A boolean expression is an expression that is either true or false.
print(5==5)
print(5==6)
True
False
Note that here we use a double equal sign $==$.
True and False are special values that belong to the class bool, not strings.
print(type(True))
print(type(False))
bool
bool
There are other ways of comparison.
x != y # x is not equal to y
x > y # x is greater than y
x < y # x is less than y
x >= y # x is greater than or equal to y
x <= y # x is less than or equal to y
x is y # x is the same as y
x is not y # x is not the same as y
print(True and True)
print(True and False)
print(False and False)
print(5 > 3 and 3 > 1)
True
False
False
True
print(True or True)
print(True or False)
print(False or False)
print(5 > 3 or 1 > 3)
True
True
False
True
print(not True)
print(not False)
print(not 1>2)
False
True
True
Any nonzero number is interpreted as "true".
print(17 and True)
True
x = 6
y = 2
print(x >= 2 and (x/y) > 2)
True
x = 1
y = 0
print(x >= 2 and (x/y) > 2)
False
x = 6
y = 0
print(x >= 2 and (x/y) > 2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
x = 1
y = 0
print(x >= 2 and y!= 0 and (x/y) > 2)
False
x = 6
y = 0
print(x >= 2 and y!= 0 and (x/y) > 2)
#y!= 0 acts as a guard.
False
x = 6
y = 0
print(x >= 2 and (x/y) > 2 and y != 0)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
x = 10
if x > 0:
print('x is positive')
The boolean expression after the if statement is called the condition.
We end the if statement with a colon character :
If the logical condition is true, then the indented statement gets executed.
There is no limit on the number of statements, but there must be at least one.
If there is no statement, you can use pass statement, which does nothing.
if x < 0:
pass
# need to handle negative values!
x=10
if x%2 == 0 :
print('x is even')
else :
print('x is odd')
x=5
y=10
if x < y:
print('x is less than y')
elif x > y:
print('x is greater than y')
else:
print('x and y are equal')
elif is an abbreviation of “else if.”
No limit on the number of elif statements. If there is an else clause, it has to be at the end, but there does not have to be one.
Each condition is checked in order. If the first is false, the next is checked. If one of them is true, then it executes, and the statement ends.
if x == y:
print('x and y are equal')
else:
if x < y:
print('x is less than y')
else:
print('x is greater than y')
inp = input('Enter Fahrenheit Temperature: ')
fahr = float(inp)
cel = (fahr - 32.0) * 5.0 / 9.0
print(cel)
Enter Fahrenheit Temperature:72
22.22222222222222
Enter Fahrenheit Temperature:fred
Traceback (most recent call last):
File "fahren.py", line 2, in <module>
fahr = float(inp)
ValueError: could not convert string to float: 'fred'
The idea of try and except is that you know that some sequence of instruction(s) may have a problem and you want to add some statements to be executed if an error occurs. These extra statements (the except block) are ignored if there is no error.
inp = input('Enter Fahrenheit Temperature:')
try:
fahr = float(inp)
cel = (fahr - 32.0) * 5.0 / 9.0
print(cel)
except:
print('Please enter a number')
Enter Fahrenheit Temperature:72
22.22222222222222
Enter Fahrenheit Temperature:fred
Please enter a number
print("BMI指数计算器\n")
inp_1 = input('请输入您的体重(kg):\n')
inp_2 = input('请输入您的身高(cm):\n')
weight = float(inp_1)
height = float(inp_2)
BMI = weight/(height/100)**2
if BMI<18.5:
print("您的体型偏瘦")
elif BMI<24 and BMI>=18.5:
print("您的体型正常")
elif BMI<28 and BMI>=24:
print("您的体型偏胖")
elif BMI<32 and BMI>=28:
print("您的体型肥胖")
elif BMI>=32:
print("您的体型过于肥胖")
A list is a sequence of values.
[10, 20, 30, 40]
['apple', 'banana', 'orange']
['spam', 2.0, 5, [10, 20]]
A list that contains no elements is called an empty list; you can create one with empty brackets, [].
