day-18: c++ condition, loop

if-statement - [c++ if-conditions]

if is a c++ condition statement.

Syntax:

if (<<condition-expression>>)
{
	<<evaluate-statement-1>>;
	<<evaluate-statement-2>>;
	// ... more statements
}
else
{
	<<evaluate-statement-1>>;
	<<evaluate-statement-2>>;
	// ... more statements
}

<<condition-expression>> is an expression that can be judged as true or false.
<<evaluate-statement>> etc. are some statements that will be executed by your program.
The else section can be omitted.

c++ example:

int x;
x = 4;
if (x == 4)
{
	std::cout << "x is 4" << std::endl;
}
else
{
	std::cout << "x is not 4" << std::endl;
}

Because x is 4, so x == 4 is true, so std::cout << "x is 4" << std::endl; will be executed, std::cout << "x is not 4" << std::endl; will be not executed.

switch branches - [c++ if-conditions]

switch is another c++ condition statement, switch is a more powerful if

Syntax:

switch (<<value-expression>>)
{
case <<value1>>:
	<<evaluate-statement-1>>;
	break;
case <<value2>>:
	<<evaluate-statement-2>>;
	break;
// more case ...
// more case ...
default:
	<<evaluate-statement-N>>;
	break;
}

If <<value-expression>> is equal to <<value1>>, it will execute <<evaluate-statement-1>>;
If <<value-expression>> is equal to <<value2>>, it will execute <<evaluate-statement-2>>;
....
....
If all of the above cases are not met, the <<evaluate-statement-N>>; "inside" th default will be executed.
break means "break and quit the switch immediately", otherwise another case might match the condition, and an unexpected statement will be executed, so you'd better add a break "inside" every case.

c++ three-op-operator :? [c++ if-conditions]

three-op-operator :? is also another c++ if-condition. It is more short for value judgement.

?:

Syntax:

auto result = <<condition>> ? <<expression-1>> : <<expression-2>> ;

If <<condition>> is true, the <<expression-1>> will be executed, and the result is <<expression-1>>
If <<condition>> is false, the <<expression-2>> will be executed, and the result is <<expression-2>>

c++ example:

bool x = 3;
std::string result = (x==3)?"true":"false";

for-loop - [c++ loops]

c++ for loop

Syntax:

// for-loop:
for (<<init-statement>>; <<judge-condition>>; <<change-expression>>)
{ // loop-body begins
	<<evaluate-statement-1>>;
	<<evaluate-statement-2>>;
	// ... more statement
	// ... more statement
} // loop-body ends

for-loop will execute the loop-body again and again, ...
<<init-statement>> is used to set the initial status, it is used to initialize a variable's value commonly.
<<judge-condition>> is an expression that can be judged to true or false, if it is true, the loop-body will be executed again; if it is false, the for-loop will quit.
<<change-expression>> is used to change the status of something. You don't want to execute the same loop-body commonly, so <<change-expression>> can be used to change the status.
Infinite loop - If the <<judge-condition>> is always true, the for-loop will run forever until some external thing interrupts it. This is commonly used in a game-rendering loop, a user click might trigger an event to interrupt the loop.
The brackets {} can be omitted, if it has only one <<evaluate-statement>>; otherwise the brackets can not be omitted.

Run orders:

step-1: Run <<init-statement>>
step-2: Run <<judge-condition>>
step-3: Run <<loop-body>>
step-4: Run <<change-expression>>
step-5: Run <<judge-condition>>
step-6: Run <<loop-body>>
step-7: Run <<change-expression>>
Repeat step-5 -> step-6 -> step-7 again and again
When it runs at <<judge-condition>>, if the condition is not met (aka. the condition result is false), the for-loop will stop and quit; otherwise it will repeat the loop.
A break; statement can be inserted inside the loop-body .

c++ example:

for (int i=0; i<5; ++i)
{
	std::cout << "i is " << i << std::endl;
}

c++ Range-based for loop - [c++ loops]

c++ range-based for loop can be also called c++ for each.

Syntax:

for (<<declare-a-variable>>: <<initializing-value-list>>)
{ // loop-body begins
	<<evaluate-statement-1>>;
	<<evaluate-statement-2>>;
	// ... more statement
	// ... more statement
} // loop-body ends

<<declare-a-variable>> is used to declare a variable .
The declared variable will be assigned with a value from the <<initializing-value-list>> one by one (iteration)
The <<loop-body>> will be executed one by one.
When all values of the <<initializing-value-list>> are iterated, the range-based for loop is finished and quits.
A break; statement can be inserted inside the loop-body .

c++ example:

for (std::string str: {"c++", "world", "hello world"})
{
	std::cout << "str is " << str << std::endl;
}

c++ while-loop [c++ loops]

c++ while loop

Syntax:

while (<<judge-condition>>)
{ // loop-body begins
	<<evaluate-statement-1>>;
	<<evaluate-statement-1>>;
	// ... more statement
	// ... more statement
} // loop-body ends

If <<judge-condition>> is true, run the loop-body again; Otherwise, while-loop stops and quits.
Infinite loop - If the <<judge-condition>> is always true, the while-loop will run forever until some external thing interrupts it. This is commonly used in a game-rendering loop, a user click might trigger an event to interrupt the loop.
Put a <<change-statement>> anywhere inside the loop-body to change the status .
If the <<judge-condition>> is true at the very first time of running the while-loop, the while-loop will stop and quit, so the loop-body will be never executed.
A break; statement can be inserted inside the loop-body .

c++ example:

int x = 0;
while (x < 10)
{
	std:cout << "x is " << x << std::endl;
	++x;	// change-statement
}

do-while-loop - [c++ loops]

c++ do-while loop

Syntax:

do
{
	<<evaluate-statement-1>>;
	<<evaluate-statement-2>>;
	// ... more statement
	// ... more statement
}
while (<<judge-condition>>);

do-while-loop is similar with while-loop.
The first running is the loop-body, it will always run at the first time, whatever the <<judge-condition>> result is.
The ; should be placed at the end of do-while-loop .

c++ example:

int i = 0;
do
{
	std::cout << "i is " << i << std::endl;
	++i;
}
while (i<10);

Written on Dec 06, 2024

Back to index

Index

cpp/c++

c++ std::exception:

std::cout.write(err.data(), err.size());

std::cout << std::endl;

caught:

  ===================================
  #  The c++ programming language.  #
  #                                 #
  #  Join c++ Discord: yZcauUAUyC   #
  #  Deck                           #
  ===================================

Home: cppfx.xyz