12/12/2024
BRANCHING,
ITERATION
a1
a2
LAST TIME
syntax and semantics
scalar objects
simple operations
expressions, variables and values
1
Diapositive 2
a1 amina; 05/10/2024
a2 amina; 05/10/2024
12/12/2024
TODAY
string object type
branching and conditionals
indentation
iteration and loops
STRINGS
letters, special characters, spaces, digits
enclose in quotation marks or single quotes
hi = "hello there"
concatenate strings
name = "ana"
greet = hi + name
greeting = hi + " " + name
do some operations on a string as defined in Python docs
silly = hi + " " + name * 3
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Example
###################
## EXAMPLE: strings
###################
#hi = "hello there"
#name = "ana"
#greet = hi + name
#print(greet)
#greeting = hi + " " + name
#print(greeting)
#silly = hi + (" " + name)*3
#print(silly)
INPUT/OUTPUT: print
used to output stuff to console
keyword is print
x = 1
print(x)
x_str = str(x)
print("my fav num is", x, ".", "x =", x)
print("my fav num is " + x_str + ". " + "x = " + x_str)
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Example
####################
## EXAMPLE: output
####################
#x = 1
#print(x)
#x_str = str(x)
#print("my fav number is", x, ".", "x=", x)
#print("my fav number is", x_str + "." + "x=" + x_str)
#print("my fav number is" + x_str + "." + "x=" + x_str)
INPUT/OUTPUT: input("")
prints whatever is in the quotes
user types in something and hits enter
binds that value to a variable
text = input("Type anything... ")
print(5*text)
input gives you a string so must cast if working
with numbers
num = int(input("Type a number... "))
print(5*num)
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COMPARISON OPERATORS ON
int, float, string
i and j are variable names
comparisons below evaluate to a Boolean
i > j
i >= j
i < j
i <= j
i == j equality test, True if i is the same as j
i != j inequality test, True if i not the same as j
LOGIC OPERATORS ON
bools
a and b are variable names (with Boolean values)
not a True if a is False
False if a is True
a and b True if both are True
a or b True if either or both are True
A B A and B A or B
True True True True
True False False True
False True False True
False False False False
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COMPARISON EXAMPLE
pset_time = 15
sleep_time = 8
print(sleep_time > pset_time)
derive = True
drink = False
both = drink and derive
print(both)
If right clear, If right blocked, If right and If right , front,
go right go forward front blocked, left blocked,
go left go back
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12/12/2024
If right clear, If right blocked, If right and If right , front,
go right go forward front blocked, left blocked,
go left go back
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CONTROL FLOW - BRANCHING
if <condition>: if <condition>:
<expression> <expression>
<expression> <expression>
... ...
elif <condition>:
if <condition>: <expression>
<expression> <expression>
<expression> ...
... else:
else: <expression>
<expression> <expression>
<expression> ...
...
<condition> has a value True or False
evaluate expressions in that block if <condition> is True
INDENTATION
matters in Python
how you denote blocks of code
x = float(input("Enter a number for x: "))
y = float(input("Enter a number for y: "))
if x == y:
print("x and
print("x and yy are
are equal"
equal")
if y !=
if y != 0 0:
:
print("therefore, xx // yy is",
print("therefore, is", x/y
x/y)
elif x < y:
print("x is smaller")
print("x is smaller"
print("y is smaller"
else:
print("y is smaller")
print("thanks!")
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12/12/2024
= vs == INDENTATION
matters in Python
how you denote blocks of code
x = float(input("Enter a number for x: "))
y = float(input("Enter a number for y: "))
if x == y:
print("x and y are equal")
if y != 0:
print("therefore, x / y is", x/y)
elif x < y:
print("x is smaller")
else:
print("y is smaller")
print("thanks!")
