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CS 140: Introduction
to Computer Programming
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Defining
Functions
Just as with the functions you probably remember
from your math classes, a function in
programming is a block of code that takes one or
more inputs, does something with them, and then
(usually) returns a value of some sort.
The basic format looks like this.
def block(letter):
print letter*3
print letter*3
This function takes a string (presumably a
single character) and then prints a block of
this letter; two rows of three letters
each. So including the line block("c")
within your program will result in a block of c's
being printed to the screen. Or one could
"blockify" an entire word by typing
word = raw_input("Enter a word: ")
for
letter in
word:
block(letter)
For the record, function definitions usually
appear at the start of a program.
Functions usually return a value based on the
inputs. For instance, we could define a
function that counted the number of times we can
divide a given number by 3 before dropping below
1 as follows.
def divbythree(value):
count = 0
while value >=
1:
value = value/3
count = count + 1
return count
The return
command sends the output back to the main
program. So we are now able to very
efficiently check whether a particular number
can be divided by 3 exactly twice within a
program by typing if
divbythree(number) == 2:,
for instance. This is one of the main
advantages of implementing functions within a
program—it often results in code that is both
more efficient and more readable.
As a final illustration, let's define a function
that takes two inputs, an interest rate and a
yearly contribution, and determines how much
money will accrue over a forty year span in an
account that adds interest once a year just
before a new contribution is made.
def value(contrib,rate):
total = 0
for j in range(0,40):
interest = float(total)*rate/100
total = total + interest + contrib
return total
We could now neatly make a chart of account
values based on various interest rates and
annual contributions using just a few lines of
code.
for j in range(1000,10000,1000):
for k in range(2,7):
print
value(j,k),
print
This is another big advantage of using
functions; namely, better organized programs.
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