WEEK01: intro to python, unix, cs21
-----------------------------------
F: using editor (vim), loops, range, sequences
Announcements:
- Lab 0 due Saturday night
- Lab 1 will be posted sometime Sunday
PRACTICE UNIX (change username to your username):
pwd
ls
update21
ls cs21/inclass
ls cs21/labs
cd cs21/labs/w01-intro
cat JK-takehomepay.py
REVIEW of JK-takehomepay.py
"""
estimate montly take home pay
J. Knerr
Fall 2013
"""
salary = raw_input("Salary: ")
salary = float(salary)
# caclulate monthly take home pay
mthp = (salary/12.0) * 0.7 # assumes gvmt takes 30%
print "Approx monthly take-home-pay = $" + str(mthp)
- how many functions are called in this program?
- what does the function raw_input() return?
- what are the comments in this program? why do we add comments?
LOOPS:
example of a program that uses a loop:
$ python squares.py
I will print out the squares of all numbers from 1 to n!
Please enter n: 6
----------------------------------
1 x 1 = 1
2 x 2 = 4
3 x 3 = 9
4 x 4 = 16
5 x 5 = 25
6 x 6 = 36
----------------------------------
for < var > in < sequence >:
do this
and this
and any indented line
over and over and over
code here is not part of loop
- code block is the 4 indented lines above
- code block is executed with var = 1st item in sequence
- then again with var = 2nd item in sequence
- then again with var = 3rd item in sequence
- and so on...
< var > is a variable you choose (i, x, foo, ch, etc)
< sequence > is a special python set of values, which
can be any of these:
- a string of characters, like "Hello!!"
- a list of integers, like [1,2,3,4,5]
- a list of anything, like [1,"hi",5.6,"no]
- all lines in a file (week 7)
>>> for i in [1,2,3,4,5,6,7]:
... print i
... print i*"HI! "
... print "============="
...
1
HI!
=============
2
HI! HI!
=============
3
HI! HI! HI!
=============
4
HI! HI! HI! HI!
=============
5
HI! HI! HI! HI! HI!
=============
6
HI! HI! HI! HI! HI! HI!
=============
7
HI! HI! HI! HI! HI! HI! HI!
=============
an easy way to create a list is with the range function:
>>> range(5)
[0, 1, 2, 3, 4]
>>> range(1,10)
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> range(1,20,2)
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
and try help(range) to get help with the built-in range() function:
>>> help(range)
Help on built-in function range in module __builtin__:
range(...)
range([start,] stop[, step]) -> list of integers
Return a list containing an arithmetic progression of integers.
range(i, j) returns [i, i+1, i+2, ..., j-1]; start (!) defaults to 0.
When step is given, it specifies the increment (or decrement).
For example, range(4) returns [0, 1, 2, 3]. The end point is omitted!
These are exactly the valid indices for a list of 4 elements.
and here's a for loop example using range:
>>> for i in range(20,1,-2):
... print "----->" + str(i)
...
----->20
----->18
----->16
----->14
----->12
----->10
----->8
----->6
----->4
----->2
---> can you write this program?
$ python blastoff.py
countdown start: 7
. . . . 7
. . . . 6
. . . . 5
. . . . 4
. . . . 3
. . . . 2
. . . . 1
blastoff!!