Remove characters from the first string which are present in the second string

Write an efficient C function that takes two strings as arguments and removes the characters from first string which are present in second string (mask string).

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Algorithm: Let first input string be”test string” and the string which has characters to be removed from first string be “mask”
1: Initialize:
res_ind = 0 /* index to keep track of processing of each character in i/p string */
ip_ind = 0 /* index to keep track of processing of each character in the resultant string */

2: Construct count array from mask_str. Count array would be:
(We can use Boolean array here instead of int count array because we don’t need count, we need to know only if character is present in mask string)
count[‘a’] = 1
count[‘k’] = 1
count[‘m’] = 1
count[‘s’] = 1

3: Process each character of the input string and if count of that character is 0 then only add the character to the resultant string.
str = “tet tringng” // ’s’ has been removed because ’s’ was present in mask_str but we we have got two extra characters “ng”
ip_ind = 11
res_ind = 9

4: Put a ‘\0′ at the end of the string?

Implementations:

C

#include <stdio.h> 
#include <stdlib.h> 
#define NO_OF_CHARS 256 
  
/* Returns an array of size 256 containg count 
   of characters in the passed char array */
int *getCharCountArray(char *str) 
{ 
   int *count = (int *)calloc(sizeof(int), NO_OF_CHARS); 
   int i; 
   for (i = 0; *(str+i);  i++) 
      count[*(str+i)]++; 
   return count; 
} 
  
/* removeDirtyChars takes two string as arguments: First 
string (str)  is the one from where function removes dirty 
characters. Second  string is the string which contain all 
dirty characters which need to be removed  from first string */
char *removeDirtyChars(char *str, char *mask_str) 
{ 
  int *count  = getCharCountArray(mask_str); 
  int ip_ind  = 0, res_ind = 0; 
  while (*(str + ip_ind)) 
  { 
    char temp = *(str + ip_ind); 
    if (count[temp] == 0) 
    { 
        *(str + res_ind) = *(str + ip_ind); 
        res_ind++; 
    } 
    ip_ind++; 
  }     
  
  /* After above step string is ngring.  
    Removing extra "iittg" after string*/
  *(str+res_ind) = '\0';     
  
  return str; 
} 
  
/* Driver program to test getCharCountArray*/
int main() 
{ 
    char str[]         = "geeksforgeeks"; 
    char mask_str[]  = "mask"; 
    printf("%s", removeDirtyChars(str, mask_str)); 
    return 0; 
} 

Java

// Java program to remove duplicates, the order of 
// characters is not maintained in this program 
  
public class GFG  
{ 
    static final int NO_OF_CHARS = 256; 
      
    /* Returns an array of size 256 containg count 
       of characters in the passed char array */
    static int[] getCharCountArray(String str) 
    { 
       int count[] = new int[NO_OF_CHARS]; 
       for (int i = 0; i<str.length();  i++) 
          count[str.charAt(i)]++; 
         
       return count; 
    } 
      
    /* removeDirtyChars takes two string as arguments: First 
    string (str)  is the one from where function removes dirty 
    characters. Second  string is the string which contain all 
    dirty characters which need to be removed  from first string */
    static String removeDirtyChars(String str, String mask_str) 
    { 
      int count[]  = getCharCountArray(mask_str); 
      int ip_ind  = 0, res_ind = 0; 
        
      char arr[] = str.toCharArray(); 
        
      while (ip_ind != arr.length) 
      { 
        char temp = arr[ip_ind]; 
        if(count[temp] == 0) 
        { 
            arr[res_ind] = arr[ip_ind]; 
            res_ind ++; 
        } 
            ip_ind++; 
          
      }     
  
      str = new String(arr); 
        
      /* After above step string is ngring.  
      Removing extra "iittg" after string*/
        
      return str.substring(0, res_ind); 
        
    } 
      
    // Driver Method 
    public static void main(String[] args) 
    { 
        String str = "geeksforgeeks"; 
        String mask_str = "mask"; 
        System.out.println(removeDirtyChars(str, mask_str)); 
    } 
} 

Python

# Python program to remove characters from first string which 
# are present in the second string 
NO_OF_CHARS = 256
  
# Utility function to convert from string to list 
def toList(string): 
    temp = [] 
    for x in string: 
        temp.append(x) 
    return temp 
  
# Utility function to convert from list to string 
def toString(List): 
    return ''.join(List) 
  
# Returns an array of size 256 containing count of characters 
# in the passed char array 
def getCharCountArray(string): 
    count = [0] * NO_OF_CHARS 
    for i in string: 
        count[ord(i)] += 1
    return count 
  
# removeDirtyChars takes two string as arguments: First 
# string (str)  is the one from where function removes dirty 
# characters. Second  string is the string which contain all 
# dirty characters which need to be removed  from first string 
def removeDirtyChars(string, mask_string): 
    count = getCharCountArray(mask_string) 
    ip_ind = 0
    res_ind = 0
    temp = '' 
    str_list = toList(string) 
  
    while ip_ind != len(str_list): 
        temp = str_list[ip_ind] 
        if count[ord(temp)] == 0: 
            str_list[res_ind] = str_list[ip_ind] 
            res_ind += 1
        ip_ind+=1
  
    # After above step string is ngring. 
     # Removing extra "iittg" after string 
    return toString(str_list[0:res_ind]) 
  
# Driver function to test the above functions 
mask_string = "mask"
string = "geeksforgeeks"
print removeDirtyChars(string, mask_string) 
  
# This code is contributed by Bhavya Jain 

C#

// C# program to remove  
// duplicates, the order  
// of characters is not  
// maintained in this program 
using System; 
class GFG  
{ 
    static int NO_OF_CHARS = 256; 
      
    /* Returns an array of size  
    256 containg count of  
    characters in the passed  
    char array */
    static int[] getCharCountArray(String str) 
    { 
        int[] count = new int[NO_OF_CHARS]; 
        for (int i = 0; i < str.Length; i++) 
            count[str[i]]++; 
              
        return count; 
    } 
      
    /* removeDirtyChars takes two 
    string as arguments: First 
    string (str) is the one from  
    where function removes dirty 
    characters. Second string is  
    the string which contain all 
    dirty characters which need  
    to be removed from first string */
    static String removeDirtyChars(String str,  
                                   String mask_str) 
    { 
    int[] count = getCharCountArray(mask_str); 
    int ip_ind = 0, res_ind = 0; 
      
    char[] arr = str.ToCharArray(); 
      
    while (ip_ind != arr.Length) 
    { 
        char temp = arr[ip_ind]; 
        if(count[temp] == 0) 
        { 
            arr[res_ind] = arr[ip_ind]; 
            res_ind ++; 
        } 
        ip_ind++; 
          
    }  
  
    str = new String(arr); 
      
    /* After above step string  
    is ngring. Removing extra  
    "iittg" after string*/
    return str.Substring(0, res_ind); 
    } 
      
    // Driver Code 
    public static void Main() 
    { 
        String str = "geeksforgeeks"; 
        String mask_str = "mask"; 
        Console.WriteLine(removeDirtyChars(str, mask_str)); 
    } 
} 
  
// This code is contributed by mits 

Output:

geeforgee

Time Complexity: O(m+n) Where m is the length of mask string and n is the length of the input string.

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My Personal Notes

We strongly recommend that you click here and practice it, before moving on to the solution.

C

Java

Python

C#

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