# C++ Programming/Code/Standard C Library/String & Character
#### Standard C String & Character

The Standard C Library includes also routines that deals with characters and strings. You must keep in mind that in C, a string of characters is stored in successive elements of a character array and terminated by the NULL character.
Even if outdated this C string and character functions still appear in old code and more so than the previous I/O functions.

## atof

| Syntax | #include <cstdlib> double atof( const char *str ); |
The function atof converts str into a double, then returns that value. str must start with a valid number, but can be terminated with any non-numerical character, other than "E" or "e". For example,
results in x being set to 42.0.

### Related topics

    atoi - atol - strtod (Standard C I/O) sprintf

## atoi

| Syntax | #include <cstdlib> int atoi( const char *str ); |
The atoi function converts str into an integer, and returns that integer. str should start with a whitespace or some sort of number, and atoi will stop reading from str as soon as a non-numerical character has been read. For example:
All five of the above assignments to the variable i would result in it being set to 512.
If the conversion cannot be performed, then atoi will return zero:
A complete C++ implementation of atoi.

### Related topics

    atof - atol (Standard C I/O) sprintf

## atol

| Syntax | #include <cstdlib> long atol( const char *str ); |
The function atol converts str into a long, then returns that value. atol will read from str until it finds any character that should not be in a long. The resulting truncated value is then converted and returned. For example,
results in x being set to 1024L.

### Related topics

    atof - atoi - strtod (Standard C I/O) sprintf

#### isalnum

| Syntax | #include <cctype> int isalnum( int ch ); |
The function isalnum returns non-zero if its argument is a numeric digit or a letter of the alphabet. Otherwise, zero is returned.

##### Related topics

    isalpha - iscntrl - isdigit - isgraph - isprint - ispunct - isspace - isxdigit

#### isalpha

| Syntax | #include <cctype> int isalpha( int ch ); |
The function isalpha returns non-zero if its argument is a letter of the alphabet. Otherwise, zero is returned.

##### Related topics

    isalnum - iscntrl - isdigit - isgraph - isprint - ispunct - isspace - isxdigit

#### iscntrl

| Syntax | #include <cctype> int iscntrl( int ch ); |
The iscntrl function returns non-zero if its argument is a control character (between 0 and 0x1F or equal to 0x7F). Otherwise, zero is returned.

##### Related topics

    isalnum - isalpha - isdigit - isgraph - isprint - ispunct - isspace - isxdigit

#### isdigit

| Syntax | #include <cctype> int isdigit( int ch ); |
The function isdigit returns non-zero if its argument is a digit between 0 and 9. Otherwise, zero is returned.

##### Related topics

    isalnum - isalpha - iscntrl - isgraph - isprint - ispunct - isspace - isxdigit

#### isgraph

| Syntax | #include <cctype> int isgraph( int ch ); |
The function isgraph returns non-zero if its argument is any printable character other than a space (if you can see the character, then isgraph will return a non-zero value). Otherwise, zero is returned.

##### Related topics

    isalnum - isalpha - iscntrl - isdigit - isprint - ispunct - isspace - isxdigit

#### islower

| Syntax | #include <cctype> int islower( int ch ); |
The islower function returns non-zero if its argument is a lowercase letter. Otherwise, zero is returned.

##### Related topics

    isupper

#### isprint

| Syntax | #include <cctype> int isprint( int ch ); |
The function isprint returns non-zero if its argument is a printable character (including a space). Otherwise, zero is returned.

##### Related topics

    isalnum - isalpha - iscntrl - isdigit - isgraph - ispunct - isspace

#### ispunct

| Syntax | #include <cctype> int ispunct( int ch ); |
The ispunct function returns non-zero if its argument is a printing character but neither alphanumeric nor a space. Otherwise, zero is returned.

##### Related topics

    isalnum - isalpha - iscntrl - isdigit - isgraph - isspace - isxdigit

#### isspace

| Syntax | #include <cctype> int isspace( int ch ); |
The isspace function returns non-zero if its argument is some sort of space (i.e. single space, tab, vertical tab, form feed, carriage return, or newline). Otherwise, zero is returned.

##### Related topics

    isalnum - isalpha - iscntrl - isdigit - isgraph - isprint - ispunct - isxdigit

#### isupper

| Syntax | #include <cctype> int isupper( int ch ); |
The isupper function returns non-zero if its argument is an uppercase letter. Otherwise, zero is returned.

##### Related topics

    islower - tolower

#### isxdigit

| Syntax | #include <cctype> int isxdigit( int ch ); |
The function isxdigit returns non-zero if its argument is a hexadecimal digit (i.e. A-F, a-f, or 0-9). Otherwise, zero is returned.

