C++ Program to Print the First Letter of Each Word of a String

String str is given which contains lowercase English letters and spaces. It may contain multiple spaces. Get the first letter of every word and return the result as a string. The result should not contain any space.
Examples:

Input: str = "geeks for geeks"
Output: gfg

Input: str = "happy   coding"
Output: hc

Source: https://www.geeksforgeeks.org/amazon-interview-set-8-2/

The idea is to traverse each character of string str and maintain a boolean variable, which was initially set as true. Whenever we encounter space we set the boolean variable is true. And if we encounter any character other than space, we will check the boolean variable, if it was set as true as copy that charter to the output string and set the boolean variable as false. If the boolean variable is set false, do nothing. 
Algorithm: 

1. Traverse string str. And initialize a variable v as true.
2. If str[i] == ' '. Set v as true.
3. If str[i] != ' '. Check if v is true or not.
   a) If true, copy str[i] to output string and set v as false.
   b) If false, do nothing.

C++

// C++ program to find the string
// which contain the first character
// of each word of another string.
#include<bits/stdc++.h>
using namespace std;

// Function to find string which has
// first character of each word.
string firstLetterWord(string str)
{
    string result = "";

    // Traverse the string.
    bool v = true;
    for (int i = 0; i < str.length(); i++)
    {
        // If it is space, set v as true.
        if (str[i] == ' ')
            v = true;

        // Else check if v is true or not.
        // If true, copy character in output
        // string and set v as false.
        else if (str[i] != ' ' && v == true)
        {
            result.push_back(str[i]);
            v = false;
        }
    }

    return result;
}

// Driver code
int main()
{
    string str = "geeks for geeks";
    cout << firstLetterWord(str);
    return 0;
}

Output

gfg

Output:

gfg

Time Complexity: O(n)

Space Complexity: O(1), if space of storing resultant string is taken in account it will be O(n).

Another Approach: 

In this approach, we will first split the input string based on the spaces. The spaces in the strings can be matched using a regular expression. The split strings are stored in an array of strings. Then we can simply add the first character of each split string in the result.  

C++

// C++ implementation of the above
// approach
#include <bits/stdc++.h>
using namespace std;

string processWords(char *input)
{
    /* We are splitting the input based on
       spaces (s)+ : this regular expression
       will handle scenarios where we have words
       separated by multiple spaces */
    char *p;
    vector<string> s;

    p = strtok(input, " ");
    while (p != NULL)
    {
        s.push_back(p);
        p = strtok(NULL, " ");
    }

    string charBuffer;

    for (string values : s)

        /* charAt(0) will pick only the
           first character from the string
           and append to buffer */
        charBuffer += values[0];

    return charBuffer;
}

// Driver code
int main()
{
    char input[] = "geeks for geeks";
    cout << processWords(input);
    return 0;
}

Output

gfg

Method: Using Recursion

C++

#include<bits/stdc++.h>
using namespace std;

// Function to find string which has first character of each word.
string firstLetterWord(string str, int index, string result)
{
    // base case: if index is out of range, return the modified string
    if (index == str.length())
        return result;

    // check for space

    if (str[index] == ' ')
    {
        // copy the character to result string
        result.push_back(str[index+1]);
    }
        // recursively call the function for the next index
        return firstLetterWord(str, index + 1, result);

}

// Driver code
int main()
{
    string str = "geeks for geeks";
   cout<< firstLetterWord(str, 0, "g");
    return 0;
}

//This code is contributed by Vinay Pinjala.

Output

gfg

Time Complexity: O(n)
Auxiliary Space: O(n)
Please refer complete article on String containing the first letter of every word in a given string with spaces for more details!