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CS 140: Introduction
to Computer Programming
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Manipulating
Strings
As we have seen, a string is a sequence
of characters, including letters, numbers,
symbols, and even spaces, usually typed on a
keyboard. We specify strings by placing
them within quotes, such as "Big Bang Theory",
"90210",
or "#&@$(%!".
For the most part our strings will involve only
letters, numbers and spaces.
We can assign a string to a variable name, just
as we can for numbers. We can also print
strings to the screen. Therefore the code
place = "carnival"
print "Let's go to the",
print place,
print "!"
would yield the output Let's go to the
carnival !
We can "add" two or more strings with a +
sign. This is known as concatenation
and has the effect of creating one long string
consisting of the smaller strings placed
immediately next to one another. Thus
defining word1="moun"
and word2="tain"
would result in word3=word1+word2+"s"
being "mountains".
Concatenation can be used to fine tune output
format since it does not introduce an extra
space, unlike the comma. For instance,
notice the (unwanted) space between the word
carnival and the exclamation point above.
If we had instead typed
place = "carnival"
print "Let's go to the",
place+"!"
then the output would read Let's go to the
carnival! as
intended.
Python also understands how to "multiply" a
string by a positive integer; it simply produces
that many copies of the string. Thus "la"*5
would result in "lalalalala".
This construction provides a shortcut for
tabbing over repeatedly or inserting lots of
carriage returns. Thus to skip seven lines
and tab over six times, type
print "\n"*7, "\t"*6,
Similarly, one can easily print a precise amount
of white space. Thus print " "*9
gives exactly nine spaces.
Python will tell you how long a particular
string is via the len
function. For example, len("carnival")
gives 8, but len("
carnival ") is 10,
because spaces count as characters. Along
the same lines, the program
word = raw_input("Enter
the name of a country: ")
print "Your
country has", len(word), "letters."
will count letters in a word entered by the
user.
Finally, Python will allow you to pinpoint a
single character or row of characters within a
string once you indicate the position (or index)
of the desired characters. The catch is
that you must count positions like a computer,
which means starting with 0. Thus the
string "carnival"
has a 'c'
in position 0, an 'a'
in position 1, all the way up to an 'l'
in position 7.
To pick off a single character within a string,
put its position in square brackets following
the string. Thus word[0]
will give the first letter of the word, while word[3]
is the fourth letter. One can also type word[-1]
to obtain the last letter, or word[-2]
for the next to last letter, and so forth.
On a slightly fancier note, you can obtain an
entire range of characters by specifying a pair
of positions, such as in word[2:5].
BEWARE: this
only returns the letters in positions 2, 3, 4; not
the letter in position 5! (So this range
behaves a lot like the range in for
loops.) So if word="carnival"
then word[2:5]
is equal to "rni".
Let's illustrate all of these ideas by creating
a SLU username:
first = raw_input("Enter your first name:
")
middle = raw_input("Enter your middle name:
")
last = raw_input("Enter your last name:
")
year = raw_input("Enter your starting year
at SLU: ")
username =
first[0]+middle[0]+last[0:4]+year[2:4]
print "Your username is",
username+"."
So if Fred Ivan Shingledecker started at SLU in
2012, the above program would assign him the
username fishin12.
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