C++ Primer Chapter 1: Control Flow and Classes

1.4 Control Flow

“Control flow” refers to the “order of program execution”—by default, programs execute in a “top-to-bottom, left-to-right” manner; however, in practical applications, we need to “determine whether to execute a segment of code based on conditions” (branching) or “repeat a segment of code” (looping). This section introduces the two most basic control flow statements in C++:if (branching) and for (looping).

1.4.1 `if` Statement: Conditional Branching

The if statement serves to "execute a segment of code if a certain condition is met; otherwise, execute another segment of code (optional)." 

Syntax Structure

// Single branch: execute if condition is trueif (condition_expression) {    code_block1;  // executed when condition is true}// Double branch: execute code_block1 if condition is true, otherwise execute code_block2if (condition_expression) {    code_block1;  // executed when condition is true} else {    code_block2;  // executed when condition is false}

Condition expression: must be of “boolean type” (bool)—true (true, non-zero values are treated astrue) or false (false, 0 is treated asfalse).
Code block: if there is only one statement, {} can be omitted; however, for clarity and to avoid errors in future modifications (such as mistakenly adding statements), it is recommended to always include {} regardless of the number of statements.

Example: Determine if a number is positive

#include<iostream>using namespace std;int main() {    int num;    cout << "Please enter an integer:";    cin >> num;    if (num > 0) {  // condition: num is greater than 0        cout << num << " is positive" << endl;    } else if (num < 0) {  // additional branch: num is less than 0 (else if can be multiple)        cout << num << " is negative" << endl;    } else {  // remaining case: num equals 0        cout << num << " is zero" << endl;    }    return 0;}

1.4.2 `for` Statement: Looping

The for statement serves to "repeat a segment of code until a termination condition is met"—especially suitable for scenarios with a "known number of iterations" (e.g., iterating through integers from 1 to 10).

Syntax Structure

for (initialization_expression; condition_expression; update_expression) {    loop_body;  // executed when condition is true, updates expression after execution}

Initialization expression: executed once before the loop starts, usually used to define and initialize the loop variable (e.g., int i = 0).
Condition expression: checked before each execution of the loop body—if it is true, the loop body is executed; if it is false, the loop exits.
Update expression: executed after each execution of the loop body, usually used to update the loop variable (e.g., i++, which increments i by 1).

Example: Calculate the sum from 1 to 10

#include<iostream>using namespace std;int main() {    int sum = 0;  // store total sum, initial value is 0    // Loop: i from 1 to 10 (10 times total)    for (int i = 1; i <= 10; ++i) {        sum += i;  // equivalent to sum = sum + i, adding i to sum each time    }    cout << "The sum from 1 to 10 is:" << sum << endl;  // output result: 55    return 0;}

Scope of Loop Variable

In the above code, int i = 1 is defined in the for “initialization expression”—according to the C++11 standard, the i variable’s scope is limited to the for loop body (after the loop ends, i can no longer be used), which is the recommended practice (to avoid variable pollution in the outer scope).
If i is defined outside the for loop (e.g., int i; for (i = 1; ...)), then the i variable’s scope extends beyond the for loop, which is not recommended.

1.5 Introduction to Classes

C++ is an “object-oriented programming language,” and “classes” are the core of object orientation—the role of a class is to “encapsulate data (attributes) and functions (methods) that operate on the data,” forming a “custom type.” This section does not delve into the implementation of classes but rather introduces readers to “how to use classes” through the Sales_item class from the C++ standard library, with detailed explanations of class definitions and encapsulation in subsequent chapters.

1.5.1 `Sales_item` Class Functionality

Sales_item is a “class for handling book sales data,” encapsulating the following information (data) and operations (methods):

Data: ISBN number of the book (unique identifier for a book), sales quantity, unit price, total sales amount.
Methods:

Read sales records (from cin).
Output sales records (to cout).
Accumulate sales records (adding the sales quantity and total sales amount of two books with the same ISBN).

