week 1-learn python 1

say hello:

    statement in python 2

        print " "

    statement in python 3

        print (" ")

define a string

    While double-quotes (“) and single-quotes (‘) are both acceptable ways to define a string, a string needs to be opened and closed by the same type of quote mark

    We can combine multiple strings using +, like so: print "This is " + "a good string"

BUG 1

    SyntaxError: EOL while scanning a string literal

  This means that a string wasn’t closed, or wasn’t closed with the same quote-character that started it.

Python treats words not in quotes as commands, like the print statement. If it fails to recognize these words as defined (in Python or by your program elsewhere) Python will complain the code has a NameError. 

Variables

Python uses variables to define things that are subject to change.

    statement

    greeting_message = "Welcome to Codecademy!"

    current_excercise = 5

arithmetic

    The modulo operator is indicated by % and returns the remainder after division is performed.返回除法后的余数

Updating variables

Updating a variable by adding or subtracting a number to its original value is such a common task that it has its own shorthand to make it faster and easier to read.

    money_in_wallet = 40

    sandwich_price = 7.50

    sales_tax = .08 * sandwich_price

    sandwich_price += sales_tax

    money_in_wallet -= sandwich_price

comments

A line of text preceded by a # is called a comment.

number

integer/ float(You can define floats with numbers after the decimal point or by just including a decimal point at the end:)(You can also define a float using scientific notation, with e indicating the power of 10:

    statement

    # this evaluates to 150:

    float4 = 1.5e2)

division

    python 2 when we divide two integers, we get an integer as a result. if the numbers do not divide evenly, the result of the division is truncated into an integer.To yield a float as the result instead, programmers often change either the numerator or the denominator (or both) to be a float

    statement

    quotient1 = 7./2

    # the value of quotient1 is 3.5

    quotient2 = 7/2.

    # the value of quotient2 is 3.5

    quotient3 = 7./2.

    # the value of quotient3 is 3.5

   # An alternative way is to use the float() method:

    quotient1 = float(7)/2

    # the value of quotient1 is 3.5

multi-line string

    If we want a string to span multiple lines, we can also use triple quotes:

statement

    address_string = """136 Whowho Rd

    Apt 7

    Whosville, WZ 44494"""

booleans

    A boolean is actually a special case of an integer. A value of True corresponds to an integer value of 1, and will behave the same. A value of False corresponds to an integer value of 0.

    statement

    a = True

    b = False

BUG 2 ValueError

 if we wanted to print out an integer as part of a string, we would want to convert that integer to a string first. We can do that using str()

int() #转换为整数型变量

float()#转换为float型变量