Question

In a single cpp file, provide efficient implementations for the following functions:

// Precondition: none

// Postcondition: the integer returned indicates where letter appears in target string,

// with the least significant bit set if letter appears in the last

// position in the string, the second least significant bit set if

// letter appears in the second to last position in the string, etc

int get_mask(string word, char letter);

Example: If word = "abcfab", get_mask(word,'a') should return 34, get_mask(word,'e') should return 0, and get_mask(word,'f') should return 4.

// Precondition: word and mask are of the same length, and mask either has

// the same contents as word, or a '?' where it differs from word

// Postcondition: wherever letter appears in word, the '?' in mask will be updated

// with letter

void update_mask(string word, string& mask, char letter);

Example: If word = "abcfab", mask = "??c???", after the call update_mask(word, mask,'a'), mask should equal "a?c?a?".

Note: update_mask must make a call to get_mask in order to determine where letter appears. It should not scan the string word directly!

// Precondition: word and mask are of the same length

// Postcondition: return true if word and mask have the same contents, and false otherwise

bool guessed(string word, string mask);

Write a main method that contains a robust set of tests and uses assert for each test.

Enforce the preconditions with assert statements.

Answer 1

Code:

#include<iostream>
using namespace std;

int get_mask(string word,char letter){
   int cnt = 0;
   for(int i=0;i<word.length();i++){
       if(word[i] == letter){
           //Using the bitwise OR operator
           //to form the binary number
           //The power at index i will be 2*(len-i-1)
           cnt = cnt | (1 << (word.length() - i - 1));
       }
   }
   return cnt;
}

void update_mask(string word,string& mask,char letter){
   //Return if the lengths are not equal
   if(word.length() != mask.length())
       return;
   int position = get_mask(word,letter);
   if(position == 0)
       return;
  
   int idx = mask.length() - 1;
      
   while(position!=0){
       //If the current bit is 1, then if it is ? in the mask string
       //then we can insert the letter at that index.
       if(position & 1){
           if(mask[idx] == '?')
               mask[idx] = letter;
       }
       idx--;
       position = position >> 1;
   }
}

bool guessed(string word,string mask){
   if(word.length() != mask.length())
       return false;
   //If all the corresponding letters are same, then return true
   //else return false  
   for(int i=0;i<word.length();i++){
       if(word[i] != mask[i])
           return false;
   }
   return true;
}

int main(){
   string word = "abcfab";
   string mask = "??c???";
   cout<<"get_mask(word,'a') = "<<get_mask(word,'a')<<endl;
   cout<<"get_mask(word,'e') = "<<get_mask(word,'e')<<endl;
   cout<<"get_mask(word,'f') = "<<get_mask(word,'f')<<endl;
   update_mask(word, mask,'a');
   cout<<"update_mask(word, mask,'a') = "<<mask<<endl;
   update_mask(word, mask,'e');
   mask = "??c???";
   cout<<"update_mask(word, mask,'e') = "<<mask<<endl;
}

Output:

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