- The header file of
string manipulation functions
in the C standard library:
#include <strings.h>
The header file contains declarations (function prototypes of the string manipulation functions
- Some string manipulation functions
in the C standard library:
- strlen(s):
returns the
length
(number of characters not including the
sentinel '\0') in the string s
- strcpy(s1, s2):
copy
the string s2 into the string s1
- strncpy(s1, s2, n):
copy n characters
from the string s2 into the string s1
- strcat(s1, s2):
append
(catenate)
the string s2 after the string s1
(string s2 is unchanged)
- strncat(s1, s2):
append (catenate)
n characters of
the string s2 after the string s1
(string s2 is unchanged)
- strcmp(s1, s2):
compare the strings s1 and s2.
- Returns a value < 0 if s1 is lexicographically smaller than s2,
- returns the value 0 if s1 and s2 are equal and
- return a value > 0 if s1 is lexicographically greater than s2.
- strncmp(s1, s2):
same as
strcmp(s1, s2),
except the result
is based only on
the first n characters in the strings.
- strcasecmp(s1, s2):
same as
strcmp(s1, s2), except
the
case (upper/lower) is ignored.
- strncasecmp(s1, s2): same as strcmp(s1, s2), except the result is based only on the first n characters in the strings.
- strlen(s):
returns the
length
(number of characters not including the
sentinel '\0') in the string s
- Syntax:
char *strcpy( char s1[], char s2[] ) or: char *strcpy( char *s1, char *s2 ) // Recall that char a[] <==> char *a in C !!!
Copy the string s2 to the string s1 (Function returns s1)
- The naive way to
implement the strcpy()
function:
char *strcpy( char s1[], char s2[] ) { int i = 0; /* ---------------------------------- '\0' marks the end of the string ! ---------------------------------- */ while ( s2[i] != '\0' ) { s1[i] = s2[i]; // Copy character at position i i++; // Move to next character } s1[i] = '\0'; // Mark the end of the s1 string return(s1); }
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: gcc strcpy1.c
- To run: ./a.out
- The C intermediate way to implement the
strcpy() function
(it is a direct transcription
from the previous version using
pointer arithmetic):
char *strcpy( char *s1, char *s2 ) { while ( *s2 != '\0' ) *s1++ = *s2++; *s1 = '\0'; // Mark the end of the s1 string return(s1); }
- The C Pro way to implement the
strcpy() function:
char *strcpy( char *s1, char *s2 ) { char *r = s1; // Save return value while ( (*s1++ = *s2++) != '\0' ); return(r); }
Or (after taking advantage of C's way to interpret non-zero as TRUE): char *strcpy( char *s1, char *s2 ) { char *r = s1; // Save return value while ( (*s1++ = *s2++) ); return(r); }Explanation:
- The input parameters
s1 and
s2 points to
2 different
character arrays:
+---+---+---+---+---+---+---+ +---+---+---+---+---+ | | | | | | | | ... | a | b | c | \0| | ... +---+---+---+---+---+---+---+ +---+---+---+---+---+ ^ ^ | | s1 s2
- Iteration 1 of while loop....
The expression of the while-statement must be evaluated from the inside out:
1. ( *s1++ = *s2++ ) // *s2++ returns 'a' and then increments s2 // *s1++ = 'a' will store 'a' in first position of s1 // and then increment s1 // The entire expression ( *s1++ = *s2++ ) // evaluates to 'a' (assignment operator // returns the value assigned !) // // Result: copies the first char // from s2 to s1 AND // increment s1 and s2 +---+---+---+---+---+---+---+ +---+---+---+---+---+ | a | | | | | | | ... | a | b | c | \0| | ... +---+---+---+---+---+---+---+ +---+---+---+---+---+ ^ ^ | | s1 s2
2. ( *s1++ = *s2++ ) != '\0' // replace ( *s1++ = *s2++ ) with return value ==> 'a' != '\0' // TRUE while loop repeats....
- Iteration 2 of while loop....
Again, evaluate the expression of the while-statement:
1. ( *s1++ = *s2++ ) // *s2++ now returns 'b' and then increments s2 // *s1++ = 'b' will store 'b' in next position of s1 // and then increment s1 // The entire expression ( *s1++ = *s2++ ) // evaluates to 'b' (assignment operator // returns the value assigned !) // // Result: copies the second char // from s2 to s1 AND // increment s1 and s2 +---+---+---+---+---+---+---+ +---+---+---+---+---+ | a | b | | | | | | ... | a | b | c | \0| | ... +---+---+---+---+---+---+---+ +---+---+---+---+---+ ^ ^ | | s1 s2
2. ( *s1++ = *s2++ ) != '\0' // replace ( *s1++ = *s2++ ) with return value ==> 'b' != '\0' // TRUE while loop repeats....
