python basis - and while cycling for

for loop iterates

for i in xxx:

    Block

i: is a variable, the value is the one which is taken out xxx stored inside the i

XXX: may be a sequence (SEQ comprising: a list of tuples, string), the object may be an iterative

>>> for i in "abc":
...     print(i)
...
a
b
c

# Debugging techniques Example: list of requirements [1,2,3,4] and print out the results of each step

Result = 0 >>>
>>> I for in [1,2,3,4]:
... Print ( "value of I", I)
... I = Result +
... Print ( "computing result ", result)
...
value i 1
results 1
i 2 value of
the calculation result 3
i value of 3
results. 6
i value of 4
results 10
>>> Print (result)
10

 

# Iterate through the index cycle

>>> fruits = ['banana','apple','mango']
>>> for index in range(len(fruits)):
...     print(fruits[index])
...
banana
apple
mango
>>>

The above examples we use the built-len () function and Range (), len () function returns the length of the list, i.e. the number of elements. range returns a sequence number.

 

 

while loop iterates

while True:

    Block

True: the conditions, as long as the condition is True code block will always be executed

Infinite loop scenarios:

    1 server: the general infinite loop

    2 You do not know how many times the cycle time is necessary to use an infinite loop

>>> a = 0
>>> while a<=5:
...     print(a)
...     a+=1   #相当于a=a+1
...
0
1
2
3
4
5

# Exercises:

Achieved with infinite loop:

　　A user input number, odd or even is determined

　　When the input number is 100 times, the end of the cycle

The while. 1 >>>:
... user_num = int (INPUT ( "Enter a number:"))
... IF user_num% 2 == 0:
... Print ( "% S is an even number"% user_num) # template string
... the else:
... Print ( "% S is an odd number"% user_num)
... IF user_num == 100:
... BREAK
...
Please enter a number: 2
2 being even
enter a digital: 3
3 is an odd number
, enter a number: 4
. 4 is an even number
, enter a number: 5
5 is an odd number
, enter a number: 6
6 is an even number
, enter a number: 100
100 is an even number
>>>

 

 

break和continue

break: a complete end to the cycle
continue: out of this cycle, the next execution time

Comparative Example:

>>> for I in Range (10 ): 
...      IF I == 3 : 
...          BREAK # for the entire cycle is ended,> = 3 is not a digital printing 
...      Print (I) 
... 
0
 . 1 
2 
>>> for i in the Range (10 ): 
...      IF i == 3 : 
...          the continue # i == 3 skip this cycle does not print, for circulation loop continues back 
...      Print (i) 
... 
0
 . 1 
2 
. 4 
. 5 
. 6 
. 7 
. 8 
. 9 
>>>

 

 

for / while ... else ... with a strange usage:

# No BREAK 
>>> for i in the Range (5 ): 
...      IF i == 3 : 
...          the Continue 
...      Print (i) 
... the else : #else statement is executed 
...      Print ( " break NO " ) 
... 
0
 1 
2 
4 
NO break 

# break there

>>> for i in range(5):
...     if i == 3:
...         break
...     print(i)
... else:   #else没有被执行
...     print("no break")
...
0
1
2
>>>

Summary: As long as there is no break execution cycle internal body else will execute the code section

 

 

Example: 

Title: [ 1,2,3,4,5,7 ] You enter a number, a judgment whether or not this list, execute three times this logic. 

Algorithm:

 1, 3 cycles ( for ) 

for I in Range (3 )

 2 , a digital input (INPUT), to a variable stored in 

user_num = int (INPUT ( " Enter a number: " ))

 3, is determined this figure is not [1,2,3,4,5,7 ] 

desc_list = [1,2,3,4,5,7 ]

 . 4 , if the printing, what; no longer be printed out. 

IF user_num in desc_list: 

　　Print ( " % S in " % user_num) 

the else : 

　　Print ( " % S not" % User_num) 

achieved:

 >>> desc_list = [1,2,3,4,5,7 ]
 >>> for I in Range (. 3 ): 
... user_num = int (INPUT ( " Please enter a number: " )) 
...      IF user_num in desc_list: 
...          Print ( " % S in " )% user_num # template string expression"% SA Number "%. 1 '. 1 A Number' 
...      the else : 
...          Print ( " % S is not "% User_num)  
...
Please enter a number:. 5 
. 5 In 
Please enter a number: 2 
2 In 
Please enter a number: . 1 
. 1 in
 >>>