Convert String to Integer in C++

Convert String to Integer in C++ (5 Programs).

BeginnerString Conversion ProgramsExample 3 of 11

convert-string-to-integer.cpp

1#include <iostream>
2#include <string>
3#include <sstream>
4using namespace std;
5
6int main() {
7    string str = "12345";
8    
9    // Method 1: Using stoi()
10    int num1 = stoi(str);
11    
12    // Method 2: Using stringstream
13    stringstream ss(str);
14    int num2;
15    ss >> num2;
16    
17    cout << "String: " << str << endl;
18    cout << "Integer (method 1): " << num1 << endl;
19    cout << "Integer (method 2): " << num2 << endl;
20    
21    return 0;
22}

Output

String: 12345
Integer (method 1): 12345
Integer (method 2): 12345

What's going on

The program converts a string like "12345" into an integer value 12345. This conversion is necessary because:

User input from cin is often read as strings

File data is typically read as strings

Network data comes as strings

You need integers for mathematical operations

Example:

Input string: "12345"

Output integer: 12345

Header Files Used

1.#include <iostream>

Provides cout and cin for input/output operations.

2.#include <string>

Essential for using the string data type.

Required for stoi() function and string operations.

3.#include <sstream>

Provides stringstream class for string-based input/output operations.

Allows treating strings as streams, similar to cin/cout.

Declaring Variables

The program declares:

string str = "12345";

string is the data type for storing text.

str stores the string "12345", which contains numeric characters.

Note: "12345" is text, not a number, until converted.

Method 1: Using stoi() (String to Integer)

int num1 = stoi(str);

This is the most modern and recommended method in C++:

stoi() stands for "string to integer"

It's part of the <string> header (C++11 and later)

It automatically handles the conversion

It throws exceptions for invalid input

How it works:

stoi("12345") reads the string and converts it to integer 12345

If the string contains non-numeric characters, it converts up to the first invalid character

Example: stoi("123abc") returns 123

Advantages:

Simple and clean syntax

Built-in error handling (throws exceptions)

Handles leading/trailing whitespace automatically

Most efficient for simple conversions

Method 2: Using stringstream

stringstream ss(str);

int num2;

ss >> num2;

This method uses stringstream, which treats a string like an input stream:

stringstream ss(str) creates a stream from the string

ss >> num2 extracts the integer from the stream, just like cin >> num

How it works:

1.Create a stringstream object from the string

2.Use the extraction operator (>>) to read the integer

3.The stream automatically converts the string to integer

Advantages:

Very flexible - can convert multiple values

Similar syntax to cin, making it familiar

Can handle multiple types in one stream

Example for multiple values:

stringstream ss("123 456 789");

int a, b, c;

ss >> a >> b >> c; // a=123, b=456, c=789

Other Methods (Mentioned but not shown in code)

Method 3: Using atoi() (C-style)

int num3 = atoi(str.c_str());

atoi() is a C function that converts C-style strings (char arrays)

Requires converting C++ string to C-string using .c_str()

Returns 0 for invalid input (no error checking)

Less safe than stoi()

1#include <cstdlib>

Method 4: Using sscanf() (C-style)

int num4;

sscanf(str.c_str(), "%d", &num4);

sscanf() reads formatted input from a string

Similar to scanf() but reads from a string

More control over format, but more complex

1#include <cstdio>

Method 5: Manual Conversion

int num5 = 0;

for (char c : str) {

if (c >= '0' && c <= '9') {

num5 = num5 * 10 + (c - '0');

}

Manually processes each character

Most control but most code

Useful for learning how conversion works internally

How Manual Conversion Works (Step-by-Step)

Understanding manual conversion helps you understand the concept:

For string "12345":

1.Start with num = 0

2.Read '1': num = 0 * 10 + 1 = 1

3.Read '2': num = 1 * 10 + 2 = 12

4.Read '3': num = 12 * 10 + 3 = 123

5.Read '4': num = 123 * 10 + 4 = 1234

6.Read '5': num = 1234 * 10 + 5 = 12345

The formula: num = num * 10 + (character - '0')

Multiply by 10 shifts digits left

(c - '0') converts character to digit (e.g., '5' - '0' = 5)

Displaying Results

The program prints:

Output:

String: 12345

Integer (method 1): 12345

Integer (method 2): 12345

Both methods produce the same integer value.

1cout << "String: " << str << endl;
2cout << "Integer (method 1): " << num1 << endl;
3cout << "Integer (method 2): " << num2 << endl;

Error Handling

Different methods handle errors differently:

stoi()

: Throws std::invalid_argument or std::out_of_range exceptions

try {

int num = stoi("abc");

} catch (const std::exception& e) {

}

1// Handle error

atoi()

: Returns 0 for invalid input (no way to distinguish error from actual 0)

stringstream

: Sets failbit on error

if (ss.fail()) {

}

1// Handle error

Best Practice

: Use stoi() with try-catch for robust error handling.

When to Use Each Method

stoi()

: Best for most cases - simple, safe, modern C++

stringstream

: Best when converting multiple values or mixing types

atoi()

: Only for legacy code or when you need C compatibility

sscanf()

: When you need complex format parsing

Manual

: For learning or when you need custom conversion logic

return 0;

This ends the program successfully.

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