substring function

[Radhika Sambamurti]

Hi,
I've written a substring function.
The prototype is: int substr(char s1[], char s2[])
Returns 1 if s2 is a substring of s1, else it returns 0.
I have written this program, but Im sure there is an easier way to do this. I am hoping someone can point me to a more elegant way of writing this same function.

//// code follows ///////////

//implement a substring function, where if string2 is a sub of string 1, then the value returned is 1 else, 0

#include<iostream>
#include<cstring>
using namespace std;

const int MAX = 10;
int substring(char *str1, char *str2); //prototype

int main()
{
char response = 'y';
char str1[MAX];
char str2[MAX];
int result = 0;

while(response == 'Y' || response == 'y')
{
cout << "Please enter string 1: " ;
cin >> str1;

cout << "\nPlease enter string 2: ";
cin >> str2;

cout << endl;

result = substring(str1, str2); //function call

if(result == 1)
cout << "string " << str2 << " is a substring of " << str1 << endl;
else
cout << "string " << str2 << " is NOT a substring of " << str1 << endl;

cout << "would you like to enter another? (Y/N) " << endl;
cin >> response;

}

return 0;
}

//''''''''''''''''''''''''''''''''''''''''''
//Function definition substring

int substring(char *str1, char *str2)
{
int len1, len2, i=0, j = 0, k, n;
len1=strlen(str1); //determine the lengths of the strings
len2=strlen(str2);


while(i <len1)
{

while( str2[j] == str1 )
{

for(k=1, n=i+1; k<len2; k++, n++)

//n is assigned i, which is where the match //starts in string 1- starting at the next letter

{
if( str2[k] != str1[n] )
return 0; //the match fails after having made an initial match
}
return 1;

j++;
}

i++;
}


}

 

[Joe C]

Hi,
I've written a substring function.
The prototype is: int substr(char s1[], char s2[])
Returns 1 if s2 is a substring of s1, else it returns 0.
I have written this program, but Im sure there is an easier way to do

this. I am hoping someone can point me to a more elegant way of writing this
same function.
Why not use the std::string member functions?

find( )
find_first_of( )
find_last_of( )
find_first_not_of( )
find_last_not_of( )
rfind( )

These return the starting position of the where the match is found, or a -1
cast to unsigned int if no match is found.

 

[Julie J.]

How about this:

int substring(char *str1, char *str2)
{
return (strstr(str1, str2) != NULL);
}

-jj-

 

[Default User]

Hi,
I've written a substring function.
The prototype is: int substr(char s1[], char s2[])
Returns 1 if s2 is a substring of s1, else it returns 0.

Why? Is this for a school exercise, or self-teaching? The above function
is almost, but not quite, the same as the C standard function strstr().

Why even mess with C-style strings?

I have written this program, but Im sure there is an easier way to do this. I am hoping someone can point me to a more elegant way of writing this same function.

int substr(char s1[], char s2[])
{
if (strstr (s1, s2))
return 1;
return 0;
}

int substring(char *str1, char *str2)
{
int len1, len2, i=0, j = 0, k, n;
len1=strlen(str1); //determine the lengths of the strings
len2=strlen(str2);

These calls to strlen() are going to whack your efficiency. They
terminate C-style strings for a reason, use that in your routine. You
don't need to know the lengths, just the endpoints.

while(i <len1)
{

while( str2[j] == str1 )
{

for(k=1, n=i+1; k<len2; k++, n++)

//n is assigned i, which is where the match //starts in string 1- starting at the next letter

{
if( str2[k] != str1[n] )
return 0; //the match fails after having made an initial match
}
return 1;

j++;
}

i++;
}


}

Here's one to consider:


int substr(const char *string, const char *substring)
{
const char *a = string;
const char *b = substring;

b = substring;
if (*b == 0)
{
return 1;
}
for ( ; *string != 0; string += 1)
{
if (*string != *b)
{
continue;
}
while (1)
{
if (*b == 0)
{
return 1;
}
if (*a++ != *b++)
{
break;
}
}
}
return 0;
}


Brian Rodenborn

 

[Default User]

int substr(const char *string, const char *substring)
{
const char *a = string;
const char *b = substring;

b = substring;

The above line is superfluous and should have been removed when I move
the assignments up to initializers.
Sorry.




Brian Rodenborn

 

[Radhika Sambamurti]

Hi,
I've written a substring function.
The prototype is: int substr(char s1[], char s2[])
Returns 1 if s2 is a substring of s1, else it returns 0.

Why? Is this for a school exercise, or self-teaching? The above function
is almost, but not quite, the same as the C standard function strstr().

Thanks! Yes, this was a class exercise.

 

[Jorge Rivera]

These return the starting position of the where the match is found, or a -1
cast to unsigned int if no match is found.

I know that this is practically irrelevant, but the correct return value
is std::string::npos, not -1.

 

[Default User]

Hi,
I've written a substring function.
The prototype is: int substr(char s1[], char s2[])
Returns 1 if s2 is a substring of s1, else it returns 0.

Why? Is this for a school exercise, or self-teaching? The above function
is almost, but not quite, the same as the C standard function strstr().

Thanks! Yes, this was a class exercise.

Ok. What you had wasn't too bad, although much more appropriate for a C
class than C++. I'm suspicious of the quality of instruction you are
getting.




Brian Rodenborn