Basics of String Manipulation

A string is a sequence of characters. In other words, a string is an array of character data type. An instance of a string is called a string literal. For instance in C++: string s = "HackerEarth"; $s$ is a string literal.

String Manipulation is a class of problems where a user is asked to process a given string and use/change its data. An example question would be a great way to understand the problems that are usually classified under this category.

• Given a string $S$ of length $N$, shift each character of the string by $K$ positions to the right, where $K \le N$.

For example: Say $S$ = "hacker" and $K = 2$. Here $N = 6$.
Shifting each character in $S$ by $2$ positions to the right would result into $erhack$.
Note that $S[0]$ i.e. 'h' is moved by 2 positions to the $S[2]$. Also, $S[5]$ i.e. 'r', which is the last character in $S$ comes round-about back to $S[1]$ as there is no space for 'r' to go beyond the limits of string length.

Approach:

• Declare another auxillary string $shiftedS$ that is of the same size as $S$.
• Copy $i^{th}$ element of $S$ to the $(K+i)^{th}$ position in $shiftedS$. This means, $shiftedS[i+K] = S[i]$ where $0 \le i \lt N$.
• Make sure that $i+K$ never exceeds $N$, because that will try to access a memory location which is not declared in $shiftedS$. There's a simple trick to ensure that - use $(i+K) mod\; N$ .

Implementation:

void shiftByK(char S[], char shiftedS[], int N, int K) {
    // Iterate through the length of given string
    for(int i=0; i<N; i++) {
        // Find the index for this current character in shiftedS[]
        int idx = (i+K) % N;
        // Copy that character at the found index idx
        shiftedS[idx] = S[i];
    }
    // Add a NULL character to mark the end of string
    shiftedS[N] = '\0';
}

Every character array in C/C++ ends with a '\0' (NULL) character. It marks the end of the string. If it is not added in the end, then the code may produce garbage characters after the string.