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Python November 14, Unit 7 Python • Hello world, in class Strings • • A string is a bunch of characters In >>>print “Hello, world!”, the string is: Hello, world! • • We can have numbers inside strings >>>print “Today is my friend’s 35th birthday!” >>>print “4+4” Strings are one type of value that python can manipulate Values and Type • Values are some of the basic items that your program can manipulate • “Hello, world!” is a value of the type string • >>>print 1+1, the value of this is 2 and is of the type integer • So what is a type? – Values are categorized according to their type Types • It’s easy to see that “Hello, world!” and the value of 4+4 are of different types – “Hello, world!” is a string – 4+4 produces 8, which is a number • They have different types • 1.2 and 4 also have different types – 4 is an integer (whole number can be positive or negative) – 1.2 is called a float (or floating-point number) • Can also be positive or negative • If it has a decimal point it’s usually a float • Like real numbers • “4+4” is of the type string – If it’s in quotes, it is a string What’s so Important about Types? • The type of value dictates what you can do with it • >>>print 8-4 – Has a value of 4 • >>>print “Hello, world!”-H – We may think this means take away the “H” in “Hello, world!” – But the interpreter has no idea what we mean – We can’t subtract a letter from a string in this way • One of the reasons for using types is that different types of information are stored differently – Characters are not stored the same way as integers – Integers and floating point numbers aren’t stored the same way Mathematical Expressions • Should all be familiar with a mathematical expression – 4+4 –8*2 – 24 / 2 + 6 * 3 • An expression is any kind of calculation that returns a result Expressions, in General • We can have things other than mathematical expressions • A value by itself is considered an expression – >>>4 – >>>”Hello, world!” • Expressions are any combination of values, variables, and operators – We’ll get more into this later Mathematical Expressions, cont. • Mathematical expressions have two parts: – Operators – Operands • Operators are the symbols we use for math – – – – – – + addition - subtraction / division * multiplication ** power (exponent) () • Operands are the values we use these operators on – 4+8/6*3 • Operands are: 4, 8, 6, 3 • Operators are: + , /, * – Operandscan also be variables Evaluating Mathematical Expressions • In python (like in most programming languages), mathematical expressions are evaluated like we learned in grammar school • PEMDAS – Parenthesis, Exponents, Multiplication, Division, Addition, and Subtraction – Please Explain My Dear Aunt Sally – Multiplication and Division have the same precedence • As do Subtraction and Addition • Evaluated left to right • Doesn’t really matter 3+4-2 – 7-2 = 5 – 3+2=5 Mathematical Expressions • In class example • Integer division: – – – – – When you divide one integer by another Produces another integer Always rounds down 2/3 = 0 4/3 = 1 • Floating point division: – If we want the decimal information at least one of the two operands must be a float • 2.0 /3 • 2 / 3.0 • 2.0 / 3.0 In-Class Examples • Playing with mathematical expressions Variables • Variables are a way to store data so that we can use it over and over >>>x = 4 + 8 >>>print x 12 – x is the variable • We can store data of any type in a variable >>>name=“Sarah Brown” >>>print name Variables, cont. • Let’s say we have the expression “8 *2 +4” and we want to store it in a variable called “x” >>>x = 8 *2 + 4 • The value of 8 *2 + 4 is calculated, resulting in 20 • This is then assigned to the variable x using the “=“ operator • This is called an assignment statement Variables, cont. • We can assign the value of one variable to another variable >>> y = x • We can assign an expression using a variable to another variable >>>y = x*2 + 7 • Variables do not store the expression – They store the value of the expression – If x = 4, y has the value of 15 – y does not have the value “x *2 + 7” User Input • Now that we have a way to store data, we can get input from users and manipulate it • To prompt the user for information we are going to use raw_input(“Whatever you want to ask for”) and assign that to a variable • Example: >>>name = raw_input(“What is your name”) >>>print name, “is such a pretty name” raw_input, cont. • Another example: >>>age = raw_input(“How old are you? “) >>>print “you are “, age,”years old! That is SO old!” • So when printing out the above statement we want to put the value for age in between two strings – One way to do this is to use a “,” – Items in python separated by a comma are considered items in a list – Just prints them in order raw_input, cont. • One of the issues with using raw_input is that all input from the user is considered to be a string • Take the following: >>>base = raw_input(“Enter a number”) >>>print “ 2 times base is: “, base • The out put would be : “2 times base is: basebase” • If the user entered the number 4 “2 times base is 44” • The user could have entered a name if they had wanted and it would print it twice – If base = “pizza” – “2 times base is pizzapizza” Another Example with raw_input • Let’s say instead of multiplying base by 2 we want to output base+1 >>>base = raw_input(“Enter a number”) >>>print base+1 • This would cause an error – We can’t add the number 1 to a string • We need to convert the value in base from a string to a number – We are going to use that concept of types Type Function • We can use the function type to determine the type of a variable >>>base = raw_input (“enter a number”) >>>type(base) Will output <type ‘str’> >>>type(4) will output <type ‘int’> >>>type(4.0) will output <type ‘float’> Type Conversion • We can use different functions to convert from one type to another – int() converts whatever is inside the () to an integer – float() converts the contents of the () to a floating point number – str() converts contents of () to a string • We can use these function to convert the input from the user to the type that we need Type Conversion, example >>>base = int(raw_input(“enter a number”)) >>>print base +1 If the user enters the number 5, 6 will be output In Class Example • Getting input from the user – Storing input in a variable • Using multiple variables • Converting from one type to another Concatenation • So far when we want to print multiple things we’ve been using a comma to separate the items • Better way is to concatenate the items • Concatenation is like string addition • >>>print “Hello,” + “world!” – Outputs “Hello, world!” • We can only concatenate strings >>>x = 4+8 >>>print “4 + 8 = “ + x Causes an error Using str() • Like we can change the type of input from the user from a string to a number, we can go back in the other direction >>>x = 4 + 8 >>>print “4 + 8 = “ + str(x) Would output: “4 + 8 = 12” In Class Examples • Putting it all together • Converting input to number then back to a string for printing Questions • The main points from today’s lecture: – Types (int, float, string) – Mathematical Expressions • PEMDAS – Variables • Assignment statements – User Input (raw_input()) • Always produces a string – Type conversion