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Branching structure

The running mode of different forward paths is selected according to the result of judgment condition

Single branched structure

if> < conditions: true < block > -- > > < conditions - > [block] - > false | ^ | __________________ |Copy the code

# Single branch structure example
guess = eval(input())
if guess == 99:
	print("Got it right.")
    
    
    
    
if Ture
	print("The conditions are right.")
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Binary structure

if> < conditions: | -- -- -- -- -- > [block2] - | < block1>     false|                      |
elseConditions: - > < > | - > < block2> true | | | -- -- -- -- -- > [block1] - |Copy the code

# Example of two-branch structure
guess = eval(input())
if guess == 99:
	print("Got it right.")
else:
	print("Guess wrong.")
    
    
    
    
    
if Ture:
    print ("Block 1")
else:
    print("Statement block 2")
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Compact form

The two-branch structure is suitable for simple expression

The < expression1> ifConditions of < >elseThe < expression2>
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guess = eval(input())

print("Guess {}".format("To" if guess==99 else "Wrong"))
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Note: The output of an if else in compact form is not a statement, but an expression, which is part of the statement.

Multibranched structure

A branching structure in which multiple blocks of statements are selected based on conditions

if> < conditions: | -- -- -- -- -- -- -- -- -- - | -- - -- -- -- -- -- - | < block1> [Statement block1[statement block n-1]          |
elif: | false | | < block2> -- -- -- > < conditions1> -- >... N - - > < conditions1> -- >[block N] -- >........ tureelse: < statement block2>
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# Grading problems for different scores
score = eval(input())
if score>=85:
	print("A")
elif score>=70:
	print("B")
elif score>=60:
	print("C")
else:
	print("D")
    
# Another way of writing (foemat)
score = eval(input())
if score>=90:
	grade="A"
elif score>=80:
	grade="B"
elif score>=70:
	grade="C"
elif score>=60:
	grade="D"
print("Input score belongs to level ()".format())
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• Note the inclusion relationships between multiple conditions
• Note the coverage of variable values

Conditional judgment and combination

The operator

Three reserved words

Three reserved words for conditional combinations

# sample
guess = eval(input())
if guess > 99 or guess < 99:
	print("Guess wrong.")
else :
	print("Got it right.")
    

    
# sample
    if not Ture:
    print("Statement block 2")
else:
    print("Block 1")
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Exception handling of the program

# sample
num = eval(input(Please enter an integer:))
print(num**2)
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# exception handling
Tracebak (most recent call last):
 File "t.py",line 1.in <module>  #line 1 indicates the number of lines in which the exception code occurred
   num = eval(input(Please enter an integer:))
 File "<string>", line 1.in <module>
NameError: name 'abc' is not defined # Exception type and exception content hint
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Two mechanisms for python exception handling

• Use the reserved try word
• Use the reserved word except

Basic use of exception handlingtry: < statement block1>
except: < statement block2> Further distinguish exception typestry: < statement block1>
except< exception type >: < statement block2>
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The sample atry :
	num = eval(input(Please enter an integer:))
	print(num**2)
expect :
	print("Input is not an integer") sampletry :
	num = eval(input(Please enter an integer:))
	print(num**2)
expect NameError:     
	print("Input is not an integer")
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• After an exception type is annotated, only the exception is responded to
• Exception type names are equivalent to variables

Advanced use of exception handling

We can boot blocks 3 and 4 with else and finally after try except

try: < statement block1>
except: < statement block2>
else: < statement block3>
finally: < statement block4>
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• Finally block 4 must be executed
• Else block 3 is executed when no exception occurs