Repetition

循环

​Parts of a Loop

• Initialization

  • Usually of a loop control variable

• Continuation condition

  • Boolean expression specifying when to continue executing the loop

• Update

  • Usually of a loop control variable

• Loop Body

  • Sequence of the statement to execute

Creating a Loop

1. What do things have to look like before entering the loop?

   Initialization (control variable)

   Initialization (loop body)

2. How do you exit/end the loop?

   Control variable

   Continuation condition

   Interrupt: break, return, exception

3. What do you do to ensure you eventually exit/end the loop?

   Update

4. What does the loop do?

   Loop Body

While

//initialize 
While (continuation condition)
{
    //loop body
    //update
}

While 循环

For

//A. initialize
//B. continuation condition
//C. update

for (A; B; C) {
 //loop body
 }

for 循环（其实都差不多）

Types of control

• Counting

  • Control variable increments by set amount each iteration

• Sentinel

  • Control variable is set to a value obtained during the loop each iteration

• Flag

  • Control variable is Boolean varaible tha represents a condition each iteration

  • Special case of sentinel

Do While

//initialize
do {
    //loop body
    //update (can be initialization)
} while (continuation conidtion);

do while 循环

while 和 do while 的区别

1. while是先判断条件，条件为true之后才会执行内部代码

2. do while是先执行内部代码，再判断条件，条件为true继续执行，为false自动推退出循环

3. 所以正常情况下do while会比while多执行一次内部代码，假设两个判断条件为false的情况

Loop

三种循环的区别

Loop conversion

• Most loops can be converted to any other type of loop

  • What can't? (Well you could but it would be bad practice)

• good exercise that can help you build good loop structurea

• Remember the guidance for building a loop:

  • Initialize control variable(s)

  • Specify continuation condition

  • Specify loop body

  • Define update for control varaible

The Increment and Decrement Operators

• ++ is the increment operator

  it adds one to a variable

  • val++, is the same as val = val + 1;

• ++ can be used before (prefix) or after (postfix) a varaible

  • ++val:

  • val++;

• -- is the decrement operator

  it subtracts one from a variable

  • val--, is the same as val = val - 1;

• -- can be used before (prefix) or after (postfix) a varaible

  • --val:

  • val--;

Prefix vs. Postfix

1. 当他们处在表达式中一定要注意，val++是先运算表达式，表达式处理结束后自己再加一，++val是先加一，再运算表达式。 a = (val++) + 1; a = (++val) + 1;

2. ++val 相当于 val = val + 1 val++ 相当于 int tem；tem = val + 1； val = tem;

3. 效率差别，不言而喻。

• ++ and -- operators can be used in complex atatements and expressions

• In prefix mode (++ val, -- val) the operator increments or decrements, then rturns the value of the variable

• In post fix mode (val++, val--) the operator returns the value of the variable, then increments or decrements

Example:

int num, val = 12;
cout << val++; //display 12, val is now 13;
cout << ++val; //sets val to 14, then display it(14);
num = --val; //sets val to 13, stores 13 in num
num = val--; //store 13 in num, sets val to 12

Deciding Which Loop to Use

• The while loop is a conditional pretest loop

  • Iterates as long as a certain condition exits

  • Validating input

  • Reading lists of data terminated by a sentinel

• The do-while loop is a conditional posttest loop

  • Always iterates at least once

  • Repeating a menu

• The for loop is a pretest loop

  • Built-in expressions for initializing, testing, and updating

  • Situations where the exact number of iterations is known