Check out example codes for "z algo preapre z array". It will help you in understanding the concepts better.

Code Example 1

``````// A C++ program that implements Z algorithm for pattern searching
#include<iostream>
using namespace std;

void getZarr(string str, int Z[]);

// prints all occurrences of pattern in text using Z algo
void search(string text, string pattern)
{
// Create concatenated string "P\$T"
string concat = pattern + "\$" + text;
int l = concat.length();

// Construct Z array
int Z[l];
getZarr(concat, Z);

// now looping through Z array for matching condition
for (int i = 0; i < l; ++i)
{
// if Z[i] (matched region) is equal to pattern
// length we got the pattern
if (Z[i] == pattern.length())
cout << "Pattern found at index "
<< i - pattern.length() -1 << endl;
}
}

// Fills Z array for given string str[]
void getZarr(string str, int Z[])
{
int n = str.length();
int L, R, k;

// [L,R] make a window which matches with prefix of s
L = R = 0;
for (int i = 1; i < n; ++i)
{
// if i>R nothing matches so we will calculate.
// Z[i] using naive way.
if (i > R)
{
L = R = i;

// R-L = 0 in starting, so it will start
// checking from 0'th index. For example,
// for "ababab" and i = 1, the value of R
// remains 0 and Z[i] becomes 0. For string
// "aaaaaa" and i = 1, Z[i] and R become 5
while (R<n && str[R-L] == str[R])
R++;
Z[i] = R-L;
R--;
}
else
{
// k = i-L so k corresponds to number which
// matches in [L,R] interval.
k = i-L;

// if Z[k] is less than remaining interval
// then Z[i] will be equal to Z[k].
// For example, str = "ababab", i = 3, R = 5
// and L = 2
if (Z[k] < R-i+1)
Z[i] = Z[k];

// For example str = "aaaaaa" and i = 2, R is 5,
// L is 0
else
{
// else start from R and check manually
L = i;
while (R<n && str[R-L] == str[R])
R++;
Z[i] = R-L;
R--;
}
}
}
}

// Driver program
int main()
{
string text = "GEEKS FOR GEEKS";
string pattern = "GEEK";
search(text, pattern);
return 0;
}``````

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