CS1-FULL-FALL2021
for
Syntax…
for a_variable in set_of_values:
# code block of for
while condition:
# code block of the while
Value is a *variable* declared in the cycle for,
In each iteration (cycle) it takes a successive value in
the set of elements specified after the word in
Result
The same in another way…
The list can be inside a variable …
What prints this program?
A.None
B.[123, 345, 456, 567]
C.[61.5, 172.5, 228.0, 283.5]
What prints this program?
A.None
B.[123, 345, 456, 567]
C.[61.5, 172.5, 228.0, 283.5]
The lists are types “by reference”, that is, by
passing them to a function as a parameter,
the function receives a reference and not a
copy. If the function modifies the list, then
the list is modified after the function has
been executed.
!
How can we access randomly an
element of a list?
?
Strings
Strings
• In Python a string is a list with characters
• So we can use the concept of a list to find patterns on a
string or to find words on a big string
The split() function
• In many situations, we need to divide a string into
chunks.
• For example, if we have a list of names in a string:
• ”John, Mary, Mike, Gabriel, Bill”
• How do we transform this string into a list and access
the fourth name (Gabriel)?
• split() – returns a list with the words of the original
substring that are delimited by spaces
• split(separator) – returns a list with the words of the
original substring that are delimited by the substring
separator
The split() function
• ”John, Mary, Mike, Gabriel, Bill”
• How do we transform this string into a list and access
the fourth name (Gabriel)?
• We use the function split () to get a list with the
individual names.
It’s fine, but it’s
not exactly what
we want
split () is
separating
the elements
considering
the spaces as
separators
How do we remove the commas?
?
The problem is that
now it leaves us
spaces
Unlike the previous
case, we specify that
the list is delimited by a
comma (,)
How do we remove the spaces?
?
The strip() function
The strip () function returns
the clean string of spaces on
the left and on the right
We keep the coma a
separator from the list
Let’s review the
Functions:
• find(x) – delivers the value of the index that the substring occupies
equal to x within the string
• find(x, start) – gives the value of the index that occupies the
substring equal to x within the string, looking from the start
position onwards
• lower() – returns a copy of the original string with all the letters in
lower case, it does not modify the original string
• upper() – returns a copy of the original string with all the letters in
upper case, it does not modify the original string
• replace(old, new) – returns a copy of the original string with all the
letters in lower case, it does not modify the original string
Dividing a list (sublist)
• We can divide any list in any size
• To do that we use the square brackets [] an the : to say the data we
want
y = [99, 999, 77, 777, 88, 888]
x= y[0 : 3]
x = [99, 999, 77]
a = y[2 : len(y)]
a = [77, 777, 88, 888]