lexicographical_compare() in C++

C++ STL offer many utilities to solve basic common life problems. Comparing values are always necessary, but sometimes we need to compare the strings also. Therefore, this article aims at explaining about “lexicographical_compare()” that allows to compare strings. This function is defined in “algorithm” header. It has two implementations.

Implementation 1 : lexicographical_compare(iter1 beg1, iter1 end1, iter2 beg2, iter2 end2)

Template:
template 
  bool lexicographical_compare(iter1 beg1, iter1 end1, 
                               iter2 beg2, iter2 end2)
{
  while (beg1!=end1)
  {
    if (beg2==end2 || *beg2<*beg1) return false;
    else if (*beg1<*beg2) return true;
    ++beg1; ++beg2;
  }
  return (beg2!=end2);
}
Parameters : 
beg1 :  Input iterator to initial position of first sequence.
end1 :  Input iterator to final position of first sequence.

beg2 :  Input iterator to initial position of second sequence.
end2 :  Input iterator to final position of second sequence.

Return value : 
Returns a boolean true, if range1 is strictly lexicographically 
smaller than range2 else returns a false.



// C++ code to demonstrate the working of
// lexicographical_compare(), implementation 1
#include <iostream>
#include <algorithm> // for lexicographical_compare()
using namespace std;

int main()
{
    // initializing char arrays
    char one[] = "geeksforgeeks";
    char two[] = "gfg";

    // using lexicographical_compare for checking
    // is "one" is less than "two"
    if (lexicographical_compare(one, one + 13, two, two + 3)) 
        cout << "geeksforgeeks is lexicographically less "
                                                 "than gfg";
    else 
        cout << "geeksforgeeks is not lexicographically "
                                         "less than gfg";    
}

Output:

geeksforgeeks is lexicographically less than gfg

Implementation 2 : lexicographical_compare(iter1 beg1, iter1 end1, iter2 beg2, iter2 end2, Compare comp)

Template:
template 
  bool lexicographical_compare(iter1 beg1, iter1 end1, 
                               iter2 beg2, iter2 end2)
{
  while (beg1!=end1)
  {
    if (beg2==end2 || *beg2<*beg1) return false;
    else if (*beg1<*beg2) return true;
    ++beg1; ++beg2;
  }
  return (beg2!=end2);
}
Parameters : 
beg1 :  Input iterator to initial position of first sequence.
end1 :  Input iterator to final position of first sequence.

beg2 :  Input iterator to initial position of second sequence.
end2 :  Input iterator to final position of second sequence.

comp : The comparator function that returns a boolean
true/false of the each elements compared. This function 
accepts two arguments. This can be function pointer or 
function object and cannot change values.

Return value : 
Returns a boolean true, if range1 is strictly lexicographically smaller 
than range2 else returns a false.

// C++ code to demonstrate the working of
// lexicographical_compare(), implementation 2

#include <iostream>
#include <algorithm> // for lexicographical_compare()
using namespace std;

// helper function to convert all into lower case:
bool comp(char s1, char s2)
{
    return tolower(s1) < tolower(s2);
}

int main()
{
    // initializing char arrays
    char one[] = "geeksforgeeks";
    char two[] = "Gfg";

    // using lexicographical_compare for checking
    // is "one" is less than "two"
    // returns false as "g" has larger ASCII value than "G"
    if (lexicographical_compare(one, one + 13, two, two + 3)) 
        cout << "geeksforgeeks is lexicographically less "
                                             "than Gfg\n";
    else 
        cout << "geeksforgeeks is not lexicographically "
                                        "less than Gfg\n";
    

    // using lexicographical_compare for checking
    // is "one" is less than "two"
    // returns true this time as all converted into lowercase
    if (lexicographical_compare(one, one + 13, two, two + 3, comp)) {
        cout << "geeksforgeeks is lexicographically less  ";
        cout << "than Gfg( case-insensitive )";

    } else {
        cout << "geeksforgeeks is not lexicographically less ";
        cout << "than Gfg( case-insensitive )";
    }
}

Output:



geeksforgeeks is not lexicographically less than Gfg
geeksforgeeks is lexicographically less than Gfg( case-insensitive )

Possible application : Comparing strings can be generally used in dictionary, where we need to place words in lexicographical order. Example of this can be to find the word which occurs 1st in dictionary among given set of words.

// C++ code to demonstrate the application of 
// lexicographical_compare()
#include<bits/stdc++.h>
using namespace std;

int main()
{
    // initializing char arrays
    char list[][100]={
        {'a','b','a','c','u','s'},
        {'a','p','p','l','e'},
        {'c','a','r'},
        {'a','b','b','a'}
    };
    
    char min[100] = "zzzzzz";
    
    // using lexicographical_compare for checking 
    // the smallest
    for (int i=0; i<4; i++)
        if( lexicographical_compare(list[i], list[i]
                + strlen(list[i]), min, min+strlen(min)))       
         strcpy(min,list[i]);
    
    // prints "abacus"
    cout << "The smallest string is : ";
    for(int i = 0; min[i]!='\0'; i++)    
        cout<<min[i];     
}

Output:

The smallest string is : abacus

This article is contributed by Manjeet Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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