Introduction to Python

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Math and String Operators

+, -, *, /: math operators

Math operators behave as you might expect.

var_int = 5
var2_float = 10.3

var3_float = 5 + 10.3       # int plus a float:  15.3, a float

var4_float = 10.3 - 0.3     # float minus a float:  15.0, a float

var5_float = 15.0 / 3       # float divided by an int:  5.0, a float

• The general rule on type is that if a float is involved, the result will be a float.
• If only ints are involved, the result will be int.
• However one exception is integer division: this will always return float type, regardless of remainder.

Ex. 2.2

identifying type through an operation

Every operation or function call results in a predictable type.

With two integers, the result is integer. If a float is involved, it's always float.

vari = 7
vari2 = 3
varf = 3.0

var3 = var * var2      # 35, an int.

var4 = var + var2      # 10.0, a float

However when an integer is divided into another integer, the result is always a float, even if there is no remainder.

var = 6
var2 = 3

var3 = var / var2      # 2.0, a float

• division always returns float to maintain consistency with other division operations
• we usually don't worry too much about ints vs. floats, because they work well together
• however, it is important to always be aware of type when evaluating the result of any operation

pythonreference

** exponentiation operator

The exponentiation operator (**) raises its left operand to the power of its right operand and returns the result as a float or int.

var = 11 ** 2     # "eleven raised to the 2nd power (squared)"
print(var)        # 121

var = 3 ** 4
print(var)        # 81

% Modulus Operator

The modulus operator (%) shows the remainder that would result from division of two numbers.

var = 11 % 2      # "eleven modulo two"
print(var)        # 1   (11/2 is 5, with a remainder of 1)


var2 = 10 % 2     # "ten modulo two"
print(var2)       # 0   (10/2 is 5, with a remainder of 0)

• modulus shows the remainder of a division
• modulus with 2 can be useful because it shows us whether a number is even or odd

+ operator with strings: concatenation

The plus operator (+) with two strings returns a concatenated string.

aa = 'Hello, '
bb = 'World!'

cc = aa + bb     # 'Hello, World!'

Note that this is the same operator (+) that is used with numbers for summing. Python uses the type of the operands (values on either side of the operator) to determine behavior and result. Ex. 2.5

* operator with one string and one integer: string repetition

The "string repetition operator" (*) creates a new string with the operand string repeated the number of times indicated by the other operand:

aa = '!'
bb = 5

cc = aa * bb       # '!!!!!!'

Note that this is the same operator (*) that is used with numbers for multiplication. Python uses the type of the operands to determine behavior and result. Ex. 2.6

+ operator "overloading"

Object types determine behavior.

int or float "added" to int or float: addition

tt = 5            # assign an integer value to tt
zz = 10.0         # assign a float value to zz

qq = tt + zz      # compute 5 plus 10 and assign float 15.0 to qq

str "added" to str: concatenation

kk = '5'          # assign a str value (quotes mean str) to kk
rr = '10.0'       # assign a str value to rr

mm = kk + rr      # concatenate '5' and '10.0'
                  # to construct a new str object, assign to mm

print(mm)         # '510.0'

• the plus operator serves double duty depending on what types are used
• we call this type of behavior 'overloaded'
• it also refers to an Object-Oriented term: 'polymorphism'

* operator "overloading"

Again, object types determine behavior.

int or float "multipled" by int or float: multiplication

tt = 5            # int, 5
zz = 10           # int, 10

qq = tt * zz      # int, 50 (5 * 10)
print(qq)         # 50

str "multiplied" by int: string repetition

aa = '5'          # str, '5'
bb = 3            # int, 3

cc = aa * bb      # str, '555' ('5' * 3)

• once again, object types determine what is possible (and not possible) in the language
• this is why it's so important to know the type of every variable

introduce the concept of labs

studying for the quizzes and the midterm and final exam

the exercises and weekly assignments are practice for the exams

• the midterm and final will consist of short programs to write and test
• you will be graded on the proper use of coding features
• you will not be permitted to use features that have not been discussed in the course
• the best way to study is to complete each of the weekly exercises
• if you have limited time, you can concentrate on the lab exercises (those named as _lab.py)
• make note of the features used, and how particular tasks are done
• the problems given in the midterm and final will use the same features as those needed for the exercises
• again, the coding assignments in the midterm and final must be done using features we have covered
• you will not get credit for working programs that use outside features!