C++ Arrays and Strings: Complete Guide With Examples, std::vector, std::string & DSA (2025)

#include <iostream>
using namespace std;

int main() {
    // 1. Declare, then assign
    int arr[5];
    arr[0]=10; arr[1]=20; arr[2]=30; arr[3]=40; arr[4]=50;

    // 2. Initialize at declaration
    int primes[5] = {2, 3, 5, 7, 11};

    // 3. Omit size — deduced from initializer
    double temps[] = {98.6, 99.1, 100.2};

    // 4. C++11 uniform initialization
    char grades[3]{'A', 'B', 'C'};

    // 5. Partial init — rest zeroed
    int partial[5] = {1, 2};  // {1, 2, 0, 0, 0}

    // 6. Zero-initialize entire array
    int zeroes[5] = {};  // all 0

    // Range-based for (C++11)
    cout << "Primes: ";
    for (int p : primes) cout << p << " ";
    cout << endl;

    // Classic for loop
    cout << "arr: ";
    for (int i = 0; i < 5; i++) cout << arr[i] << " ";
    cout << endl;

    return 0;
}

int arr[5] = {10,20,30,40,50};

// arr decays to &arr[0]
cout << arr       << endl;   // address of first element
cout << &arr[0]   << endl;   // same address

// arr[i] == *(arr + i)
cout << arr[2]       << endl;   // 30
cout << *(arr + 2)   << endl;   // 30 — pointer arithmetic

// Passing array to function — decays to pointer, size lost!
void process(int* arr, int n) {  // must pass size separately
    for (int i = 0; i < n; i++) cout << arr[i] << " ";
}

#include <iostream>
using namespace std;

int main() {
    int A[3][3] = {{1,2,3},{4,5,6},{7,8,9}};
    int B[3][3] = {{9,8,7},{6,5,4},{3,2,1}};
    int C[3][3] = {};
    for (int i=0; i<3; i++)
        for (int j=0; j<3; j++)
            C[i][j] = A[i][j] + B[i][j];
    cout << "A + B:\n";
    for (int i=0; i<3; i++) {
        for (int j=0; j<3; j++) cout << C[i][j] << " ";
        cout << endl;
    }
    return 0;
}

#include <iostream>
#include <string>
#include <cstring>
using namespace std;

int main() {
    char cs1[] = "Hello";
    char cs2[20];
    strcpy(cs2, cs1);
    strcat(cs2, " World");
    cout << cs2 << endl;
    cout << strlen(cs1) << endl;
    cout << strcmp(cs1, "Hello") << endl;

    string s1 = "Hello";
    string s2 = s1 + " World";
    cout << s2 << endl;
    cout << s1.length() << endl;
    cout << (s1 == "Hello") << endl;

    const char* to_cs = s1.c_str();
    cout << to_cs << endl;
    return 0;
}

#include <iostream>
#include <string>
using namespace std;

int main() {
    string word, full_name;
    int age;

    cout << "Enter a word: ";
    cin >> word;
    cout << "Word: " << word << endl;

    cin.ignore();  // discard leftover \n

    cout << "Enter full name: ";
    getline(cin, full_name);
    cout << "Full name: " << full_name << endl;

    cout << "Enter age: ";
    cin >> age;
    cin.ignore();

    cout << "Enter message: ";
    string msg;
    getline(cin, msg);
    cout << "Age: " << age << ", Message: " << msg << endl;

    return 0;
}

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int binarySearch(const vector<int>& v, int target) {
    int lo = 0, hi = v.size() - 1;
    while (lo <= hi) {
        int mid = lo + (hi - lo) / 2;
        if (v[mid] == target) return mid;
        if (v[mid] < target)  lo = mid + 1;
        else                   hi = mid - 1;
    }
    return -1;
}

int main() {
    vector<int> v = {1,3,5,7,9,11,15};
    cout << "Index of 7: " << binarySearch(v, 7) << endl;
    cout << "Index of 6: " << binarySearch(v, 6) << endl;
    cout << boolalpha << binary_search(v.begin(), v.end(), 9) << endl;
    auto it = lower_bound(v.begin(), v.end(), 9);
    cout << "lower_bound(9) at index: " << it - v.begin() << endl;
    return 0;
}

#include <iostream>
#include <string>
#include <algorithm>
#include <cctype>
using namespace std;

bool isPalindrome(const string& s) {
    int l = 0, r = (int)s.length() - 1;
    while (l < r) {
        if (s[l] != s[r]) return false;
        l++; r--;
    }
    return true;
}

bool isPalindromeClean(const string& s) {
    int l = 0, r = (int)s.length() - 1;
    while (l < r) {
        while (l < r && !isalnum(s[l])) l++;
        while (l < r && !isalnum(s[r])) r--;
        if (tolower(s[l]) != tolower(s[r])) return false;
        l++; r--;
    }
    return true;
}

int main() {
    cout << boolalpha;
    cout << isPalindrome("racecar") << endl;
    cout << isPalindrome("hello") << endl;
    cout << isPalindromeClean("A man, a plan, a canal: Panama") << endl;
    string s = "Hello World";
    reverse(s.begin(), s.end());
    cout << s << endl;
    return 0;
}

#include <iostream>
#include <vector>
using namespace std;

int rangeSum(const vector<int>& pre, int l, int r) {
    return pre[r] - (l > 0 ? pre[l-1] : 0);
}

int maxSumK(const vector<int>& arr, int k) {
    int n = arr.size(), window = 0, maxSum = 0;
    for (int i=0; i<k; i++) window += arr[i];
    maxSum = window;
    for (int i=k; i<n; i++) {
        window += arr[i] - arr[i-k];
        maxSum = max(maxSum, window);
    }
    return maxSum;
}

int main() {
    vector<int> arr = {3,1,4,1,5,9,2,6};
    int n = arr.size();
    vector<int> pre(n);
    pre[0] = arr[0];
    for (int i=1; i<n; i++) pre[i] = pre[i-1] + arr[i];
    cout << "Sum [2,5]: " << rangeSum(pre, 2, 5) << endl;
    cout << "Max sum k=3: " << maxSumK(arr, 3) << endl;
    return 0;
}

#include <iostream>
#include <string>
using namespace std;

bool isAnagram(const string& s, const string& t) {
    if (s.length() != t.length()) return false;
    int freq[26] = {};
    for (char c : s) freq[c - 'a']++;
    for (char c : t) freq[c - 'a']--;
    for (int f : freq) if (f != 0) return false;
    return true;
}

int main() {
    cout << boolalpha;
    cout << isAnagram("listen", "silent") << endl;
    cout << isAnagram("hello",  "world")  << endl;
    cout << isAnagram("rat",    "car")    << endl;
    return 0;
}