C++ Code Snippets & Useful Techniques

Common Operations and Functions

1. Find the Position of an Element

To find the position of an element in a string:

std::cout << s.find() << std::endl;

2. Removing Duplicates from a Vector

To remove duplicates from a vector, use the unique function:

unique(v.begin(), v.end());

3. Built-in Functions and Libraries

__builtin_popcount

Counts the number of 1’s in the XOR of two elements in an array:

__builtin_popcount(arr[i] ^ arr[k]);  // Counts the number of 1s in the XOR of arr[i] and arr[k]

Shift Operations:

• a << b is equivalent to a * b ^ b
• a >> b is equivalent to a / b ^ b

4. Accumulating Values in a Vector

To sum up the elements in a vector:

long long sum = accumulate(a.begin(), a.end(), 0ll);

5. String Terminator with c_str()

The c_str() method in C++ provides a pointer to the character array terminated with a null character ('\0'):

const char* str = s.c_str();  // Returns a C-style string

6. Using lower_bound (STL)

lower_bound is useful for finding the first element that is not less than the given value:

auto it = lower_bound(v.begin(), v.end(), value);

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String and Map Operations

7. Finding an Element in a Map

To check if an element exists in a map, you can use:

if (st.find(temp) != st.end()) {
    // Element found in map
}

Or check a specific map element:

if (!m[10]) {
    // Element not found or value is zero
}

8. Accessing Last Element of a String

To get the last element of a string:

char last = s.back();  // Returns the last character in the string

To add a value to the end of a string:

s.push_back('a');  // Adds 'a' to the end of the string

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Mathematical Operations

9. Finding Minimum from a List of Values

To find the minimum value from a list of values:

int mini = min({1, 3, 4, 5});

10. Counting Occurrences in a Vector

To count the occurrences of a specific element in a vector:

int count = count(v.begin(), v.end(), 2);

11. Combinatorics

• Permutations (nPr):

  // nPr = n! / (n - r)!

• Combinations (nCr):

  // nCr = n! / (r! * (n - r)!)
  // For the case where r doesn't matter (e.g., when selecting all elements):
  // nCr = n * (n - 1) / 2

  This formula is commonly used to find the total number of subarrays in an array of size n.

12. Sum of First N Natural Numbers

The sum of all elements from 1 to n is given by:

n * (n + 1) / 2;

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Mathematical Functions

13. Ceiling and Floor Functions

• Ceiling: ⌈a / b⌉ = (a + b - 1) / b
• Floor: ⌊a / b⌋

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Bit Manipulation

14. Exploring a Number in Bits

To explore a number in bits:

bitset<10> temp(30); 
cout << temp;  // Output the binary representation of 30

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Array and Matrix Operations

15. Traversing 2D Array in Custom Order

If you need to traverse a 2D array like a[3][2] in a custom order (e.g., traversing column-wise):

for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
        cout << a[j][i];  // Traverses the array column-wise
    }
}

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Sorting and Checking Sorted Order

16. Checking if an Array is Sorted

To check if an array is sorted:

if (is_sorted(a, a + n)) {
    cout << "Yes";  // Array is sorted
} else {
    cout << "No";   // Array is not sorted
}

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Maximum Subarray Problem

17. Finding Maximum Subarray (Sliding Window Technique)

To find the largest subarray sum with size m:

for (int i = 0; i < m; i++) {
    sum[0] += arr[i];
}
for (int i = 1; i <= (n - m); i++) {
    sum[i] = sum[i - 1];
    sum[i] -= arr[i - 1];
    sum[i] += arr[m + i - 1];
}

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Vector of Pairs

18. Storing Pairs in a Vector

To store pairs of characters and integers in a vector:

vector<pair<char, int>> vp;
vp.push_back({s[i], i});  // Adding a pair to the vector