Introduction
This article describe how to implement several function with respect to the index of string, including startsWith, endsWith, indexOf, indexOf with a shift and lastIndexOf.
The Program
startsWith_endsWith_indexOf_lastIndexOf.c
#include <stdio.h>
#include <string.h>
#include <stdbool.h>
/**
*
* Tested with Dev-C++ 4.9.9.2
*
* Practice of string compare.
*
* char * strstr ( const char *, const char * );
* Locate substring
* Returns a pointer to the first occurrence of str2 in str1,
* or a null pointer if str2 is not part of str1.
* bool startsWith (char* base, char* str);
* Custom function for detecting whether base is starts with str
* bool endsWith (char* base, char* str);
* Custom function for detecting whether base is ends with str
* int indexOf (char* base, char* str)
* Custom function for getting the first index of str in base
* -1 denotes not found
* int indexOf_shift (char* base, char* str, int startIndex)
* Custom function for getting the first index of str in base
* after the given startIndex
* -1 denotes not found
* int lastIndexOf (char* base, char* str)
* Custom function for getting the last index of str in base
* -1 denotes not found
* References:
* http://www.cplusplus.com/reference/cstring/strstr/
*
*/
bool startsWith (char* base, char* str);
bool endsWith (char* base, char* str);
int indexOf (char* base, char* str);
int indexOf_shift (char* base, char* str, int startIndex);
int lastIndexOf (char* base, char* str);
int main () {
// a char array without '\0'
char* chArrOne = "abcdefabcdef";
printf("chArrOne starts with abc?\n%s\n\n", startsWith(chArrOne, "abc")? "true" : "false");
printf("chArrOne starts with def?\n%s\n\n", startsWith(chArrOne, "def")? "true" : "false");
printf("chArrOne ends with def?\n%s\n\n", endsWith(chArrOne, "def")? "true" : "false");
printf("first index of abc in chArrOne?\n%d\n\n", indexOf(chArrOne, "abc"));
printf("first index of def in chArrOne?\n%d\n\n", indexOf(chArrOne, "def"));
printf("first index of abc in chArrOne after index 2?\n%d\n\n", indexOf_shift(chArrOne, "abc", 2));
printf("first index of def in chArrOne after index 5?\n%d\n\n", indexOf_shift(chArrOne, "def", 5));
printf("last index of abc in chArrOne?\n%d\n\n", lastIndexOf(chArrOne, "abc"));
printf("last index of def in chArrOne?\n%d\n\n", lastIndexOf(chArrOne, "def"));
system("PAUSE");
return 0;
}
/** detecting whether base is starts with str
*/
bool startsWith (char* base, char* str) {
return (strstr(base, str) - base) == 0;
}
/** detecting whether base is ends with str
*/
bool endsWith (char* base, char* str) {
int blen = strlen(base);
int slen = strlen(str);
return (blen >= slen) && (0 == strcmp(base + blen - slen, str));
}
/** getting the first index of str in base
*/
int indexOf (char* base, char* str) {
return indexOf_shift(base, str, 0);
}
int indexOf_shift (char* base, char* str, int startIndex) {
int result;
int baselen = strlen(base);
// str should not longer than base
if (strlen(str) > baselen || startIndex > baselen) {
result = -1;
} else {
if (startIndex < 0 ) {
startIndex = 0;
}
char* pos = strstr(base+startIndex, str);
if (pos == NULL) {
result = -1;
} else {
result = pos - base;
}
}
return result;
}
/** use two index to search in two part to prevent the worst case
* (assume search 'aaa' in 'aaaaaaaa', you cannot skip three char each time)
*/
int lastIndexOf (char* base, char* str) {
int result;
// str should not longer than base
if (strlen(str) > strlen(base)) {
result = -1;
} else {
int start = 0;
int endinit = strlen(base) - strlen(str);
int end = endinit;
int endtmp = endinit;
while(start != end) {
start = indexOf_shift(base, str, start);
end = indexOf_shift(base, str, end);
// not found from start
if (start == -1) {
end = -1; // then break;
} else if (end == -1) {
// found from start
// but not found from end
// move end to middle
if (endtmp == (start+1)) {
end = start; // then break;
} else {
end = endtmp - (endtmp - start) / 2;
if (end <= start) {
end = start+1;
}
endtmp = end;
}
} else {
// found from both start and end
// move start to end and
// move end to base - strlen(str)
start = end;
end = endinit;
}
}
result = start;
}
return result;
}
The Result
Reference
strstr
http://www.cplusplus.com/reference/cstring/strstr/
Download
startsWith_endsWith_indexOf_lastIndexOf.c at github
https://github.com/benbai123/C_Cplusplus_Practice/blob/master/C_StringProcessing/utils/startsWith_endsWith_indexOf_lastIndexOf.c