1.5.2 Prerequisites for Using `Sales_item` Class

To use the Sales_item class, two conditions must be met:

Include the header file:#include "Sales_item.h"—note that double quotes are used here"", not angle brackets<><code>:

Angle brackets<>: used to include standard library header files (e.g., <iostream>), the compiler will look in the “system standard library directory”.
Double quotes"": used to include custom header files (e.g., Sales_item.h<code>, usually written by yourself or third-party library header files), the compiler will first look in the "directory of the current source file" and if not found, will then look in the system standard library directory.

Obtain the Sales_item.h file: this file is not a default file in the C++ standard library and needs to be downloaded from the official website of “C++ Primer” (or accompanying resources) and placed in the same directory as the source file.

1.5.3 Example: Using `Sales_item` to Process Sales Data

#include<iostream>#include"Sales_item.h"  // include the header file for Sales_item classusing namespace std;int main() {    Sales_item book1, book2;  // define two variables of Sales_item type (similar to int a;)    // read sales records for two books with the same ISBN    cout << "Please enter the sales record for the first book (ISBN quantity unit price):";    cin >> book1;  // call the input method of Sales_item    cout << "Please enter the sales record for the second book (ISBN quantity unit price):";    cin >> book2;    // check if the ISBNs of the two books are the same (the == operator is overloaded by Sales_item)    if (book1.isbn() == book2.isbn()) {        Sales_item total = book1 + book2;  // accumulate sales records (+ operator is overloaded)        cout << "Total sales record for the two books:" << total << endl;  // output total record        return 0;    } else {        cerr << "The ISBNs of the two books are different, cannot accumulate!" << endl;  // cerr: standard error stream        return -1;  // return non-zero value, indicating abnormal program termination    }}

Key Concept: Operator Overloading

In the above code, cin >> book1, book1 + book2, book1.isbn() == book2.isbn() and other operations appear to be “using standard operators on class objects”—this is the C++ feature of “operator overloading”: classes can customize the behavior of >>, +, == and other operators, making object manipulation more intuitive (just like manipulating int, double and other basic types).
cerr: is the “standard error stream,” and the difference from cout is:<code>cerr is unbuffered, error messages are displayed immediately; while cout is buffered, which may delay display—usually, <code>cerr is used to output error messages.

C++ Primer Chapter 1: Control Flow and Classes

1.4 Control Flow

1.4.1 `<span><span>if</span></span>` Statement: Conditional Branching

Syntax Structure

Example: Determine if a number is positive

1.4.2 `<span><span>for</span></span>` Statement: Looping

Syntax Structure

Example: Calculate the sum from 1 to 10

Scope of Loop Variable

1.5 Introduction to Classes

1.5.1 `<span><span>Sales_item</span></span>` Class Functionality

1.5.2 Prerequisites for Using `<span><span>Sales_item</span></span>` Class

1.5.3 Example: Using `<span><span>Sales_item</span></span>` to Process Sales Data

Key Concept: Operator Overloading

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1.4 Control Flow

1.4.1 <span><span>if</span></span> Statement: Conditional Branching

Syntax Structure

Example: Determine if a number is positive

1.4.2 <span><span>for</span></span> Statement: Looping

Syntax Structure

Example: Calculate the sum from 1 to 10

Scope of Loop Variable

1.5 Introduction to Classes

1.5.1 <span><span>Sales_item</span></span> Class Functionality

1.5.2 Prerequisites for Using <span><span>Sales_item</span></span> Class

1.5.3 Example: Using <span><span>Sales_item</span></span> to Process Sales Data

Key Concept: Operator Overloading

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1.4.1 `<span><span>if</span></span>` Statement: Conditional Branching

1.4.2 `<span><span>for</span></span>` Statement: Looping

1.5.1 `<span><span>Sales_item</span></span>` Class Functionality

1.5.2 Prerequisites for Using `<span><span>Sales_item</span></span>` Class

1.5.3 Example: Using `<span><span>Sales_item</span></span>` to Process Sales Data