Python algorithm practice week2 branch programming

0x00 Branching structure

• Problem presentation Enter two integers to store and a and b in , bring a The data stored in is less than b Data stored in .
• analysis
  • Enter two numbers a and b
  • a It may be less than b It may also be greater than b
    • If a Less than b, Do nothing
    • conversely , In exchange for a and b
  • This type of problem requires different treatment for different situations , Questions like this , Sequence structure cannot be solved
• Three basic forms of branching structure ( Single branch 、 Two branches and many branches )

Single branch structure

• The grammatical form of a single branch structure

if ( Conditional expression )：
Sentence block
# The conditional expression can be any expression , As long as the result is not 0 The idea that True, Otherwise False
# Sentence block ： It could be a statement , It can also be multiple statements 

• The solution of the above problems
  • Restatement of the problem ： Enter two integers to store and a and b in , bring a The data stored in is less than b Data stored in .
  • Python Language solution

# Enter two integers to store and a and b in , bring a The data stored in is less than b Data stored in .
# Use branch structure
a = int(input(' Please enter a:'))
b = int(input(' Please enter b:'))
print(' Before processing ')
print('a={},b={}'.format(a, b))
if a > b:
a, b = b, a # In exchange for a,b A variable's value
print(' After processing ')
print('a={},b={}'.format(a, b))

Two branch structure

• The grammatical form of the double branch result

• problem ： Compare the size of two numbers , Enter two numbers , Output a large number

• problem ： Enter the length of the three sides , Judge whether it can form a triangle

• problem ： Rowing problem , A teacher takes a x A student went boating , Each ship can carry at most 4 personal , Ask how many ships are needed .

Multi branch structure

• The grammatical form of a multi branch structure

• problem ： Calculate the value of the piecewise function , When x>1 when ,y=x, When x<-1 when ,y=-x, When x Be situated between -1 and 1 Between time ,y=1
  • algorithm flow chart
  • Python Language implementation

1. = float(input(' Please enter x:'))
2. x > 1: y = x
3. x < -1: y = -x else: y = 1 print('y={}'.format(y))

• problem ： Know the percentage of a course mark, Convert it into a five grade system

• problem ： Known coordinate points (x,y), Judge its quadrant

## Known coordinate points (x,y), Judge its quadrant
x = float(input(' Please enter x Coordinate value ：'))
y = float(input(' Please enter y Coordinate value ：'))
if x == 0 and y == 0:
print(' The point is at the origin ')
elif x == 0:
print(' The point is at y On the shaft ')
elif y == 0:
print(' The point is at x On the shaft ')
elif x > 0 and y > 0:
print(' First quadrant ')
elif x < 0 and y > 0:
print(' Beta Quadrant ')
elif x < 0 and y < 0:
print(' The third quadrant ')
elif x < 0 and y > 0:
print(' Quadrant four ')

• problem ： Determine if a year is a leap year The condition for judging leap years is ： The year can be 4 Divide but not be 100 to be divisible by , Or can be 400 to be divisible by .
  • flow chart

# Determine if a year is a leap year
# The condition for judging leap years is ： The year can be 4 Divide but not be 100 to be divisible by , Or can be 400 to be divisible by .
# Method 1： Use a multi branch structure
y = int(input(' Please enter the year :'))
if y % 4 == 0 and y % 100 != 0:
print('{} Year is a leap year '.format(y))
elif y % 400 == 0:
print('{} Year is a leap year '.format(y))
else:
print('{} Year is not a leap year '.format(y))
# Method 2： With the help of logical operators
y = int(input(' Please enter the year :'))
if (y % 4 == 0 and y % 100 != 0) or (y % 400) == 0:
print('{} Year is a leap year '.format(y))
else:
print('{} Year is not a leap year '.format(y))

• Discuss the following conditional expressions

if (y % 4 == 0 and y % 100 != 0) or (y % 400) == 0:
if (y % 4 == 0 and y % 100 ) or (y % 400) == 0:
if (not(y % 4) and y % 100 ) or (y % 400) == 0:

The above three conditional expressions all have the same effect , But the first one is more Simple and easy to understand

0x01 The three indicators of a good program

• Simple and easy to understand
• Extensibility is strong
• Be faithful to your algorithm