
8

int iscntrl( int c )

Returns 1 if c is a control character, such as newline ('\n'), form feed ('\f'), carriage return ('\r'), horizontal tab ('\t'), vertical tab ('\v'), alert ('\a'), or backspace ('\b')and 0 otherwise.
9

int ispunct( int c )

Returns 1 if c is a printing character other than a space, a digit, or a letter and 0 otherwise.
10

int isprint( int c )

Returns 1 if c is a printing character including space (' ') and 0 otherwise.
11

int isgraph( int c )

Returns 1 if c is a printing character other than space (' ') and 0 otherwise.

Examples

The following example demonstrates the use of the functions isdigit, isalpha, isalnum and isxdigit. Function isdigit determines whether its argument is a digit (09). The function isalpha determines whether its argument is an uppercase letter (A-Z) or a lowercase letter (az). The function isalnum determines whether its argument is an uppercase, lowercase letter or a digit. Function isxdigit determines whether its argument is a hexadecimal digit (AF, af, 09).

Example 1

 void setup () { Serial.begin (9600); Serial.print ("According to isdigit:\r"); Serial.print (isdigit( '8' ) ? "8 is a": "8 is not a"); Serial.print (" digit\r" ); Serial.print (isdigit( '8' ) ?"# is a": "# is not a") ; Serial.print (" digit\r"); Serial.print ("\rAccording to isalpha:\r" ); Serial.print (isalpha('A' ) ?"A is a": "A is not a"); Serial.print (" letter\r"); Serial.print (isalpha('A' ) ?"b is a": "b is not a"); Serial.print (" letter\r"); Serial.print (isalpha('A') ?"& is a": "& is not a"); Serial.print (" letter\r"); Serial.print (isalpha( 'A' ) ?"4 is a":"4 is not a"); Serial.print (" letter\r"); Serial.print ("\rAccording to isalnum:\r"); Serial.print (isalnum( 'A' ) ?"A is a" : "A is not a" ); Serial.print (" digit or a letter\r" ); Serial.print (isalnum( '8' ) ?"8 is a" : "8 is not a" ) ; Serial.print (" digit or a letter\r"); Serial.print (isalnum( '#' ) ?"# is a" : "# is not a" ); Serial.print (" digit or a letter\r"); Serial.print ("\rAccording to isxdigit:\r"); Serial.print (isxdigit( 'F' ) ?"F is a" : "F is not a" ); Serial.print (" hexadecimal digit\r" ); Serial.print (isxdigit( 'J' ) ?"J is a" : "J is not a" ) ; Serial.print (" hexadecimal digit\r" ); Serial.print (isxdigit( '7' ) ?"7 is a" : "7 is not a" ) ; Serial.print (" hexadecimal digit\r" ); Serial.print (isxdigit( '$' ) ? "$ is a" : "$ is not a" ); Serial.print (" hexadecimal digit\r" ); Serial.print (isxdigit( 'f' ) ? f is a" : "f is not a"); } void loop () { }

Result

 According to isdigit: 8 is a digit # is not a digit According to isalpha: A is a letter b is a letter & is not a letter 4 is not a letter According to isalnum: A is a digit or a letter 8 is a digit or a letter # is not a digit or a letter According to isxdigit: F is a hexadecimal digit J is not a hexadecimal digit 7 is a hexadecimal digit $ is not a hexadecimal digit f is a hexadecimal digit

We use the conditional operator (?:) with each function to determine whether the string " is a " or the string " is not a " should be printed in the output for each character tested. For example, line a indicates that if '8' is a digiti.e., if isdigit returns a true (nonzero) valuethe string "8 is a " is printed. If '8' is not a digit (i.e., if isdigit returns 0), the string " 8 is not a " is printed.

