【CPP】表达式

1-if Statement

if and if-else

• Statements are exected conditionally

# if.cpp
#include 
using namespace std;

int main()
{
    int num  = 10;
    if (num < 5)
        cout << "The number is less than 5. " << endl;
    
    if (num == 5 )
    {
        cout << "The number is 5." << endl;
    }
    else
    {
        cout << "The number is not 5." << endl;
    }

    if (num < 5)
        cout << "The number is less than 5." << endl;
    else if (num > 10)
        cout << "The number is greater than 10." << endl;
    else
        cout << "The number is in range [5, 10]." << endl;

    if(num < 20)
    if(num < 5)
    cout << "The number is less than 5" << endl;
    else
    cout << "Where I'm?" << endl;

    int * p = new int[1024];
    if (p)
        cout << "Memory has been allocated." << endl;

    return 0;
}
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• When will “Where I’m?” be printed?
• How to make the code easier to understand

? : operator

• When can we use the ternary conditional operator?

#ternary.cpp
#include 
using namespace std;

int main()
{
    bool isPositive = true;
    int factor = 0;
    //some operations may change isPositive's value
    if(isPositive)
        factor = 1;
    else
        factor = -1;
    //the if-else statement can be replaced by a ternary conditional operation
    factor = isPositive ? 1 : -1;

    //sometimes the following code can be more efficient.
    factor = isPositive * 2 - 1;

    return 0;
}
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2-Conditions

Condition

• What should be a condition

int num = 10;
if (num < 5)
   cout << "The number is less than 5." << endl;
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• The condition should be an expression which is convertible to bool

Its value can be bool, char, int, float

Relational Expressions

• The condition can be a relational expression
• The 6 relational/comparison operators

• Return 1 if the condition (such as a == b) is true
• Return 0 if the condition is false

Logical Expressions

• If an operands is not bool, it will be converted to bool implicitly

  

• Precedence: ! > && > ||

• What’s the value of the follow expression?

if (-2 && true)
  cout << "The condition is true." << endl;
if (!-2)
  cout << "(!-2) is true, really?" << endl;
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Non-boolean Expressions

• They will be converted to bool implicitly if it is feasible.

float count = 0.2f;
if (count).  // not recommend to use a float-point number
   cout << "There are some."  << endl;
   
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• Pointers are also frequently used as conditions

int * p = new int[1024];
if (!p) // if (p == NULL)
  cout << "Memory allocation failed." << ends;
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3- While loop

While loop

-Syntax:

While (expression)
{
}
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• If the condition is true, the statement (loop body) will be executed

#while.cpp
#include 
using namespace std;
int main()
{
    int num = 10;
    while(num > 0)
    {
        cout << "num = " << num << endl;
        num--;
    }

    // num = 10;
    // do
    // {
    //     cout << "num = " << num << endl;
    //     num--;
    // }while (num > 0);

    // num = 10;
    // while (num > 0)
    // {
    //     if (num == 5)
    //         break;
    //     cout << "num = " << num << endl;
    //     num--;
    // }
    return 0;
} 
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do-while loop

• The test takes place after each iteration in a do-while loop
• The test takes place before each iteration in a while loop

#include 
using namespace std;
int main()
{
    // int num = 10;
    // while(num > 0)
    // {
    //     cout << "num = " << num << endl;
    //     num--;
    // }

    int num = 10;
    do
    {
        cout << "num = " << num << endl;
        num--;
    }while (num > 0);

    // num = 10;
    // while (num > 0)
    // {
    //     if (num == 5)
    //         break;
    //     cout << "num = " << num << endl;
    //     num--;
    // }
    return 0;
} 
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break statement

• Terminate a loop

#include 
using namespace std;
int main()
{
    // int num = 10;
    // while(num > 0)
    // {
    //     cout << "num = " << num << endl;
    //     num--;
    // }

    // int num = 10;
    // do
    // {
    //     cout << "num = " << num << endl;
    //     num--;
    // }while (num > 0);

    int num = 10;
    while (num > 0)
    {
        if (num == 5)
            break;
        cout << "num = " << num << endl;
        num--;
    }
    return 0;
} 
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continue statement

• Skip the remaining part of the loop body and continue the next iteration

#include 
using namespace std;
int main()
{
    // int num = 10;
    // while(num > 0)
    // {
    //     cout << "num = " << num << endl;
    //     num--;
    // }

    // int num = 10;
    // do
    // {
    //     cout << "num = " << num << endl;
    //     num--;
    // }while (num > 0);

    // int num = 10;
    // while (num > 0)
    // {
    //     if (num == 5)
    //         break;
    //     cout << "num = " << num << endl;
    //     num--;
    // }

    int num = 10;
    while (num > 0)
    {
        if (num == 5)
            continue;
        cout << "num = " << num << endl;
        num--;
    }
    return 0;
} 
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死循环！

The Condition, Be Careful

• Can you find any problem from the code?

size_t num = 10;
while(num >= 0)
{
  cout << "num = " << num << endl;
  num--;
}
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死循环！！

size_t. 无符号， 减到0再减一则变成最大的数，没有负数，则循环停不下来

bool flag = true;
int count = 0;
while (flag = true)
{
  cout << "Count = " << count++ << endl;
  // and do sth
  if (count == 10)   // meet a condition
  flag = false;     // set flag to false to break the loop 
}
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while (flag = true)

why?

• Expression 3+4 has a value
• Expression a+b has a value
• Expression (a==b) has value (true or false)
• a = b is an assignment, also an expression and has a value
• The follow code can be compiled successfully

int b = 0;
int m = (b = 8);
cout << "m=" << m << endl;

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4- for loop

for loop

• Syntax:

for (init-clause; cons-expression; iteration-expression)
loop-statement

• Example

#for.cpp
#include 
using namespace std;
int main()
{
    int sum = 0;
    for(int i = 0; i < 10; i++)
    {
        sum += i;
        cout << "Line " << i << endl;
    }
    cout << "sum = "  << sum << endl;

    return 0;
}
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for loop VS while loop

Endless loop

• Sometimes we need it
  

5-goto-switch

goto statement

• Jump to a desired location
• An unrecommended statement

#goto.cpp
#include 

using namespace std;

float mysquare(float value)
{
    float result = 0.0f;

    if(value >= 1.0f || value <= 0)
    {
        cerr << "The input is out of range." << endl;
        goto EXIT_ERROR;
    }
    result = value * value;
    return result;

  EXIT_ERROR:
    //do sth such as closing files here
    return 0.0f;
}

int main()
{
    float value;
    cout << "Input a floating-point number." << endl;
    cin >> value;

    float result = mysquare(value);

    if (result > 0)
        cout << "The square is " << result << "." << endl;

    return 0;
}
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switch statement

• Execute one of several statements, depending on the value of a condition
• break prevents to execute some following statements. Don’t forget break!
• More similar with goto, not if-else

#include 
using namespace std;

int main()
{
    unsigned char input_char = 0;

    cout << "Please input a character to start." << endl;
    cin >> input_char;
    while (input_char != 'q')
    {
        switch (input_char)
        {
            case 'a':
            case 'A':
                cout << "Move left. Input 'q' to quit." << endl;
                break;
            case 'd':
            case 'D':
                cout << "Move right. Input 'q' to quit." << endl;
                break;
            default: 
                cout << "Undefined key. Input 'q' to quit." << endl;
                break;
        }
        cin >> input_char;
    }
}
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