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Announcements
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ECS10
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10/10
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Interest Compounded
Monthly
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Costs you more than interest compounded
annually.
Program Crash
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Why is this program
crashing?
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It tells us the line:
monthlyRate = annualRate
annualRate/12.0
/12.0
It tells us what it doesn’t like:
unsupported operand type(s) for /: ''str
str'' and
'float‘
Function raw_input
raw_input()
() returns a string
„ Cannot divide a string by 12.0
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First midterm a week from today, in class
Open book, open notes
No computers, no calculators
Bring a Scantron 2000 form, available in
bookstore
We will also ask you to write a short program
Last year’s midterm and program on Web site
Python stops and refuses to run your program
any more because it contains an error.
Nasty red error messages
Y
Your
goall as a programmer is for
f your
programs never to crash.
crash.
Windows crashes sometimes. IDLE crashes
sometime. So annoying! Bad programming!
Converting strings to
numbers
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Use Python functions:
„ int
int()
()
„ float()
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Examples:
x = int(“26”)
int(“26”) # x now contains the integer
26
y = float(“7.5”) # y now contains the float
7.5
1
Still crashes!
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The input to float() has to be a string that
represents a float.
The input to int()
int() has to be a string that
represents an integer
float(“2.366”) # does not crash
float(“cow”) # crashes!
int(“3.45”)
int
(“3.45”) # crashes!
We’ll solve this with a new function
function..
Operators and Functions
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x = 66--7
x = 8.0 / 3.44
print
rint ‘Molly’+’,’
‘M ll ’+’ ’
if (x < 20) and (x >= 10):
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We can’t control what the user enters!
Need to check user’s input before we do
anything with it that might cause a crash.
P h doesn’t
Python
d
’ have
h
a function
f
to checks
h k
whether a string can be converted to a float
or to an int
There is a way to do this, but we haven’t
learned the right parts of Python yet….
A helpful module
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Helper.py
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Our module will be called helper
It’s in the file helper.py
You need to have this file in the same folder
as your program so that
h Python
P h can find
f d it.
To get access, at top of program file:
Some functions:
functions:
reply = raw_input
raw_input(‘Enter
(‘Enter weight: ‘)
weight = int(reply)
int(reply)
How to fix?
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Some operators:
operators:
You need a checking function
We’ll give you a checking function
We put it in a module that you can import
A module is a file that extends Python (a plugplugin, an add
add--on).
Anybody can write a module!
Ability to extend the language makes it really
useful.
Two functions in helper
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helper.isFloat(), helper.isInt()
helper.isFloat(),
helper.isInt()
Both take a string as input
Both return a Boolean value as output
isFloat(“9.2”) # is True
helper.isFloat (“12”) # True
helper.isFloat(“three”) # False
import helper
2
Looping for input
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The program is not crashing.
It tells the user what is wrong and asks again
for input.
Parts of program
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Getting input
Doing computation – while loop
Writing output
Prettier Output
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You can ask Python to cut off extra decimal
places.
‘% 2f’ % totalInterest
‘%.2f’
lI
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First part says output will be a string with two
digits after decimal place, made out of a float
% operator connects format and floating point
data
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