In the previous lesson, we implemented the “output function” in C++ by printing text, adding comments, and inserting line breaks. However, a program that only repeats fixed content is of little practical use—like a calculator that only displays “Hello, World!”. The introduction of variables addresses this issue. Variables can be understood as small “containers” or “boxes” within a program that can store numbers, text, and even boolean values like “true/false”. By changing the content inside the “box”, the program can immediately become flexible and responsive.
1: The 4 Most Common Variable Types
If we compare variables to labeled “jars”, different “jars” can hold different types of data (integers, decimals, text, true/false answers). Here are the 4 most commonly used variable types in C++: a) int (Integer Type) Purpose: Store integers (e.g., counting the number of cookies). Importance: A core type for everyday counting and simple mathematical operations. In most modern systems, the range is approximately -2 billion to +2 billion.
#include <iostream>
using namespace std;
int main() {
// Data type is int, variable name is apples, assigned value is 5
int apples = 5;
cout << "I have " << apples << " apples\n";
return 0;
}b) float & double (Floating Point Type, i.e., Decimal) float: Stores decimal data (e.g., weighing an object), with medium precision. double: Similar to float but can store more decimal places (e.g., meeting scientific calculation precision requirements).
#include <iostream>
using namespace std;
int main() {
float temperature = 36.5f; // f indicates this value is of float type
double preciseValue = 3.1415926535;
cout << "Temp: " << temperature << "\nPi: " << preciseValue << "\n";
return 0;
}c) bool (Boolean Type) Purpose: Store “true” or “false”, suitable for determining “yes/no” questions (e.g., whether a task is completed). Note: When outputting, false will display as 0, and true will display as 1.
#include <iostream>
using namespace std;
int main() {
bool isRaining = false;
cout << "Is it raining? " << isRaining << "\n"; // Output will be 0
return 0;
}d) string (String Type) Purpose: Store text, sentences, and other textual data (e.g., names, message content).
Note: Using the string type requires including the header file <string>.
#include <iostream>
#include <string> // Include the header file required for string type
using namespace std;
int main() {
string message = "Hello, C++ world!";
cout << message << "\n";
return 0;
}Understanding these 4 types is like knowing your ingredients before cooking—once you master them, you can flexibly combine them to write powerful dynamic programs.
2: Declaring, Assigning, and Modifying Variables
After understanding the main data types, let’s learn how to create and use variables in C++. This process can be likened to: labeling the “box” (declaring the variable) → putting something in the “box” (assigning a value). In C++, there are two common ways to create variables:
a) Declare first, then assign
#include <iostream>
using namespace std;
int main() {
int age; // Step 1: Declare the variable (create an empty box named "age")
age = 20; // Step 2: Assign a value (put 20 in the "age" box)
cout << "Age: " << age << "\n";
return 0;
}b) Declare and assign in one step
#include <iostream>
using namespace std;
int main() {
int age = 20; // Simultaneously complete "variable declaration" and "assignment"
cout << "Age: " << age << "\n";
return 0;
}c) Modifying variable values
The core feature of a variable is “modifiable”—it is not fixed after creation.
#include <iostream>
using namespace std;
int main() {
int age = 20;
cout << "Initial age: " << age << "\n"; // Initial value: 20
age = 25; // Modify the variable value
cout << "New age: " << age << "\n"; // After modification: 25
return 0;
}Output: Initial age: 20New age: 25
Comparison: How does Python handle variables? Python’s variable rules are more flexible than C++: there is no need to declare the type in advance; you can directly write the variable name and assign a value.
name = "Adam"
age = 20
print("name: ", name)
print("Age:", age)
# Modify variable values
name = "Kevin"
age = 25
print("name: ", name)
print("New Age:", age)Output: name: AdamAge: 20name: KevinNew Age: 25
Core Differences:
C++: Must declare the variable type first (e.g., int, string). Python: Automatically recognizes the type based on the assigned content.
3: Best Practices for Variable Naming
Good variable names make code more readable (for yourself and others). It’s like labeling a “jar” clearly—no one wants to open a jar labeled “x” only to find cookies inside.
Recommended Practices:
a) Use meaningful names (self-explanatory).
int age = 25; // Clear: represents “age”
string userName = “Rahul”; // Clear: represents “username”
b) Use camelCase or snake_case to enhance readability.
c) Names should be “short yet descriptive” (avoid being too long or too short).
Avoid Practices:
a) Naming with meaningless characters (syntactically correct but poor readability):int x = 25; // ✅ Syntactically valid, but cannot determine what “x” represents
b) Starting with a number:int 1stUser = 10; // ❌ Syntactically invalid
c) Using special characters or spaces:string user-name = “Rahul”; // ❌ Syntactically invalid (contains “-“)
d) Using C++ keywords (e.g., int, return, class, etc.).
Conclusion
In this lesson, we unlocked the core building blocks of a program—variables. The content includes: 1. 4 core data types (int, float/double, bool, string); 2. Methods for declaring, assigning, and modifying variables; 3. Differences in variable handling between C++ and Python; 4. Best practices for variable naming. Mastering variables allows programs to achieve the functionality of “storing, modifying, and processing information”, truly gaining dynamism and practical value.