Transforming a C++ string into an integer, comparing lists versus constructors, which method is preferable or, should I say, more reliable or devoid of errors, is a query frequently posed by numerous C++ novices and even seasoned programmers. Hence, whether you opt for stoi() functions, stringstream, or manual iterations, several approaches exist. Not all methods provide the same level of efficacy! In this guide, we will explore the most effective techniques to convert a string into an integer in C++, complete with practical examples, advantages, disadvantages, and straightforward code elucidations to eliminate any uncertainty.
C++ offers a variety of basic data types to define the size and nature of the data a variable can maintain. The most prevalent and efficient data types include int and string. Int is meant for whole numbers, excluding decimal points, while string is designated for textual data. Nonetheless, string forms a component of the C++ standard library, necessitating the inclusion of #include<string>.
Data Type
Description
Example
int
Numerical integers
int a = 10;
float
Single-precision floating-point numbers
float b = 5.5;
double
Double-precision floating-point numbers
double c = 9.99;
char
A single character
char d = 'A';
bool
Boolean values (true/false)
bool e = true;
void
No value (utilized for functions)
void func();
How to Declare and Initialize an int in C++
The int data type is used to hold whole numbers, i.e., (also referred to as integer type), both positive and negative numbers without any decimal values.
Example:
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var doc = new DOMParser().parseFromString(input, “text/html”);
return doc.documentElement.textContent;
}
// Function to copy code to clipboard
function copyCodeToClipboard98739() {
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document.getElementById(“runBtn98739”).addEventListener(“click”, runCode98739);
document.getElementById(“closeoutputBtn98739”).addEventListener(“click”, closeOutput98739);
Result:
The preceding C++ program defines an integer variable age with a value of 25, and you can utilize cout in C++ to display this value.
How to Declare and Initialize a String in C++
The string data type is utilized to hold text, initialized through the std::string class. To declare the string, the <string> header must be included in your program.
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document.getElementById("runBtn21607").addEventListener("click", runCode21607);
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Result:
The above code utilizes the <string> header to declare the string. The string greeting is assigned the text “Hello, C++!”. By utilizing cout, the assigned text is printed on your console.
How to Transform a String into an Integer.
In C++, the conversion from a string to an integer is a fundamental task. These methodologies are particularly beneficial when extracting integer data from user input, files, or other text-based sources. We can convert strings to integers using various methods. Each of these approaches caters to distinct scenarios, such as performance, input accuracy, etc.
1. Utilizing stoi() Function
In C++, the stoi() function serves as a straightforward and effective means to convert a string into an int datatype. It is defined by including the <string> header and is part of the std namespace.
Example:
Cpp
Code Duplicated!
Output:
In the above code, after including the <string> header, the string declaration follows. The string str = “123” initializes the number (123) as a string using double quotes. Now, employing stoi, it transforms the string into a number; the number is displayed on the console with cout.
2. Utilizing atoi() Function
The atoi() function is a C-style method that converts a C-string into an integer. To declare this function in C++, we must incorporate the <cstdlib> header in our program.
Example:
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Outcome:
In the preceding program, we utilize atoi() to transform the C-style string “456” into an integer. The resulting value is displayed using cout.
Contrast between stoi() and atoi()
Characteristic
atoi
stoi
Full Designation
ASCII to Integer
String to Integer
Header File
<cstdlib>
<string>
Namespace
Global (C-style)
std:: (C++-style)
Input Format
C-style string (char*)
std::string
Return Format
int
int
Error Management
No inherent error management
Throws exceptions on improper input
First Appeared In
C
C++11
Sample Code
#include <cstdlib>
#include <iostream>
using namespace std;
int main() {
const char* str = "123";
int num = atoi(str);
cout << num;
return 0;
}
// Outcome: 123
#include <string>
#include <iostream>
using namespace std;
int main() {
string str = "123";
int num = stoi(str);
cout << num;
return 0;
}
// Outcome: 123
3. Utilizing stringstream
The stringstream class is defined in the header file as <sstream>, which is part of the C++ standard library. Stringstream ensures a resilient and proficient method of converting the string into any distinct data type. It serves as an alternative to atoi() and is exceptionally useful when processing formatted input.
Syntax: std::stringstream ss; ss << string_value; ss >> int_variable; Illustration:
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function decodeHTML25751(input) {
var doc = new DOMParser().parseFromString(input, "text/html");
return doc.documentElement.textContent;
}
// Function to duplicate code to clipboard
function copyCodeToClipboard25751() {
const code = editor25751.getValue(); // Retrieve code from the editor
navigator.clipboard.writeText(code).then(() => {
jQuery(".maineditor25751 .copymessage").show();
setTimeout(function() {
jQuery(".maineditor25751 .copymessage").hide();
}, 2000);
}).catch(err => {
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});
}
function runCode25751() {
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url: "https://intellipaat.com/blog/wp-admin/admin-ajax.php",
type: "post",
data: {
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code: code,
cmd_line_args: "",
variablenames: "",
action:"compilerajax"
},
success: function(response) {
var myArray = response.split("~");
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jQuery(".output25751").html("
function closeoutput25751() {
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document.getElementById("runBtn25751").addEventListener("click", runCode25751);
document.getElementById("closeoutputBtn25751").addEventListener("click", closeoutput25751);
Outcome:
In the earlier program, we use the header to declare the string stream. Now in this program, the string ”789”, which is stored in the variable str, is converted to an integer. The stringstream
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An object named ss is instantiated and assigned a string. An integer value is extracted from the stream into the variable num via the operator >>. Ultimately, the transformed number is displayed using cout.