##### Related topics

    isalnum - isalpha - iscntrl - isdigit - isgraph - ispunct - isspace

#### memchr

| Syntax | #include <cstring> void *memchr( const void *buffer, int ch, size_t count ); |
The memchr function looks for the first occurrence of ch within count characters in the array pointed to by buffer. The return value points to the location of the first occurrence of ch, or NULL if ch isn't found. For example:

##### Related topics

    memcmp - memcpy - strstr

#### memcmp

| Syntax | #include <cstring> int memcmp( const void *buffer1, const void *buffer2, size_t count ); |
The function memcmp compares the first count characters of buffer1 and buffer2. The return values are as follows:
| Return value | Explanation |
| less than 0 | buffer1 is less than buffer2 |
| equal to 0 | buffer1 is equal to buffer2 |
| greater than 0 | buffer1 is greater than buffer2 |

##### Related topics

    memchr - memcpy - memset - strcmp

#### memcpy

| Syntax | #include <cstring> void *memcpy( void *to, const void *from, size_t count ); |
The function memcpy copies count characters from the array from to the array to. The return value of memcpy is to. The behavior of memcpy is undefined if to and from overlap.

##### Related topics

    memchr - memcmp - memmove - memset - strcpy - strlen - strncpy

#### memmove

| Syntax | #include <cstring> void *memmove( void *to, const void *from, size_t count ); |
The memmove function is identical to memcpy, except that it works even if to and from overlap.

##### Related topics

    memcpy - memset

#### memset

| Syntax | #include <cstring> void* memset( void* buffer, int ch, size_t count ); |
The function memset copies ch into the first count characters of buffer, and returns buffer. memset is useful for intializing a section of memory to some value. For example, this command:
...is a very efficient way to set all values of the_array to zero.
The table below compares two different methods for initializing an array of characters: a for loop versus memset. As the size of the data being initialized increases, memset clearly gets the job done much more quickly:
| Input size | Initialized with a for loop | Initialized with memset |
| 1000 | 0.016 | 0.017 |
| 10000 | 0.055 | 0.013 |
| 100000 | 0.443 | 0.029 |
| 1000000 | 4.337 | 0.291 |

##### Related topics

    memcmp - memcpy - memmove

#### strcat

| Syntax | #include <cstring> char *strcat( char *str1, const char *str2 ); |
The strcat function concatenates str2 onto the end of str1, and returns str1. For example:
Note that strcat does not perform bounds checking, and thus risks overrunning str1 or str2. For a similar (and safer) function that includes bounds checking, see strncat.

##### Related topics

    strchr - strcmp - strcpy - strncat

## strchr

| Syntax | #include <cstring> char *strchr( const char *str, int ch ); |
The function strchr returns a pointer to the first occurrence of ch in str, or NULL if ch is not found.

### Related topics

    strcat - strcmp - strcpy - strlen - strncat - strncmp - strncpy - strpbrk - strrchr -strspn - strstr - strtok

#### strcmp

| Syntax | #include <cstring> int strcmp( const char *str1, const char *str2 ); |
The function strcmp compares str1 and str2, then returns:
| Return value | Explanation |
| less than 0 | str1 is less than str2 |
| equal to 0 | str1 is equal to str2 |
| greater than 0 | str1 is greater than str2 |
For example:
Note that if str1 or str2 are missing a null-termination character, then strcmp may not produce valid results. For a similar (and safer) function that includes explicit bounds checking, see strncmp.

##### Related topics

    memcmp - strcat - strchr - strcoll - strcpy - strlen - strncmp - strxfrm

#### strcoll

| Syntax | #include <cstring> int strcoll( const char *str1, const char *str2 ); |
The strcoll function compares str1 and str2, much like strcmp. However, strcoll performs the comparison using the locale specified by the (Standard C Date & Time) setlocale function.

##### Related topics

    strcmp - strxfrm (Standard C Date & Time) setlocale

#### strcpy

| Syntax | #include <cstring> char *strcpy( char *to, const char *from ); |
The strcpy function copies characters in the string 'from to the string to, including the null termination. The return value is to.
Note that strcpy does not perform bounds checking, and thus risks overrunning from or to. For a similar (and safer) function that includes bounds checking, see strncpy.

##### Related topics

    memcpy - strcat - strchr - strcmp - strncmp - strncpy

#### strcspn

| Syntax | #include <cstring> size_t strcspn( const char *str1, const char *str2 ); |
The function strcspn returns the index of the first character in str1 that matches any of the characters in str2.

##### Related topics

    strpbrk - strrchr - strstr - strtok

## strerror

| Syntax | #include <cstring> char *strerror( int num ); |
The function strerror returns an implementation defined string corresponding to num. If an error occurred, the error is located within the global variable errno.

