select
1. definitions
Some code is executed, the code may not perform , selectively execute some code
2. Classification
if
switch
Select _ IF
1.if simplest usage
if ( expression )
Statements ;
Function : If the expression is true, then the statement is executed ; otherwise it does
2.if range ( focus )
① IF default can only control the execution of a sentence, if you want to execute multiple statements, required braces
② control multiple statements format
if ( expression )
{
Statement A;
Statement B;
}
IF 3. ... the else ... Usage
① format
if ( expression )
Statement A;
else
Statement B;
② Note that, the else default only a statement control ( method of performing a plurality of statements and if the same ) , and the sentence A and the sentence B can not be performed at the same time
IF 4. ... the else ... IF ... the else usage
① format
if ( expression )
Statement A;
else if ( expression )
Statement B;
else
Statement C;
Note :
① How to control multiple statements execute the same if the same
② the else ... IF the number may be increased
③ statements A, B, C can only be performed where a
5.C language for true and false judgments
Non-zero is true
That is a false zero
True with a representation
False with 0 represented
6. Programs
① Note : the expression can not be written "a <x <b" form, to be written as "x> a && x <b " form
if common issues
1. Empty the problem statement
if (3>2);
Equivalent to
if (3>2)
; // This is an empty statement
Therefore, under normal circumstances if the back expression can not add ";"
Select _switch
1. Format
switch ( expression )
{
case constant expression A :
Statement 1;
break;
case constant expressions B;
Statement 2;
break;
default:
Statement N;
}
2. Usage
①break quit. For example if the sentence 1 no later BREAK , you have to execute the statement 2 . That is, from start to execution break ends.
②default can appear anywhere, or may not, but it is recommended to write at the end, it represents remove the case other circumstances match.
③ Some books also the switch as a selection structure
④ Note: Case labels must be constant expressions (constant Expression) can only be used for basic data types Switch , these types include int , char and so on. For other types, you must use the if statement.
( Supplementary ) constant expression ( const experssion ): means (1) will not change, and (2) can be obtained calculation result of expression during compilation. Literal belongs constant expression, initialized with constant expressions const object is also constant expressions.
An object (or expression) is not constant expression determined by its data type and an initial value.
example:
const int a = 1; // constant expression
cosnt int b = a + 1; // constant expression
int c = 2; // initial value literal constant, when c data type is normal int .
const int d = fun (); () // fun value to be obtained at runtime, D is not literals
3. Examples
switch(1+1)
{
case 1:
printf("1\n");
break;
case 2:
printf("2\n");
printf("...");
break;
default:
printf("。。。");
}
Output:
2
...