Queries for frequencies of characters in substrings

Given a string s and Q number of queries. Each query Q consists of l and r and a character c. Find frequency of character c in substring l to r.

Examples:

Input : s = geeksforgeeks
        4
        0 5 e
        2 6 f
        4 7 m
        0 12 e
Output : 2
         1
         0
         4
Substring from 0 to 5 is geeksf.
Here e occurs 2 times.

Input : s = apple
        2
        0 4 e
        1 2 p        
Output : 1
         2 



Naive Approach: Run a loop from l to r for Q number of queries. Count occurrence of character and return count. Overall time complexity will be Q * O(|s|).

Efficient Approach:We can pre-compute the count for each character. Store count of each character in 2-D array. Return frequency of character from 0 to r minus frequency of character in range 0 to l in O(1). Overall time complexity will be Q * O(1).

C++

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// CPP program to find occurrence
// of character in substring l to r
#include <bits/stdc++.h>
#define MAX_LEN 1005
#define MAX_CHAR 26
  
using namespace std;
  
// To store count of all character
int cnt[MAX_LEN][MAX_CHAR];
  
// To pre-procees string from
// 0 to size of string
void preProcess(string s)
{
    int n = s.length();
  
    // Initializing cnt with 0
    memset(cnt, 0, sizeof(cnt));
  
    // Store occurrence of
    // character i
    for (int i = 0; i < n; i++)
        cnt[i][s[i] - 'a']++;
  
    // Store occurrence o
    // all character upto i
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < 26; j++)
            cnt[i][j] += cnt[i - 1][j];
    }
}
  
// To return occurrence of
// character in range l to r
int findCharFreq(int l, int r, char c)
{
    // Return occurrence of character
    // from 0 to r minus its
    // occurrence from 0 to l
    return cnt[r] - cnt[l - 1];
}
  
// Driver program to test above functions
int main()
{
    string s = "geeksforgeeks";
    int Q = 4;
    preProcess(s);
  
    cout << findCharFreq(0, 5, 'e') << endl;
    cout << findCharFreq(2, 6, 'f') << endl;
    cout << findCharFreq(4, 7, 'm') << endl;
    cout << findCharFreq(0, 12, 'e') << endl;
  
    return 0;
}

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Java

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// Java program to find occurrence
// of character in substring l to r
import java.util.*;
  
class GFG
{
  
static int MAX_LEN = 1005;
static int MAX_CHAR = 26;
  
// To store count of all character
static int [][]cnt = new int[MAX_LEN][MAX_CHAR];
  
// To pre-procees string from
// 0 to size of string
static void preProcess(String s)
{
    int n = s.length();
  
    // Store occurrence of
    // character i
    for (int i = 0; i < n; i++)
        cnt[i][s.charAt(i) - 'a']++;
  
    // Store occurrence o
    // all character upto i
    for (int i = 1; i < n; i++)
    {
        for (int j = 0; j < 26; j++)
            cnt[i][j] += cnt[i - 1][j];
    }
}
  
// To return occurrence of
// character in range l to r
static int findCharFreq(int l, int r, char c)
{
    // Return occurrence of character
    // from 0 to r minus its
    // occurrence from 0 to l
    return (cnt[r][(c) - 97] - cnt[l][(c) - 97]);
}
  
// Driver Code
public static void main(String[] args) 
{
    String s = "geeksforgeeks";
    int Q = 4;
    preProcess(s);
  
    System.out.println(findCharFreq(0, 5, 'e'));
    System.out.println(findCharFreq(2, 6, 'f'));
    System.out.println(findCharFreq(4, 7, 'm'));
    System.out.println(findCharFreq(0, 12, 'e'));
}
  
// This code is contributed by PrinciRaj1992

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Python3

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# Python3 program to find occurrence 
# of character in substring l to r 
MAX_LEN, MAX_CHAR = 1005, 26
  
# To store count of all character 
cnt = [[0 for i in range(MAX_CHAR)] 
          for j in range(MAX_LEN)] 
  
# To pre-procees string from 
# 0 to size of string 
def preProcess(s): 
  
    n = len(s) 
  
    # Store occurrence of character i 
    for i in range(0, n): 
        cnt[i][ord(s[i]) - ord('a')] += 1
  
    # Store occurrence o 
    # all character upto i 
    for i in range(0, n): 
        for j in range(0, 26): 
            cnt[i][j] += cnt[i - 1][j] 
      
# To return occurrence of 
# character in range l to r 
def findCharFreq(l, r, c): 
  
    # Return occurrence of character 
    # from 0 to r minus its 
    # occurrence from 0 to l 
    return (cnt[r][ord(c) - 97] - 
            cnt[l - 1][ord(c) - 97]) 
  
# Driver Code
if __name__ == "__main__"
  
    s = "geeksforgeeks"
    Q = 4
    preProcess(s) 
  
    print(findCharFreq(0, 5, 'e')) 
    print(findCharFreq(2, 6, 'f')) 
    print(findCharFreq(4, 7, 'm')) 
    print(findCharFreq(0, 12, 'e')) 
  
# This code is contributed by Rituraj Jain

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C#

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// C# program to find occurrence 
// of character in substring l to r 
using System;
  
class GFG
{
    static int MAX_LEN = 1005;
    static int MAX_CHAR = 26;
  
    // To store count of all character
    static int[,] cnt = new int[MAX_LEN, 
                                MAX_CHAR];
  
    // To pre-procees string from 
    // 0 to size of string
    static void preProcess(string s)
    {
        int n = s.Length;
  
        // Store occurrence of 
        // character i
        for (int i = 0; i < n; i++)
            cnt[i, s[i] - 'a']++;
  
        // Store occurrence o 
        // all character upto i
        for (int i = 1; i < n; i++)
        {
            for (int j = 0; j < 26; j++)
                cnt[i, j] += cnt[i - 1, j];
        }
    }
  
    // To return occurrence of 
    // character in range l to r
    static int findCharFreq(int l, int r, char c)
    {
        // Return occurrence of character 
        // from 0 to r minus its 
        // occurrence from 0 to l
        return (cnt[r, c - 97] - cnt[l, c - 97]);
    }
  
    // Driver code 
    public static void Main(string[] args)
    {
        string s = "geeksforgeeks";
        int Q = 4;
        preProcess(s);
  
        Console.WriteLine(findCharFreq(0, 5, 'e'));
        Console.WriteLine(findCharFreq(2, 6, 'f'));
        Console.WriteLine(findCharFreq(4, 7, 'm'));
        Console.WriteLine(findCharFreq(0, 12, 'e'));
    }
}
  
// This code is contibuted by
// sanjeev2552

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Output:

2
1
0
4


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