Example 2

The following example demonstrates the use of the functions islower and isupper. The function islower determines whether its argument is a lowercase letter (az). Function isupper determines whether its argument is an uppercase letter (AZ).

 int thisChar = 0xA0; void setup () { Serial.begin (9600); Serial.print ("According to islower:\r") ; Serial.print (islower( 'p' ) ? "p is a" : "p is not a" ); Serial.print ( " lowercase letter\r" ); Serial.print ( islower( 'P') ? "P is a" : "P is not a") ; Serial.print ("lowercase letter\r"); Serial.print (islower( '5' ) ? "5 is a" : "5 is not a" ); Serial.print ( " lowercase letter\r" ); Serial.print ( islower( '!' )? "! is a" : "! is not a") ; Serial.print ("lowercase letter\r"); Serial.print ("\rAccording to isupper:\r") ; Serial.print (isupper ( 'D' ) ? "D is a" : "D is not an" ); Serial.print ( " uppercase letter\r" ); Serial.print ( isupper ( 'd' )? "d is a" : "d is not an") ; Serial.print ( " uppercase letter\r" ); Serial.print (isupper ( '8' ) ? "8 is a" : "8 is not an" ); Serial.print ( " uppercase letter\r" ); Serial.print ( islower( '$' )? "$ is a" : "$ is not an") ; Serial.print ("uppercase letter\r "); } void setup () { }

Result

 According to islower: p is a lowercase letter P is not a lowercase letter 5 is not a lowercase letter ! is not a lowercase letter According to isupper: D is an uppercase letter d is not an uppercase letter 8 is not an uppercase letter $ is not an uppercase letter

Example 3

The following example demonstrates the use of functions isspace, iscntrl, ispunct, isprint and isgraph.

  • The function isspace determines whether its argument is a white-space character, such as space (' '), form feed ('\f'), newline ('\n'), carriage return ('\r'), horizontal tab ('\t') or vertical tab ('\v').

  • The function iscntrl determines whether its argument is a control character such as horizontal tab ('\t'), vertical tab ('\v'), form feed ('\f'), alert ('\a'), backspace ('\b'), carriage return ('\r') or newline ('\n').

  • The function ispunct determines whether its argument is a printing character other than a space, digit or letter, such as $, #, (, ), [, ], {, }, ;, : or %.

  • The function isprint determines whether its argument is a character that can be displayed on the screen (including the space character).

  • The function isgraph tests for the same characters as isprint, but the space character is not included.

 void setup () { Serial.begin (9600); Serial.print ( " According to isspace:\rNewline ") ; Serial.print (isspace( '\n' )? " is a" : " is not a" ); Serial.print ( " whitespace character\rHorizontal tab") ; Serial.print (isspace( '\t' )? " is a" : " is not a" ); Serial.print ( " whitespace character\n") ; Serial.print (isspace('%')? " % is a" : " % is not a" ); Serial.print ( " \rAccording to iscntrl:\rNewline") ; Serial.print ( iscntrl( '\n' )?"is a" : " is not a" ) ; Serial.print (" control character\r"); Serial.print (iscntrl( '$' ) ? " $ is a" : " $ is not a" ); Serial.print (" control character\r"); Serial.print ("\rAccording to ispunct:\r"); Serial.print (ispunct(';' ) ?"; is a" : "; is not a" ) ; Serial.print (" punctuation character\r"); Serial.print (ispunct('Y' ) ?"Y is a" : "Y is not a" ) ; Serial.print ("punctuation character\r"); Serial.print (ispunct('#' ) ?"# is a" : "# is not a" ) ; Serial.print ("punctuation character\r"); Serial.print ( "\r According to isprint:\r"); Serial.print (isprint('$' ) ?"$ is a" : "$ is not a" ); Serial.print (" printing character\rAlert "); Serial.print (isprint('\a' ) ?" is a" : " is not a" ); Serial.print (" printing character\rSpace "); Serial.print (isprint(' ' ) ?" is a" : " is not a" ); Serial.print (" printing character\r"); Serial.print ("\r According to isgraph:\r"); Serial.print (isgraph ('Q' ) ?"Q is a" : "Q is not a" ); Serial.print ("printing character other than a space\rSpace "); Serial.print (isgraph (' ') ?" is a" : " is not a" ); Serial.print ("printing character other than a space "); } void loop () { }

Result

 According to isspace: Newline is a whitespace character Horizontal tab is a whitespace character % is not a whitespace character According to iscntrl: Newline is a control character $ is not a control character According to ispunct: ; is a punctuation character Y is not a punctuation character # is a punctuation character According to isprint: $ is a printing character Alert is not a printing character Space is a printing character According to isgraph: Q is a printing character other than a space Space is not a printing character other than a space
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