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Most Critical Mistakes & Tips in Competitive Programming

  • Last Updated : 30 Nov, 2021

The moment a beginner hears this word, an image pops up in one’s mind where students are sitting in a room full of Computers and coding some out of the world stuff, We’ll to be honest it’s nothing too hard to grasp so we will be proposing tips that help a programmer to level up faster. So let’s begin with the most underrated points on each topic where even the advanced competitive programmers slip.

Most-Critical-Mistakes-Tips-in-Competitive-Programming

So the concept of overflow can be seen in competitive coding a lot, the worst problem of not understanding it is, writing a completely perfect code but Still getting the wrong answer, this is because, at higher input ranges, the compiler strips down the result as per the data size hence it’s important to remember the basic ranges of data types.

Int => [-109  To 109]
Long Int / long => [-1012,1012]
Long Long Int / long long => [-1018,10^18]

Example:



C++




// C++ Program, to Illustrate General Tips In Competitive
// Programming
 
// Importing all C++ libraries
#include <bits/stdc++.h>
 
using namespace std;
 
// Main method
int main()
{
 
    // Case: Stack Overflow
    int a = 100000, b = 100000;
    // Expected answer ? 10^10 but
    // no the answer you get is 1410065408, error in
    // precision.
    int c = a * b;
    long long int d = a * b;
 
    // Note: Still error will be generated bacause
    // calcultation was done on two ints which were later
    // converted into long long ie they already
    // lost their data before converting
 
    // Print and display on console
    cout << c << " " << d << endl;
 
    // Now if we store a value more than its capacity then
    // what happens is the number line of range of value
    // bacomes a number
    // Example: Circle, If min val is 1 and max is 9, so if
    // we add 1 to 9 it will result in 1, it looped back to
    // starting, this is overflow
 
    int p = INT_MAX;
 
    // An example of overflow
    cout << "An example of overflow " << p + 1 << endl;
 
    // Long long is way better than double,
    // double although can store more than long long but
    // in exchange it will cost you your precision.
    // We can simply use the below command as follows:
    // cout<<fixed(no scientific
    // notation)<<setprecision(0){removes decimal}<<variable
    // to give value same form as long long
 
    // Question where the answer came wrong coz of
    // overflow!!
    int t, n;
    cin >> t;
    while (t--) {
        cin >> n;
 
        // Here, before i used int and got wrong answer,
        // then made it long long
        long long pdt = 1, temp;
 
        for (int i = 0; i < n; i++) {
            cin >> temp;
            pdt *= temp;
        }
 
        if (pdt % 10 == 2 || pdt % 10 == 3 || pdt % 10 == 5)
            cout << "YES" << endl;
 
        else
            cout << "NO" << endl;
    }
}
Output
1410065408 1410065408
An example of overflow -2147483648

Tips For Data Structures

Tip 1: In competitive programming always declare the array globally when you need to have a function along with the main function, globally declared arrays can have a size of 10^7 as they are stored in Data Segments.

Tip 2: Whenever you declare a function where you need to return multiple items, or even in general where your goal is to return an array after some modifications, always go for Pass By Reference (eg void swap(int &a, int &b)), it helps you in two ways one, by reducing the time in making a copy of the data and then processing on it and second, as you’re directly working on the main data you can modify as many values as you wish without having to worry about returning the values and dealing with them.

  • The limit of the size of an array declared inside the main method is 10^5, coz it’s stored in Stack Memory with a memory limit of around 8MB, if the size is more than this then it results in Stack Overflow.
  • A very peculiar error occurs sometimes when you’re taking multiple strings as input after a cin input. instead of taking required inputs, it automatically takes space as input then takes the remaining two strings, to resolve this use cin.ignore() before using the getline() to take the string as input.
  • while forming a new string from another string, instead of using syntax like str = str + str2[I], ie adding character to get a string, this method’s time complexity is O(size(string)), Hence listed of adding characters to a string we use push_back(character) whose time complexity is O(n).

Tips on STL

As we already know by far now that STL stands for Standard Template Library in C++, which is a boon for competitive programmers, that contains inbuilt special Data Structures and Algorithm that reduces and optimize your code like Crazy. DO remember following below listed STL data structures tips as follows:

Tip 1: Whenever we need to sort data in a question and remove duplicates, the best practice is to store it in a Set, it automatically sorts the data and as per the property of Set, It only stores unique values.

Tip 2: In questions where we need to find the frequency of all values, the best practice is to use a Map, The property of Map makes it automatically arrange the data in lexicographical order, and just with a little trick, you’ll have the most efficient solution.

Example

C++




// C++ Program to Illustrate STL Algorithms Tips
 
// Importing all C++ libraries
// Standard command
#include <bits/stdc++.h>
 
using namespace std;
 
// Main method
// From here the code starts executing
int main()
{
 
    // Creating vector and initializing objects
    // by passing custom integer numbers
    vector<int> v
        = { 1, 4, 2, 5, 6, 3, 9, 0, 10, 3, 15, 17, 3 };
 
    // Operation 1
    // Finding the minimum element in the array,
 
    // Note: It returns the pointer/iterator
    auto min = min_element(v.begin(), v.end());
 
    // Print and display elements on console
    cout << "Min element: " << *min << endl;
 
    // Operation 2
    // // Finding the maximum  element in the array
    auto max = max_element(v.begin(), v.end());
    cout << "Max Element: " << *max << endl;
 
    // Operation 3
    // Sum of all elements, the third parameter tells us
    // initial sum ki value kya hai.
    auto sum = accumulate(v.begin(), v.end(), 0);
    cout << "The sum of all elements: " << sum << endl;
 
    // Operation 4
    // Count the frequency of an element in the array/vector
    auto cnt = count(v.begin(), v.end(), 3);
    cout << "Frequency Of 3 is: " << cnt << endl;
 
    // Operation 5
    // Finds an element and returns its pointer
    auto elem = find(v.begin(), v.end(), 3);
 
    if (elem != v.end())
        cout << "the element is at posn: " << *elem << endl;
    else
        cout << "Element not found" << endl;
 
    // Operation 6
    // Reversing a string or array/vector
    // using reverse method
    reverse(v.begin(), v.end());
 
    cout << "Reversed Vector: ";
 
    for (auto val : v) {
        cout << val << " ";
    }
 
    // New line for better readability
    cout << endl;
 
    // Operation 7
    // Sort array/vector using sort() method
    sort(v.begin(), v.end());
    cout << "Sorted Vector: ";
    for (auto val : v) {
        cout << val << " ";
    }
}
Output
Min element: 0
Max Element: 17
The sum of all elements: 78
Frequency Of 3 is: 3
the element is at posn: 3
Reversed Vector: 3 17 15 3 10 0 9 3 6 5 2 4 1 
Sorted Vector: 0 1 2 3 3 3 4 5 6 9 10 15 17 

Note: These are some popular algorithms that can make your life easy all have the same pattern of usage ie the name of the function

(lower lim, upper lim+1)




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