CSci 150: Foundations of computer science
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else clauses & methods

else clauses

Each if statement is allowed to have an else clause, specifying statements that should be done only when the if condition fails. In writing this, the e starting else should line up directly under the i in if.

password = input()
if password == 'friend':

If the user types friend, the computer will display Enter followed by Ready (skipping over any statements in the else block). If the user types something else, then the computer will display Fail followed by Ready.

for loop over lists

So far, we have only used the for loop to iterate over integers, using the range function. However, we can simply name a list after the in keyword:

primes = [23571113]
total = 1
for p in primes:
    total = total * p

Python does the for loop's body once for each element in the list. In this case, it does the body with p being 2, then with p being 3, then with 5, then with 7, then with 11, and finally with p being 13.

String methods

Thus far, we've seen a wide variety of functions.

abs int min round str
float len print sorted sum
input max range

You call a function by first writing the function name, followed by parentheses stating the arguments. The function does some work and provides a return value.

num_letters = len(name)

A method is a different way of getting Python to do some work and produce a return value. However, a method “belongs” to a value, so in calling a method you first write the value followed by a period and the method name, followed by parentheses including any arguments.

num_bs = name.count('e')

This invokes the count method of the string name, passing it the string e as an argument. As it happens, the count method counts and returns the number of times times e appears in name. So if name refers to the string Shannon, name.count('n') returns 3.

Python includes many methods — many more than the number of functions. Each type of data has its own set of methods, but for now we'll concentrate just on the methods that strings have. Here are seven worth knowing about at first.

Returns a string with all lower-case letters; shannon if s is Shannon
Returns a string with all upper-case letters; SHANNON if s is Shannon
Returns a string with all leading and trailing spaces removed; “Eva Lu” if s is “  Eva Lu 
Returns a string with all spaces on the left removed; “Eva Lu ” if s is “  Eva Lu 
Returns a string with all spaces on the right removed; “  Eva Lu” if s is “  Eva Lu 
Finds the non-space strings in s, returning a list of those “words” found; if s is “  Eva Lu  Ator ”, the list [EvaLuAtor] is returned.
Given an argument words that is a list of strings, creates a larger single string containing each string in words separated by the string s; if s is “-&-” and words is [EvaLuAtor], s.join(words) is Eva-&-Lu-&-Ator.