Library Functions - aakashpahwa/iecse-code-winter-resources-18 GitHub Wiki

A good understanding of the library functions is important for writing quick and bug-free code. As you dive into competitive programming you will come across many useful functions. Some of which are given below to get you started.

Imports

Imports for C++ STL

#include<bits/stdc++.h>

Imports for Java Utility Package

import java.util.*;
import java.io.*; // Classes for Fast I/O implementation in Java

Strings

After PSUC , you are probably used to declaring a string using an array of chars. C++ offers a string class that makes string handling easier.

The following code snippet shows how to declaring and use a string :

#include<bits/stdc++.h>
using namespace std;
int main()
{
  string str = "Coding is fun";   // Declaring and initialising string

  int len = str.length(); // the length() function returns the length of the string

  cout<<"The length of string "<<str<<" is "<<len<<endl;

  return 0;
}

Java equivalent

import java.util.*;
public class Solution{
	public static void main(String[] args){
		String str = "Coding is fun";

		int len = str.length();

		System.out.println("The length of the string "+str+" is "+len);
	}
}

Inputting , Concatenating , Comparing Strings

A major advantage of using string class instead of character array is that you don't need to know the size of the string while declaring it.

Concatenating strings is also easier with the string class. You can concatenate two strings using the '+' operator.

For comparing if two strings are equal , you can either use the relational operators('<' , '>' , '==' , "!="). Another way is using the compare function.

#include<bits/stdc++.h>
using namespace std;
int main()
{
  // Input

  string str1;   // Declaring a string
  cin>>str1; // Taking user input for the string

  int len = str1.length(); // the length() function returns the length of the string
  cout<<"The length of string "<<str1<<" is "<<len<<endl;

  // Concatenating

  string s1 = "Hello " , s2 = "World" , s3;
  s3 = s1 + s2; // Concatenating the strings
  cout<<s3<<endl;

  //Referencing strings elements

  for(int i=0 , n=s3.length() ; i < n ; i++)
  {
    cout<<"Char "<<i+1<<" of s3 is "<<s3[i]<<endl; // Referencing ith element of a string
  }

  // Comparing strings

  string s4 = "aaa" , s5 = "aab"; // s5 is greater than s4

  if(s4 <= s5) cout<<"s4 is less than s5\n"; // Use the <= operator to compare strings instead strcomp()

  if(!(s4.compare(s5)) cout<<"s4 and s5 are not equal\n"; // s4.compare(s5) returns 0 if s4 and s5 are equal

  string s6=""; // declare an empty string

  for(int i=0 ; i < 26 ; i++)
  {
    s6 += (i + 'a'); // Concatenation using compound assignemnt ( += ) also works
  }

  cout<<s6<<endl;
  return 0;
}

Java equivalent

import java.util.*;
public class Solution{
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        String str = in.next();
        
        int len = str.length();

        System.out.println("The length of the string "+str+" is "+len);
        
        String s1 = "Hello",s2 = "World",s3;

        s3 = s1 + s2; // Concatenating strings

        System.out.println(s3);
        
        for(int i=0;i<s3.length();i++){
            // ith element 
            System.out.println("Char at "+(i+1)+" of s3 is "+s3.charAt(i));
        }
        
        String s4 = "aaa" , s5 = "aab";
        
        // comparing using compareTo()
        
        if(s5.compareTo(s4)>0)
            System.out.println("s5 greater than s4");
        else if(s5.compareTo(s4)<0)
            System.out.println("s5 less than s4");
        else
            System.out.println("s5 equal to s4");
            
        String s6 = "";

        for(int i=0;i<26;i++){
            s6 += (char)(i+'a'); // shorthand operator +=
        }

        System.out.println(s6);
    }
}

Vectors

A vector is a container i.e. something you can use to store many elements together. It is similar to an array but with added functionalities.

You don't need to know the size of a vector when declaring it. You can also get a vector's length and resize a vector at runtime.

#include<bits/stdc++.h>
using namespace std;
int main()
{
  vector<int> v1(10); // this is a declaration for a vector of integers that has size 10. Note that unlike an array declaration, we use ()
                      // instead of []
  for(int i=0 ; i < 10 ; i++)
    v1[i] = i;
  }

  for(int i=0 ; i < 10 ; i++)
  {
    cout<< v1[i] << " ";
  }
  cout<< endl;

  vector<int> v2; // Here, we are not declaring the size of the vector

  int len = v2.size(); // v.size() returns the size of the vector
  cout<<"Initial size of v2 is "<<len<<endl; // Output is 0, because we have not specified the size of the vector yet.

