String in c++

7 min readOct 2, 2024

C Style char

char chr = 'A';

char implicitly represents the ascii integer value. For example, the character ‘A’ is represented by the ASCII code 65. When you store ‘A’ in a char variable, it's actually storing the integer value 65.

// Method 1: Using printf with %d format specifier
printf("ASCII value of '%c' is %d\n", ch, ch);

// Method 2: Casting to int and printing
int asciiValue = (int)ch;
printf("ASCII value of '%c' is %d\n", ch, asciiValue);

// Read digit value
printf("Digit value of '%c' is %d\n", ch, ch - '0');

For example, to check if char is digit or not can be done using following.

if(chr >= '0' && chr <= '9') {
  cout <<chr <<" is a digit"<<endl;
}

We can also use std::isdigit() function from <cctype> to check if char is a digit or not.

if(isdigit(chr)) {
  cout <<chr <<" is a digit"<<endl;
}

Following are few useful methods from cctype

isalpha(c): Returns true if c is an alphabetic character (A-Z or a-z).
isdigit(c): Returns true if c is a digit (0-9).
isalnum(c): Returns true if c is an alphanumeric character (alphabetic or digit).
isspace(c): Returns true if c is a whitespace character (space, tab, newline, etc.).
isupper(c): Returns true if c is an uppercase letter.
islower(c): Returns true if c is a lowercase letter.
toupper(c): Converts c to uppercase if it is a lowercase letter.
tolower(c): Converts c to lowercase if it is an uppercase letter.
isxdigit(c): Returns true if c is a hexadecimal digit (0-9, A-F, or a-f).

C Style string

A C-style string is essentially an array of characters. However, unlike other arrays, it’s not explicitly marked with its length. Instead, the end of the string is indicated by a special character called the null character, which has an ASCII value of 0 and is represented by the character '\0'.

char str[] = "Hello World!";

Following is code to iterate chars in string.

    for (int i = 0; str[i] != '\0'; i++) {
        std::cout << str[i] << " ";
    }

strstr(): Finds the first occurrence of a substring within a C-style string. Returns a pointer to the beginning of the found substring, or NULL if not found.

char str[] = "Hello, world!";
char substr[] = "world";

char* found = strstr(str, substr);

if (found != NULL) {
    cout << "Found 'world' at index " << found - str << endl;
} else {
    cout << "'world' not found." << endl;
}

string class

We can create string and initialize as below.

string str = "Hello, world!";
string str(5,'A'); // Creates AAAAA
string str(other_str);// copy constructor

Access char

We can access char of string as below.

char c = str[3]; // access 4th char

Modify string

str.append(" there"); // Append a string
str.insert(5, "beautiful"); // Insert at index 5th
str.erase(5,2); // erase two chars starting at index 5th
str.replace(0,5,"Good bye"); // Replace chars from 0 ending at 5(excluding)

Length and comparison

int len = str.size();  // str.length() also exist
bool eq = str == other_string;
bool less = str < other_string;

reverse api

reverse() Method: Use string being and end pointer to reverse the string

string str = "Hello, world!";
reverse(str.begin(), str.end()); // mutate str

find api

find(): Finds the first occurrence of a substring within the string. Returns the starting index of the substring if found, otherwise std::string::npos. Can optionally specify the starting position for the search.

string str = "Hello, world!";
int pos = str.find("world");

if (pos != string::npos) {
    cout << "Found 'world' at index " << pos << endl;
} else {
    cout << "'world' not found." << endl;
}

rfind(): Finds the last occurrence of a substring within the string. Returns the starting index of the substring if found, otherwise std::string::npos. Can optionally specify the starting position for the search.

string str = "Hello, world, world!";
int pos = str.rfind("world");

if (pos != string::npos) {
    cout << "Found 'world' at index " << pos << endl;
} else {
    cout << "'world' not found." << endl;
}

find_first_of(): Finds the first occurrence of any character from a given set within the string. Returns the starting index of the found character, otherwise std::string::npos.

