Write an efficient function to implement strncpy function in C. Standard strncpy() function copy given n characters from source C-string to another string.
The prototype of strncpy is:
char* strncpy(char* destination, const char* source, size_t num);
The C99 standard adds the restrict qualifiers to the prototype:
char* strncpy(char* restrict destination, const char* restrict source, size_t num);
The strncpy() function copies num characters from the null-terminated string pointed to by source to the memory pointed to by destination and finally returns the pointer destination.
C
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#include <stdio.h> #include <stddef.h> // Function to implement strncpy() function char* strncpy(char* destination, const char* source, size_t num) { // return if no memory is allocated to the destination if (destination == NULL) return NULL; // take a pointer pointing to the beginning of destination string char* ptr = destination; // copy first num characters of C-string pointed by source // into the array pointed by destination while (*source && num--) { *destination = *source; destination++; source++; } // null terminate destination string *destination = '\0'; // destination is returned by standard strncpy() return ptr; } // Implement strncpy function in C int main(void) { char* source = "Techie Delight"; char destination[20]; size_t num = 6; // Copies the first num characters of source to destination printf("%s", strncpy(destination, source, num)); return 0; } |
Output:
Techie
The time complexity of above solution is O(num).
Shorter Version:
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while (*source && num--) { *destination = *source; destination++; source++; } *destination = '\0'; |
We can replace above lines of code by single line of code. Below code will copy first num characters of source to the array pointed by destination including the terminating null character.
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while (num-- && (*destination++ = *source++)); |
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