  {
    for(int i=0 ; i < 15 ; i++)
    v2.push_back(i); // Since v[i] is not a valid index yet, we use the push_back() function to insert an element into the vector
  }

  for(int i=0 ; i < 15 ; i++) cout<<v2[i]<<" ";
  cout<<endl;
  int len1 = v1.size() , len2 = v2.size();
  cout<<"Sizes of the two vectors are "<<len1<<" and "<<len2<<endl;

}
return 0;

You can use the empty() function to check if a vector is empty.

The resize function can be sued to resize a vector one it is filled upto its current capacity.

The clear function clears the vector.

#include<bits/stdc++.h>
using namespace std;
int main()
{
  vector<string> v1(10,"Hello"); // Declaring a vector of of size 10 that holds strings.Each index in the vector
                                 // is initialized to the value "Hello"
  for(int i=0 , n = v1.size() ; i<10 ; i++)
  {
    cout<<"Replacing "<<v1[i]<<" with ";
    v1[i] = "World";
    cout<<v1[i]<<endl;
  }

  v1.resize(20); // Resizing the vector to 20

  for(int i=10 ; i<19 ; i++) v1[i] = "Hello";
  vector<string> v2 = v1 ; // Here v2 is initialized with the same size and values as v1

  for(int i=0 , n = v2.size() ; i<n ; i++) cout<<v2[i]<<" ";
  cout<<endl;

  if(!v1.empty()) cout<<"Length of v2 is "<<v2.size()<<endl; // v1.empty() returns 1/true if the vector is empty, else it returns 0/false

  v2.clear(); // After clearing the vector v2 ,  the outut of v.size() is 0

  cout<<"Size of v2 after clearing is "<<v2.size()<<endl;
  return 0;
}

Java equivalent is ArrayLists

import java.util.*;
public class Solution{
    public static void main(String[] args){
        ArrayList<Integer> v1 = new ArrayList<Integer>(Arrays.asList(new Integer[10]));

        for(int i=0;i<10;i++){
            v1.set(i,i);
        }

        for(int i=0;i<10;i++){
            System.out.println(v1.get(i));
        }

        ArrayList<Integer> v2 = new ArrayList<Integer>();

        System.out.println("Initial size is "+v2.size());

        for(int i=0;i<15;i++){
            v2.add(i); // cannot use set() as they cannot be indexed yet
        }

        for(int i=0;i<15;i++){
            System.out.println(v2.get(i));
        }

        System.out.println("Length of v1 : "+v1.size()+" and Length of v2 : "+v2.size());

        v2.ensureCapacity(50); // resize to 50

        if(!v2.isEmpty()) // isEmpty() function to check if arraylist is empty
            System.out.println("Size of v2 is "+v2.size());

        v2.clear(); // clear arraylist

        System.out.println("Size of v2 is now : "+v2.size());
    }
}

Some Useful Functions

Sorting

C++ provides the sort(pointer_to_first_ele , pointer_to_last_ele) function to sort arrays and vectors. The sort function is more efficient than sorting algorithms like bubble sort. It is also easier to use.

  int a[5] = {5 , 4 , 3 , 2 , 1};
  sort(a , a + n); // to sort an array we pass the pointer to the first and last element of the array

  vector<int> v(10);

  for(int i=10 ; i > -1 ; i--) v[i] = i;
  sort(v.begin() , v.end()) // Here also, we pass the pointer to the first and last element in the vector.
                           // v.begin() returns the pointer to the first element and v.end() returns the pointer to the last element

Java equivalent is using Arrays.sort() and Collections.sort()

   int a[] = {5 , 4 , 3 , 2 , 1};

   Arrays.sort(a); // sort an array

   for(int i=0;i<5;i++)
       System.out.println(a[i]);

   ArrayList<Integer> v = new ArrayList<Integer>();

   for(int i=4;i>=0;i--) v.add(i);

   Collections.sort(v); // sort an arraylist

   for(int i=0;i<5;i++)
       System.out.println(v.get(i));

Min, Max

The min and max functions can be used to find the minimum and maximum of multiple numbers without using many if else statements.

int a=1 , b=2 , c=3 , d=4;
int maximum = max(max(a , b) , max(c , d)); // maximum of four numbers
int minimum = min(min(a , b) , min(c , d)); // minimum of four numbers

Java equivalent

int a = 1,b = 2,c = 3,d = 4;
int maximum = Math.max( Math.max(a,b) , Math.max(c,d) );
int minimum = Math.min( Math.min(a,b) , Math.min(c,d) );

To use max(), min() directly you can use static imports

import static java.lang.Math.*;

Further Reading

A great place to get started would be geeksforgeeks, links for which are provided below.

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