4. Utilizing the sscanf() Function
The sscanf() function is part of the C standard library, designated for processing data formatted similarly to a C-string. It operates akin to printf(); however, instead of reading from the user, it retrieves data from a string.
function closeoutput75000() {
var code = editor75000.getSession().getValue();
jQuery(".maineditor75000 .code-editor-output").hide();
}
// Attach event listeners to the buttons
document.getElementById("copyBtn75000").addEventListener("click", copyCodeToClipboard75000);
document.getElementById("runBtn75000").addEventListener("click", runCode75000);
document.getElementById("closeoutputBtn75000").addEventListener("click", closeoutput75000);
Output:
The above example illustrates how to convert a C-style string “321” into an integer using the sscanf() function provided by the C standard library. The string is stored in the character array str. ” num = sscanf( str, “%d” ); The program then outputs the converted number. The cout will display the result on the console.
5. Employing a For Loop (Manual Method)
A string can be converted into digits by utilizing a manual method with a for-loop that produces a digit step-by-step. This approach is particularly effective for understanding how conversion occurs between different data types.
function closeoutput16170() {
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jQuery(".maineditor16170 .code-editor-output").hide();
}
// Attach event listeners to the buttons
document.getElementById("copyBtn16170").addEventListener("click", copyCodeToClipboard16170);
document.getElementById("runBtn16170").addEventListener("click", runCode16170);
document.getElementById("closeoutputBtn16170").addEventListener("click", closeoutput16170);
Output:
This program illustrates how to transform the string “234” into an integer through a manual loop method. You commence with num = 0, and then progress through the characters in the string, converting it to an integer by subtracting a character (‘0’) from the string. The value is computed utilizing num = num * 10 + (c – ‘0’). After completing the loop, the final number, 234, is printed. This process demonstrates the inner workings of converting a string to an integer.
6. Utilizing strtol() Function
In contrast to atoi(), the strtol() function boasts increased robustness and efficiency in terms of error verification. This function is employed to recognize invalid input or values that are out-of-bounds, enhancing its safety and dependability for real-world applications.
Example:
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editor47717.setValue(decodedContent); // Assign the default text
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editor47717.setOptions({
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function decodeHTML47717(input) {
var doc = new DOMParser().parseFromString(input, "text/html");
return doc.documentElement.textContent;
}
// Function to copy code to clipboard
function copyCodeToClipboard47717() {
const code = editor47717.getValue(); // Fetch code from the editor
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// alert("Code copied to clipboard!");
function closeoutput47717() {
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}
// Attach event listeners to the buttons
document.getElementById("copyBtn47717").addEventListener("click", copyCodeToClipboard47717);
document.getElementById("runBtn47717").addEventListener("click", runCode47717);
document.getElementById("closeoutputBtn47717").addEventListener("click", closeoutput47717);
Output:
The subsequent program transforms the string “987” into an int with the use of strtol(). Initially, it accepts the string, a pointer named end to indicate where the conversion concludes, and the base (10). Additionally, the resultant value is stored in num and displayed. Unlike
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The strtol() function, similar to atoi(), provides minimal error verification via the  end pointer.
Conclusion
In modern C++ programming, there are various methods to transform a string into an int. The stoi() function serves as a straightforward and secure option. Legacy functions such as atoi() and sscanf() may not manage errors effectively. For production-oriented applications, utilizing  strtol() or stringstream is advisable. This promotes a deeper understanding of all available methods and ultimately leads to creating more robust and reliable code.
Convert a String to an int in C++ – FAQs
1. What is the most secure method to convert a string to an int in C++?
In C++, std::stoi() guarantees safe conversion with error management.
2. Is it possible to convert a string to an int without libraries?
Yes, a string can be transformed into an int by employing a manual conversion for-loop.
3. What distinguishes atoi() from stoi()?
Both functions in C++ allow conversion of a string to an int; however, atoi() does not validate errors, while stoi() raises an exception for inaccurate input.
4. How do I convert a std::string utilizing atoi()?
In C++, utilize the .c_str() method such as atoi(str.c_str()) for the conversion process.
5. Which approach is optimal for managing invalid input?
Both the stoi() and stringstream approaches are recommended for dealing with invalid input and ensuring error validation.
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