Is there something like startsWith(str_a, str_b)
in the standard C library?
It should take pointers to two strings that end with nullbytes, and tell me whether the first one also appears completely at the beginning of the second one.
Examples:
"abc", "abcdef" -> true "abcdef", "abc" -> false "abd", "abdcef" -> true "abc", "abc" -> true
The strchr function returns a pointer to the first occurrence of character c in string or a null pointer if no matching character is found.
Implement substr() function in CThe substr() function returns the substring of a given string between two given indices. It returns the substring of the source string starting at the position m and ending at position n-1 .
In the C Programming Language, the strncmp function returns a negative, zero, or positive integer depending on whether the first n characters of the object pointed to by s1 are less than, equal to, or greater than the first n characters of the object pointed to by s2.
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.
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