### Related topics

    perror

#### strlen

| Syntax | #include <cstring> size_t strlen( char *str ); |
The strlen function returns the length of str (determined by the number of characters before null termination).

##### Related topics

    memcpy - strchr - strcmp - strncmp

#### strncat

| Syntax | #include <cstring> char *strncat( char *str1, const char *str2, size_t count ); |
The function strncat concatenates at most count characters of str2 onto str1, adding a null termination. The resulting string is returned.

##### Related topics

    strcat - strchr - strncmp - strncpy

#### strncmp

| Syntax | #include <cstring> int strncmp( const char *str1, const char *str2, size_t count ); |
The strncmp function compares at most count characters of str1 and str2. The return value is as follows:
| Return value | Explanation |
| less than 0 | str1 is less than str2 |
| equal to 0 | str1 is equal to str2 |
| greater than 0 | str1 is greater than str2 |
If there are less than count characters in either string, then the comparison will stop after the first null termination is encountered.

##### Related topics

    strchr - strcmp - strcpy - strlen - strncat - strncpy

#### strncpy

| Syntax | #include <cstring> char *strncpy( char *to, const char *from, size_t count ); |
The strncpy function copies at most count characters of from to the string to. Only if from has less than count characters, is the remainder padded with '\0' characters. The return value is the resulting string.

##### Related topics

    memcpy - strchr - strcpy - strncat - strncmp

## strpbrk

| Syntax | #include <cstring> char * strpbrk( const char *str, const char *ch ); |
The function strpbrk returns a pointer to the first occurrence of any character within ch in str, or NULL if no characters were not found.

### Related topics

    strchr - strrchr - strstr

## strrchr

| Syntax | #include <cstring> char *strrchr( const char *str, int ch ); |
The function strrchr returns a pointer to the last occurrence of ch in str, or NULL if no match is found.

### Related topics

    strchr - strcspn - strpbrk - strspn - strstr - strtok

#### strspn

| Syntax | #include <cstring> size_t strspn( const char *str1, const char *str2 ); |
The strspn function returns the index of the first character in str1 that doesn't match any character in str2.

##### Related topics

    strchr - strpbrk - strrchr - strstr - strtok

#### strstr

| Syntax | #include <cstring> char *strstr( const char *str1, const char *str2 ); |
The function strstr returns a pointer to the first occurrence of str2 in str1, or NULL if no match is found. If the length of str2 is zero, then strstr will simply return str1.
For example, the following code checks for FPIFCthe existence of one string within another string:
When run, the above code displays this output:

##### Related topics

    memchr - strchr - strcspn - strpbrk - strrchr - strspn - strtok

#### strtod

| Syntax | #include <cstdlib> double strtod( const char *start, char **end ); |
The function strtod returns whatever it encounters first in start as a double. end is set to point at whatever is left in start after that double. If overflow occurs, strtod returns either HUGE_VAL or -HUGE_VAL.
results in x being set to 42.0.

##### Related topics

    atof

#### strtok

| Syntax | #include <cstring> char *strtok( char *str1, const char *str2 ); |
The strtok function returns a pointer to the next "token" in str1, where str2 contains the delimiters that determine the token. strtok returns NULL if no token is found. In order to convert a string to tokens, the first call to strtok should have str1 point to the string to be tokenized. All calls after this should have str1 be NULL.
For example:
The above code will display the following output:

##### Related topics

    strchr - strcspn - strpbrk - strrchr - strspn - strstr

#### strtol

| Syntax | #include <cstdlib> long strtol( const char *start, char **end, int base ); |
The strtol function returns whatever it encounters first in start as a long, doing the conversion to base if necessary. end is set to point to whatever is left in start after the long. If the result can not be represented by a long, then strtol returns either LONG_MAX or LONG_MIN. Zero is returned upon error.

##### Related topics

    atol - strtoul

#### strtoul

| Syntax | #include <cstdlib> unsigned long strtoul( const char *start, char **end, int base ); |
The function strtoul behaves exactly like strtol, except that it returns an unsigned long rather than a mere long.

##### Related topics

    strtol

#### strxfrm

| Syntax | #include <cstring> size_t strxfrm( char *str1, const char *str2, size_t num ); |
The strxfrm function manipulates the first num characters of str2 and stores them in str1. The result is such that if a strcoll is performed on str1 and the old str2, you will get the same result as with a strcmp.

##### Related topics

    strcmp - strcoll

#### tolower

| Syntax | #include <cctype> int tolower( int ch ); |
The function tolower returns the lowercase version of the character ch.

##### Related topics

    isupper - toupper

#### toupper

| Syntax | #include <cctype> int toupper( int ch ); |
The toupper function returns the uppercase version of the character ch.

##### Related topics

    tolower