Problem Solving: Substring Search
Test your problem-solving skills in this challenging exercise.
Substring searching and its variants
Substring index
Write a program that takes two strings, S and sub, and returns the index of sub in S (if sub is the subset of S).
Sample program
Str1: helloaaabbc
Str2: abbc
helloaaabbcabbc
^
abbc
Found at index: 7
Sliding window technique
Let’s suppose we have a window of the length of the substring, sub length, and we are placing that window at every index of S (giving the effect of sliding by moving one step forward) and checking if the window matches exactly, at the current index. We immediately return that index. Otherwise, we return -1, meaning that the substring is not found.
Look at the animation below:
#include <iostream>
using namespace std;
#include <cstring>
int substring_index(char S[ ], char sub[])
{
// If substring found, return index otherwise return -1
// Write your code here
}
int main()
{
char S[ ] = "helloaaabbcabbc";
char sub[] = "abbc";
int fi = substring_index(S, sub);
if(fi!=-1)
{
cout << S<<endl;
for(int i=0;i<fi; i++)
cout << ' ';
cout << '^'<<endl;
for(int i=0;i<fi; i++)
cout << ' ';
cout<< sub<<endl;
cout<<"index: "<< fi<<endl;
}
return 0;
}Look at the solution below:
Substring occurrences in a string
Write a program that takes two strings, S and sub, and returns all the occurrences of sub in S.
Sample input
Str1: helloaaabbcabbc
Str2: abbc
helloaaabbcabbc
^ ^
abbc found 2 times!
What changes do we need to make in the function above?
We just need to add an int array Is[], that saves the index when every substring character matches with the string.
Instead of returning immediately after finding the substring, we’ll save that index i inside Is[] and move the window forward, and exhaust all the indexes.
Look at the complete code below:
#include <iostream>
using namespace std;
#include <cstring>
int substring_index_all(char S[ ], char sub[], int Is[])
{
int sub_length = strlen(sub);
int S_length = strlen(S);
int ii = 0;
for(int si=0; si<=S_length-sub_length; si++)
{
int count=0;
for(int i=0; i<sub_length; i++)
{ // if ith character of sub matches the ith character
if(S[si+i]==sub[i]) // of the window starting at si in S
{
count++;
}
}
// if every character of substring sub is matched
if(count==sub_length)
{
Is[ii] = si; // save this index
ii++; // increment in ii so that the next index
} // is saved at the next index in Is
}
return ii;
}
void printSpecial(int times, char S[], char sub[], int Is[])
{
if(times!=0)
{
cout << S<<endl;
int ti=0;
for(int i=0;ti!=times; i++) // Printing location where exactly found.
{
if(Is[ti]!=i)
cout << ' ';
else
cout << '^', ti++;
}
cout<< endl << sub <<" found " << times << " times!" <<endl;
}
else
{
cout<< endl << sub <<" was not found "<<endl;
}
}
int main()
{
const int capacity = 100;
char S[ ] = "helloaaabbcabbc";
char sub[] = "abbc";
int Is[capacity] = {};
int times = substring_index_all(S, sub,Is);
printSpecial(times, S, sub, Is);
return 0;
}