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12/12/2024
Legend of Zelda –
Lost Woods
keep going right,
takes you back to this
same screen, stuck in
a loop
Image Courtesy Nintendo, All Rights Reserved. This content is excluded from our
Creative Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-
use/.
if <exit right>:
<set background to woods_background>
<set background to woods_background
if <exit right>
if <exit right>: :
<set background to woods_background>
<set background to woods_background
if <exit right>
if <exit right>:
<set background to woods_background>
set background to woods_background
and so on and on and on..
and so on and on and on...
else
else:
<set background to exit_background>
set background to exit_background
else
else:
<set background to exit_background>
<set background to exit_background
else:
<set background to exit_background>
_background
exit
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12/12/2024
Legend of Zelda –
Lost Woods
keep going right,
takes you back to this
same screen, stuck in
a loop
while <exit right>:
<set background to woods_background >
<set background to exit_background>
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CONTROL FLOW:
while LOOPS
while <condition>:
<expression>
<expression>
...
<condition> evaluates to a Boolean
if <condition> is True, do all the steps inside the
while code block
check <condition> again
repeat until <condition> is False
while LOOP EXAMPLE
You are in the Lost Forest.
************
************
************
************
Go left or right?
PROGRAM:
n = input("You're in the Lost Forest. Go left or right? ")
while n == "right":
n = input("You're in the Lost Forest. Go left or right? ")
print("You got out of the Lost Forest!")
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CONTROL FLOW:
while and for LOOPS
iterate through numbers in a sequence
# more complicated with while loop
n = 0
while n < 5:
print(n)
n = n+1
# shortcut with for loop
for n in range(5):
print(n)
CONTROL FLOW: for LOOPS
for <variable> in range(<some_num>):
<expression>
<expression>
...
each time through the loop, <variable> takes a value
first time, <variable> starts at the smallest value
next time, <variable> gets the prev value + 1
etc.
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range(start,stop,step)
default values are start = 0 and step = 1 and optional
loop until value is stop - 1
mysum = 0
for i in range(7, 10):
mysum += i
print(mysum)
mysum = 0
for i in range(5, 11, 2):
mysum += i
print(mysum)
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break STATEMENT
immediately exits whatever loop it is in
skips remaining expressions in code block
exits only innermost loop!
while <condition_1>:
while <condition_2>:
<expression_a>
break
<expression_b>
<expression_c>
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break STATEMENT
mysum = 0
for i in range(5, 11, 2):
mysum +=
= i
if mysum == 5:
5
break
mysum += 1
print(mysum)
what happens in this program?
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In-Class Questions
Lecture 2
Strings
What is the value of variable `u` from the code below?
once = "umbr"
repeat = "ella"
u = once + (repeat+" ")*4
o umbrella ella ella ella
o umbrellaellaellaella
o umbrella
o umbrella4
In-Class Questions
Comparisons
What does the code below print?
pset_time = 15
sleep_time = 8
print(sleep_time > pset_time)
derive = True
drink = False
both = drink and derive
print(both)
o False then False
o False then True
o True then False
o True then True
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In-Class Questions
Branching
What’s printed when x = 0 and y = 5?
x = float(input("Enter a number for x: "))
y = float(input("Enter a number for y: "))
if x == y:
if y != 0:
print("x / y is", x/y)
elif x < y:
print("x is smaller")
else:
print("y is smaller")
o x is smaller
o y is smaller
o x / y is 0.0
In-Class Questions
While Loops
In the code below from Lecture 2, what is printed when you type
“Right”?
n = input("You're in the Lost Forest. Go left or right? ")
while n == "right":
n = input("You're in the Lost Forest. Go left or right? ")
print("You got out of the Lost Forest!")
o You’re in the Lost Forest. Go left or right?
o You got out of the Lost Forest!
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In-Class Questions
For Loops
What is printed when the below code is run?
mysum = 0
for i in range(5, 11, 2):
mysum += i
if mysum == 5:
break
mysum += 1
print(mysum)
o 5
o 6
o 21
o 24
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