In C - check if a char exists in a char array

Question

I m trying to check if a character belongs to a list array of invalid characters   Coming from a Python background  I used to be able to just say    for c in string      if c in invalid characters           do stuff  etc   How can I do this with regular C char arrays

User · Answer

The less well-known but extremely useful (and standard since C89 — meaning 'forever') functions in the C library provide the information in a single call. Actually, there are multiple functions — an embarrassment of riches. The relevant ones for this are:

7.21.5.3 The strcspn function

Synopsis
#include <string.h>
size_t strcspn(const char *s1, const char *s2);
Description

The strcspn function computes the length of the maximum initial segment of the string pointed to by s1 which consists entirely of characters not from the string pointed to by s2.

Returns

The strcspn function returns the length of the segment.

7.21.5.4 The strpbrk function

Synopsis
#include <string.h>
char *strpbrk(const char *s1, const char *s2);
Description

The strpbrk function locates the first occurrence in the string pointed to by s1 of any character from the string pointed to by s2.

Returns

The strpbrk function returns a pointer to the character, or a null pointer if no character from s2 occurs in s1.

The question asks about 'for each char in string ... if it is in list of invalid chars'.

With these functions, you can write:

size_t len = strlen(test);
size_t spn = strcspn(test, "invald");

if (spn != len) { ...there's a problem... }

Or:

if (strpbrk(test, "invald") != 0) { ...there's a problem... }

Which is better depends on what else you want to do. There is also the related strspn() function which is sometimes useful (whitelist instead of blacklist).

User · Answer

I believe the original question said      a character belongs to a list array of   invalid characters   and not      belongs to a null-terminated string   which  if it did  then strchr would indeed be the most suitable answer  If  however  there is no null termination to an array of chars or if the chars are in a list structure  then you will need to either create a null-terminated string and use strchr or manually iterate over the elements in the collection  checking each in turn  If the collection is small  then a linear search will be fine  A large collection may need a more suitable structure to improve the search times - a sorted array or a balanced binary tree for example   Pick whatever works best for you situation

User · Answer

Assuming your input is a standard null-terminated C string  you want to use strchr    include  lt string h gt   char  foo    abcdefghijkl   if  strchr foo   a      NULL         do stuff     If on the other hand your array is not null-terminated  i e  just raw data   you ll need to use memchr and provide a size    include  lt string h gt   char foo        a    b    c    d    e        note last element isn t   0  if  memchr foo   a   sizeof foo           do stuff

User · Answer

The equivalent C code looks like this    include  lt stdio h gt   include  lt string h gt      This code outputs  h is in  This is my test string  int main int argc  char  argv         const char  invalid characters    hz      char  mystring    This is my test string      char  c   mystring     while   c              if  strchr invalid characters   c                      printf   c is in    s   n    c  mystring                    c             return 0      Note that invalid characters is a C string  ie  a null-terminated char array

User · Answer

use strchr function when dealing with C strings   const char   strchr   const char   str  int character      Here is an example of what you want to do      strchr example     include  lt stdio h gt   include  lt string h gt   int main        char invalids         lt  gt       char   pch    pch strchr invalids  s     is s an invalid character    if  pch  NULL          printf   Invalid character          else           printf  Valid character           return 0      Use memchr when dealing with memory blocks  as not null terminated arrays   const void   memchr   const void   ptr  int value  size t num        memchr example     include  lt stdio h gt   include  lt string h gt   int main        char   pch    char invalids        lt  gt       pch    char   memchr  invalids   p   strlen invalids      if  pch  NULL      printf  p is an invalid character     else     printf   p valid character  n      return 0      http   www cplusplus com reference clibrary cstring memchr   http   www cplusplus com reference clibrary cstring strchr

User · Answer

You want     strchr  const char  s  int c    If the character c is in the string s it returns a pointer to the location in s   Otherwise it returns NULL   So just use your list of invalid characters as the string

User · Answer

strchr for searching a char from start  strrchr from the end      char str      This is a sample string      if  strchr str   h      NULL             h is in str

[c] In C - check if a char exists in a char array

7.21.5.3 The strcspn function

7.21.5.4 The strpbrk function

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