- Iteration 3 of while loop....
Again, evaluate the expression of the while-statement:
1. ( *s1++ = *s2++ ) // *s2++ now returns 'c' and then increments s2 // *s1++ = 'c' will store 'c' in next position of s1 // and then increment s1 // The entire expression ( *s1++ = *s2++ ) // evaluates to 'c' (assignment operator // returns the value assigned !) // // Result: copies the third char // from s2 to s1 AND // increment s1 and s2 +---+---+---+---+---+---+---+ +---+---+---+---+---+ | a | b | c | | | | | ... | a | b | c | \0| | ... +---+---+---+---+---+---+---+ +---+---+---+---+---+ ^ ^ | | s1 s2
2. ( *s1++ = *s2++ ) != '\0' // replace ( *s1++ = *s2++ ) with return value ==> 'c' != '\0' // TRUE while loop repeats....
- Iteration 4 of while loop....
Again, evaluate the expression of the while-statement:
1. ( *s1++ = *s2++ ) // *s2++ now returns '\0' and then increments s2 // *s1++ = '\0' will store '\0' in next position of s1 // and then increment s1 // The entire expression ( *s1++ = *s2++ ) // evaluates to '\0' (assignment operator // returns the value assigned !) // // Result: copies the SENTINEL char // from s2 to s1 AND // increment s1 and s2 +---+---+---+---+---+---+---+ +---+---+---+---+---+ | a | b | c | \0| | | | ... | a | b | c | \0| | ... +---+---+---+---+---+---+---+ +---+---+---+---+---+ ^ ^ | | s1 s2
2. ( *s1++ = *s2++ ) != '\0' // replace ( *s1++ = *s2++ ) with return value ==> '\0' != '\0' // FALSE while loop END (String has been copied !!!)
- The input parameters
s1 and
s2 points to
2 different
character arrays:
- Note:
- The strcpy() function is one of the classic one-liner code of C
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: gcc strcpy2.c
- To run: ./a.out
- Syntax:
int strcmp( char s1[], char s2[] ) or: int strcmp( char *s1, char *s2 ) // Recall that char a[] <==> char *a in C !!!
Compare the string s1 and s2 Returns: value < 0 if s1 < s2 value 0 if s1 equals to s2 value > 0 if s1 > s2
- The naive way to
implement the strcmp()
function:
int strcmp( char s1[], char s2[] ) { int i = 0; /* ---------------------------------- '\0' marks the end of the string ! ---------------------------------- */ while ( s2[i] != '\0' && s1[i] != 0 ) { if ( s1[i] != s2[i] ) // If s1 and s2 differ in char i break; // exit while loop ! i++; } /* ---------------------------------------------------------------- If we reach here, then: 1. one or both of s1 and s2 are exhausted ! 2. character position i in s1 and s2 are different ---------------------------------------------------------------- */ return s1[i] - s2[i]; // s1[i] - s2[i] will tell us which string // is lexicographically smaller // If they are equal, we will get // '\0' - '\0' = 0 }
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: gcc strcmp1.c
- To run: ./a.out
- The C intermediate way to implement the
strcmp() function
(it is a direct transcription
from the previous version using
pointer arithmetic):
int strcmp( char *s1, char *s2 ) { while ( *s1 != '\0' && *s2 != '\0' ) { if ( *s1 != *s2 ) break; s1++; s2++; } return( *s1 - *s2 ); }
- We can move the break test into
the if-condition to make the
strcmp() function
"sexier":
int strcmp( char *s1, char *s2 ) { while ( *s1 != '\0' && *s2 != '\0' && (*s1 - *s2) == 0 ) { s1++; s2++; } return( *s1 - *s2 ); }
- The advanced way to code
strcmp() function
is as follows:
int strcmp( char *s1, char *s2 ) { int res = 0; while ( *s1 != '\0' && *s2 != '\0' && ( res = (*s1 - *s2) ) == 0 ) { s1++; s2++; } return( res ); }
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: gcc strcmp2.c
- To run: ./a.out