C++ Day 38

STL Algorithms (Practical & Real-World Usage)

Until now, you have learned about loops, arrays, vectors, functions, and STL containers.
From today, you move into the real power of C++, where you stop writing long loops again and again and instead use ready-made logic provided by the Standard Template Library (STL).

These are called STL Algorithms.

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1. What are STL Algorithms?

STL algorithms are predefined functions that perform common operations like:

• searching

• sorting

• counting

• modifying data

• checking conditions

They work on ranges, not directly on containers.

A range is defined using:

container.begin(), container.end()

This design makes algorithms container-independent.

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2. Why Use STL Algorithms Instead of Loops?

Consider this loop:

int sum = 0;
for(int i = 0; i < v.size(); i++) {
    sum += v[i];
}

Now compare with:

int sum = accumulate(v.begin(), v.end(), 0);

Advantages:

• Less code

• Fewer bugs

• Easy to read

• Optimized by compiler

• Used in real projects and interviews

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3. Required Header Files

Most algorithms:

#include <algorithm>

For numeric operations:

#include <numeric>

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4. sort() Algorithm

Sorting in Ascending Order

vector<int> v = {4, 1, 7, 2, 9};

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

Output:

1 2 4 7 9

Sorting in Descending Order

sort(v.begin(), v.end(), greater<int>());

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Sorting Using Lambda Function

Lambda allows custom logic.

sort(v.begin(), v.end(), [](int a, int b) {
    return a > b;
});

This is commonly used in competitive programming.

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5. find() Algorithm

Used to search an element.

auto it = find(v.begin(), v.end(), 7);

if(it != v.end())
    cout << "Element Found";
else
    cout << "Not Found";

find() returns an iterator, not index.

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6. count() Algorithm

Counts how many times a value appears.

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

If 2 appears 3 times, cnt = 3.

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7. count_if() Algorithm

Used when condition is needed.

Example: Count even numbers.

int evenCount = count_if(v.begin(), v.end(), [](int x) {
    return x % 2 == 0;
});

This replaces a long loop with if condition.

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8. find_if() Algorithm

Finds first element that satisfies a condition.

auto it = find_if(v.begin(), v.end(), [](int x) {
    return x > 5;
});

Useful when exact value is not known.

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9. accumulate() Algorithm

Used to calculate:

• sum

• product

• custom accumulation

Sum of Elements

int sum = accumulate(v.begin(), v.end(), 0);

Product of Elements

int product = accumulate(v.begin(), v.end(), 1, multiplies<int>());

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10. max_element() and min_element()

int mx = *max_element(v.begin(), v.end());
int mn = *min_element(v.begin(), v.end());

Returns iterator, so * is required.

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11. reverse() Algorithm

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

No need to write swapping logic manually.

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12. for_each() Algorithm

Used to apply an operation to every element.

for_each(v.begin(), v.end(), [](int x) {
    cout << x << " ";
});

Common in functional-style coding.

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13. all_of(), any_of(), none_of()

Check if all elements are positive

bool result = all_of(v.begin(), v.end(), [](int x) {
    return x > 0;
});

Check if any element is negative

bool result = any_of(v.begin(), v.end(), [](int x) {
    return x < 0;
});

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14. Real-World Thinking

STL algorithms help you:

• write cleaner production code

• perform faster coding in contests

• clear interviews easily

• avoid logical mistakes

In professional C++ code, manual loops are avoided unless necessary.

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15. Common Mistakes to Avoid

• Forgetting #include <algorithm>

• Forgetting iterators work on ranges

• Forgetting algorithms return iterators

• Writing loops when algorithms already exist

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16. Summary (Day 38)

Today you learned:

• What STL algorithms are

• Why they are important

• How to use sort, find, count, accumulate

• How lambda functions simplify logic

• How to write clean, professional C++ code