Strings

find, npos, and substr patterns

• find returns string::npos when not found — always check before using the index.
• substr(pos) takes from pos to the end; substr(pos, n) takes up to n characters.
• Chaining find + substr is the usual “parse email / path / CSV field” pattern.
• find(sub, startPos) continues searching from an offset — useful for counting occurrences.

string s = "one,two,three";
size_t start = 0, comma;
while ((comma = s.find(',', start)) != string::npos) {
    cout << s.substr(start, comma - start) << "\n";
    start = comma + 1;
}
cout << s.substr(start) << "\n"; // last field

Capacity, SSO, and performance notes

• size() is length; capacity() is allocated storage — capacity may be larger than size.
• Many implementations use **SSO** (small string optimization): short strings stay on the stack.
• reserve(n) before many appends can reduce reallocations.
• Prefer += / append in a loop over repeated s = s + x for large builds (fewer temporaries).
• Passing const string& avoids copies; returning by value is fine (move/RVO).

string_view (C++17)

• string_view is a **non-owning** view: pointer + length — great for read-only APIs.
• Do **not** keep a string_view after the underlying string dies (dangling).
• Useful to accept both string and string literals without forcing a copy into string.

#include <string_view>
using namespace std;

size_t len(string_view sv) { return sv.size(); }

// len("hello"); len(string("hi"));

Compare, sort order, and case

• == checks equality; < / compare use lexicographic (dictionary) order by character codes.
• Uppercase letters usually sort before lowercase in ASCII ('A' < 'a').
• Case-insensitive compare is not built-in — normalize with a loop or algorithm, or use a library.
• compare returns 0 if equal, negative if *this is less, positive if greater.

string a = "Apple", b = "Banana";
if (a < b) cout << "Apple comes first\n";
cout << a.compare(b) << "\n"; // negative

Real-life advanced models

• URL path segments: split on '/' with repeated find.
• Mask a card number: keep last 4 chars, replace the rest with *.
• Slug for a blog title: replace spaces with -, erase punctuation.
• Validate password: length, and find_first_of for digits / special characters.

Create, append, and print length

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

int main() {
    string s = "Learn";
    s += " Tech";
    s.append(" Guru");
    cout << s << "\nLength = " << s.size() << "\n";
    return 0;
}

getline — full name with spaces

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

int main() {
    string name;
    cout << "Full name: ";
    getline(cin, name);
    cout << "Hello, " << name << "!\n";
    return 0;
}

find + substr — parse email

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

int main() {
    string email = " intern@learntechguru.com ";
    // trim spaces simply for demo:
    while (!email.empty() && email.front() == ' ') email.erase(0, 1);
    while (!email.empty() && email.back() == ' ') email.pop_back();

    auto at = email.find('@');
    if (at == string::npos) {
        cout << "Invalid email\n";
        return 0;
    }
    cout << "User: " << email.substr(0, at) << "\n";
    cout << "Domain: " << email.substr(at + 1) << "\n";
    return 0;
}

insert / erase / replace

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

int main() {
    string s = "Hello World";
    s.insert(5, ",");
    cout << s << "\n";
    s.erase(5, 1);
    cout << s << "\n";
    s.replace(6, 5, "C++");
    cout << s << "\n";
    return 0;
}

Count vowels with indexing

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

int main() {
    string s = "Education";
    int vowels = 0;
    for (char ch : s) {
        char c = static_cast<char>(tolower(static_cast<unsigned char>(ch)));
        if (c=='a'||c=='e'||c=='i'||c=='o'||c=='u') vowels++;
    }
    cout << "Vowels = " << vowels << "\n";
    return 0;
}

stoi / to_string conversion

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

int main() {
    string priceText = "499";
    int price = stoi(priceText);
    int total = price * 2;
    cout << "Total = " << to_string(total) << "\n";
    return 0;
}

Password checks with library functions

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

int main() {
    string pass = "Abcd@123";
    bool okLen = pass.size() >= 8;
    bool hasDigit = pass.find_first_of("0123456789") != string::npos;
    bool hasSpecial = pass.find_first_of("@#$%&*") != string::npos;
    cout << (okLen && hasDigit && hasSpecial ? "Strong\n" : "Weak\n");
    return 0;
}