Python Learning note Five-conditions and loops

5.1 If statement

Nothing to say, the IF statement syntax is as follows:

If expression:

Expr_true_suit

5.1.1 Multi-Conditional expressions

A single If statement can achieve multiple conditional or negative judgments through Boolean operator And,or,not.

If not warn and (system_load>=10):

print ' warning:losing RESOURCE '

Warn+=1

5.2 Else statement

If the IF condition is False the program will execute the ELSE after statement.

If expression:

Expr_true_suit

Else

Expr_false_suit

5.3 Elif is else-if

The elif is used to check if multiple expressions are true, if the position executes subsequent statements. Elif can have any one, but else can only have one.

The syntax is as follows

If expression1:

Expr1_true_suit

Elif expression2:

Expr2_true_suit

Elif Expression3:

Expr3_ture_suit

Else

None_of_the_above_suit

5.4 Three-dimensional expression

Python does not support ternary expressions (c?x:y) in python2.5, and is quite aesthetically pleasing. Syntax: X if C else Y

if Else y>>>smaller3

5.5 While loop

5.5.1 General Syntax

When the condition is true, the statement inside the statement block is executed until the condition is false.

While expression:

Suite_to_repeat

5.5.2 Count Cycle

Adding a count statement to a statement quickly ends when the loop executes a certain number of times, as in the following example.

Count=0

while (count<9):

print ' The index is: ', count

Count+=1

5.5.3 Wireless Loop

Condition is always true, and the program is always executed in the loop.

While True:

Suite_to_repeat

5.6 For Loop

Python provides another loop for loop, which can traverse the sequence members, which can be used in list parsing and generator, which automatically calls the next () method.

1. For sequence type

For can iterate over different sequence objects including, strings, lists, tuples.

>>>for eachletter in ' Names '

print ' Current letter: ', Eachletter

Current Letter:n

Current LETTER:A

Current Letter:m

Current Letter:e

Current Letter:s

For lists and tuples, the iterations are similar to the above.

2. Iterating through the sequence index

The index to set up the sequence is to use the range (Len (list)) to index the first

>>>string='python'>>>range (len (string)) [0,1,2,3,4,5] >>> for in range (len (string)):>>>     print  string [Index]python

3. Using Item and Index iterations

Use the built-in function enumerate () function to iterate over items and indexes at the same time.

>>>namelist=['Donn','Ben', ' David ',' Wendy ' ]>>> for in enumerate (       namelist): ... Print ' %d,%s '% (i+1, eachname)1, Donn2, Ben3, David4,wendy

5.7 Break

Break is used in the while and for loops when the execute to break statement jumps out of the yourselves loop to execute the next statement.

def factor (num):    count=num/2     while count>0:        if num%count==0:            Print count,' is largest factorof', num            break        count-=1 factor (Input ('Please enter the number:'))

The program is used to calculate the greatest common divisor of a number.

5.8 Continue

As with continue in other languages, continue means to start the next loop immediately after the current loop is completed, assuming that the cyclic prerequisites are still satisfied.

Valid=Falsecount=3 whileCount>0:input=raw_input ('Enter password:\n')     forEachpwdinchpasswdlist:ifeachpwd==Input:valid=TruePrint 'Welcome back'             Break    if  notvalid:Print 'Invalid input'Count-=1Continue    Else:         Break

The above code is an example of the use of continue.

5.9 Pass Statement

The pass statement is provided as a placeholder in Python. If there is no statement in a block of statements the compiler will error, then you can use the PASS statement as a placeholder, it does not do anything, you can later fill in the code tag.

Other uses of 5.10 else

In other languages, such as C, else does not appear outside the conditional statement, but in Python you can use else outside of the loop. This is easy to understand, because the loop is to do the first is the condition of judgment, then there must be conditions are not satisfied when the need to execute the statement.

defmaxfactor (num): Count=num/2 whileCount>1:        ifnum%count==0:Print 'largest Factor of%d is%d'%(Num,count) BreakCount-=1Else:        PrintNum'is Prime' forEachnuminchRange (10,21): Maxfactor (Eachnum)

Python Learning note Five-conditions and loops