How to check if a string starts with another string in C?
There's no standard function for this, but you can define
bool prefix(const char *pre, const char *str)
{
return strncmp(pre, str, strlen(pre)) == 0;
}
We don't have to worry about str
being shorter than pre
because according to the C standard (7.21.4.4/2):
The
strncmp
function compares not more thann
characters (characters that follow a null character are not compared) from the array pointed to bys1
to the array pointed to bys2
."
Apparently there's no standard C function for this. So:
bool startsWith(const char *pre, const char *str)
{
size_t lenpre = strlen(pre),
lenstr = strlen(str);
return lenstr < lenpre ? false : memcmp(pre, str, lenpre) == 0;
}
Note that the above is nice and clear, but if you're doing it in a tight loop or working with very large strings, it does not offer the best performance, as it scans the full length of both strings up front (strlen
). Solutions like wj32's or Christoph's may offer better performance (although this comment about vectorization is beyond my ken of C). Also note Fred Foo's solution which avoids strlen
on str
(he's right, it's unnecessary if you use strncmp
instead of memcmp
). Only matters for (very) large strings or repeated use in tight loops, but when it matters, it matters.
I'd probably go with strncmp()
, but just for fun a raw implementation:
_Bool starts_with(const char *restrict string, const char *restrict prefix)
{
while(*prefix)
{
if(*prefix++ != *string++)
return 0;
}
return 1;
}