>>> cheeses = ['Cheddar', 'Edam', 'Gouda']
>>> numbers = [17, 123]
>>> empty = [] #[] is false
>>> print(cheeses, numbers, empty)
['Cheddar', 'Edam', 'Gouda'] [17, 123] []
>>> cheeses = ['Cheddar', 'Edam', 'Gouda']
>>> print(cheeses[0])
Cheddar
>>> cheeses = ['Cheddar', 'Edam', 'Gouda']
>>> print(cheeses[-1])
Gouda
>>> print(cheeses[-2])
Edam
>>> numbers = [17, 123]
>>> numbers[1] = 5
>>> print(numbers)
[17, 5]
>>> a = [1, 2, 3]
>>> b = [4, 5, 6]
>>> c = a + b
>>> print(c)
[1, 2, 3, 4, 5, 6]
>>> [0] * 4
[0, 0, 0, 0]
>>> [1, 2, 3] * 3
[1, 2, 3, 1, 2, 3, 1, 2, 3]
>>> t = ['a', 'b', 'c', 'd', 'e', 'f']
>>> t[1:3]
['b', 'c']
>>> t[:4]
['a', 'b', 'c', 'd']
>>> t[3:]
['d', 'e', 'f']
>>> t[:]
['a', 'b', 'c', 'd', 'e', 'f']
>>> t = ['a', 'b', 'c', 'd', 'e', 'f']
>>> t[1:3] = ['x', 'y']
>>> print(t)
['a', 'x', 'y', 'd', 'e', 'f']
append adds a new element to the end of a list:
>>> t = ['a', 'b', 'c']
>>> t.append('d')
>>> print(t)
['a', 'b', 'c', 'd']
extend takes a list as an argument and appends all of the elements:
>>> t1 = ['a', 'b', 'c']
>>> t2 = ['d', 'e']
>>> t1.extend(t2)
>>> print(t1)
['a', 'b', 'c', 'd', 'e']
insert adds a new element at any position in your list
>>> t = ['a', 'b', 'c']
>>> t.insert(1, 'd')
>>> print(t)
['a', 'd', 'b', 'c']
The return value of the above three methods is None. What is None in Python?
>>> t = ['a', 'b', 'c']
>>> x = t.insert(1, 'd')
>>> print(t)
['a', 'd', 'b', 'c']
>>> print(x)
None
The None keyword is used to define a null value, or no value at all. None is not the same as 0, False, or an empty string. None is a datatype of its own (NoneType) and only None can be None.
There are several ways to delete elements from a list. If you know the index of the element you want, you can use pop
>>> t = ['a', 'b', 'c']
>>> x = t.pop(1)
>>> print(t)
['a', 'c']
>>> print(x)
b
pop modifies the list and returns the element that was removed. If you don't provide an index, it deletes and returns the last element.
>>> t = ['a', 'b', 'c']
>>> x = t.pop()
>>> print(t)
['a', 'b']
>>> print(x)
c
If you don't need the removed value, you can use the del operator
>>> t = ['a', 'b', 'c']
>>> del t[1]
>>> print(t)
['a', 'c']
>>> t = ['a', 'b', 'c', 'd', 'e', 'f']
>>> del t[1:5]
>>> print(t)
['a', 'f']
If you know the element you want to remove (but not the index), you can use remove
>>> t = ['a', 'b', 'c']
>>> t.remove('b')
>>> print(t)
['a', 'c']
The return value from remove is None. But, you can assign the element which you want to remove to a variable so that you can use it next time.
There is a possibility that the value appears more than once in the list, but the remove() method deletes only the first occurrence.
>>> t1 = ['a', 'b', 'c']
>>> t2 = t1[:]
>>> t1.remove('b')
>>> print(t1)
['a', 'c']
>>> print(t2)
['a', 'b', 'c']
sort arranges the elements of the list from low to high:
>>> t = ['d', 'c', 'e', 'b', 'a']
>>> t.sort()
>>> print(t)
['a', 'b', 'c', 'd', 'e']
>>> t = ['d', 'c', 'e', 'b', 'a']
>>> t.sort(reverse=True)
>>> print(t)
['e', 'd', 'c', 'b', 'a']
Most list methods modify the list and return None.
By .sort(), the order of the list is permanently changed.
To maintain the original order of a list but present it in a sorted order, you can use the sorted() function.
>>> t = ['d', 'c', 'e', 'b', 'a']
>>> print(sorted(t))
['a', 'b', 'c', 'd', 'e']
>>> print(t)
['d', 'c', 'e', 'b', 'a']
>>> t = ['d', 'c', 'e', 'b', 'a']
>>> t=sorted(t)
>>> print(t)
['a', 'b', 'c', 'd', 'e']
To reverse the original order of a list, you can use the reverse() method.
>>> t = ['d', 'c', 'e', 'b', 'a']
>>> t.reverse()
>>> print(t)
['a', 'b', 'e', 'c', 'd']
The reverse() method changes the order of a list permanently, but you can revert to the original order anytime by applying reverse() to the same list a second time.
You can quickly find the length of a list by using the len() function.
>>> s = ['python', 'java', ['asp', 'php'], 'scheme']
>>> len(s)
4
>>> L = []
>>> len(L)
0
>>> s = [3, 8, 10, 7]
>>> sum(s)
28
>>> min(s)
3
>>> max(s)
10
>>> x = ['banana', 'jack', 'jessica']
>>> min(x)
banana
>>> max(x)
jessica
>>> sum(x)
TypeError: unsupported operand type(s)
for +: 'int' and 'str'
The element can be number, string, list, and empty [].
Features: Ordered, Mutable, Repeatable
The indices start at 0. Index -1 means the last item.
The item can be changed by reassign a new item. Two lists can be added together. list slice :
.append(), .extend(), .insert()
.pop(), del, .remove()
.sort(), sorted(), .reverse(), len(), sum(), max(), min()