string str = "Hello, world!";
int pos = str.find_first_of("aeiou");

if (pos != string::npos) {
    cout << "First vowel found at index " << pos << endl;
} else {
    cout << "No vowels found." << endl;
}

find_last_of(): Finds the last occurrence of any character from a given set within the string. Returns the starting index of the found character, otherwise std::string::npos.

string str = "Hello, world!";
int pos = str.find_last_of("aeiou");

if (pos != string::npos) {
    cout << "Last vowel found at index " << pos << endl;
} else {
    cout << "No vowels found." << endl;
}

Clear api

str.clear();
str.empty();

Substring api

substr() Method: Extracts a substring from a given position and length. Takes two arguments:

Starting index (inclusive)
Length of the substring (optional)

string str = "Hello world!!";
string token1 = str.substr(5); // index 5 to the end
string token2 = str.substr(5,2); // index 5 to 6 (length 2)

Iterate string

str.begin();
str.end();
str.rbegin();
str.rend();

Following are few example

for (int i = 0; i < str.length(); i++) {
    std::cout << str[i] << " ";
}
// Iterating using begin() and end()
for (string::iterator it = str.begin(); it != str.end(); ++it) {
    cout << *it << " ";
}
// Iterate using for-each style
for (char chr : str) {
  cout << chr << " ";
}

Note: Iterating or reading string at given index returns char therefore be careful.

Conversion

Following are ways to convert a string representation of a number into an actual numerical value.

for integers

std::stoi: Converts to an int.
std::stol: Converts to a long.
std::stoll: Converts to a long long.

They throw exceptions (std::invalid_argument or std::out_of_range) if the conversion fails.

string str = "12345";
int num = stoi(str);
cout << num << endl; // Output: 12345

for floating-point numbers

std::stof: Converts to a float.
std::stod: Converts to a double.
std::stold: Converts to a long double.

string str = "3.14159";
double num = stod(str);
cout << num << endl; // Output: 3.14159

Convert number to string

std::to_string(): convert numerical types (int, float, double, etc.) to strings.

int age = 30;
double price = 19.99;

string ageString = to_string(age);  // "30"
string priceString = to_string(price); // "19.99"

Convert char to string

If we use std::to_string() for char then first it will convert char into corresponding ascii value then convert integer into string. Following is buggy code.

char chr = 1;
cout << to_string(chr); // It will print 49 as ascii value

To convert char to string we can take following approach.

char chr = 1;
string str = {chr};
cout << str; // It will print 1

String concatenation

The + Operator

string firstName = "John";
string lastName = "Doe";
string fullName = firstName + " " + lastName;

The += Operator

string message = "Hello";
message += ", world!";

std::string::append() : Offers variations for appending parts of strings or a specified number of characters.

string str1 = "Hello";
string str2 = " world!";
str1.append(str2);
cout << str1 << endl;   
 // Output: Hello world!

Sort string

The most efficient and straightforward way to sort a string in C++ is by using the std::sort function from the <algorithm> header. This function takes iterators to the beginning and end of the range to be sorted. For strings, these iterators are typically begin() and end().

int main() {
    string str = "hello world";

    // Sort the string in ascending order
    sort(str.begin(), str.end());

    cout << str << std::endl; // Output: dehllloorw
    return 0;
}

Split operation

C++ doesn’t have native .split() methon on string. We will have to write our own as below.

Approach #1: Using stringstream and getline

#include <bits/stdc++>
using namespace std

vector<string> split(const string& str, char delimiter) {
    vector<string> tokens;
    stringstream ss(str);
    string token;
    while (getline(ss, token, delimiter)) {
        tokens.push_back(token);
    }
    return tokens;
}

int main() {
    string str = "apple,banana,cherry";
    char delimiter = ',';
    vector<string> result = split(str, delimiter);
    for (const auto& token : result) {
        cout << token << endl;
    }
    return 0;
}

Good for: Simple cases with single-character delimiters.

How it works:

Create a std::stringstream from your string.
Use std::getline with the delimiter to extract substrings into a container (e.g., a std::vector).

Approach #2: Manual Splitting with find and string::substr

Good for: More control and flexibility, especially with multi-character delimiters.