Control statements in Python

1.1   Blocks

What is a block?

It is a group of statements.

Python uses spaces and/or tabs to create a block.

The standard width of a tab is 8 spaces, but many editors support different widths.

I would not mix tabs and spaces for Python blocks.

How many spaces do you need? You can use any number of spaces, but just be careful to be consistent in the same block. We’ll see how blocks work in the next section.

1.2   if

The if statement executes its block only if the condition is truthy

Try

x = int(input("x? "))
if x == 0:
  print('x is 0') # use 2 spaces for this block

What about this?

x = int(input("x? "))
if x > 0:
  print(x)          # 2 spaces
    print('x is 0') # 4 spaces in the same block, inconsistent!

Try

x = int(input("x? "))
if x == 0:
  print('x is 0')            # 2 spaces
if x < 0:
    print(x)                 # 4 spaces
    print('x is negative')   # 4 spaces, consistent
if x > 0:
      print(x)               # 8 spaces
      print('x is positive') # 8 spaces, consistent

1.3   else

The else statement executes its block if the previous if or elif condition is falsy. Compare these two:

x = int(input("x? "))
if x <= 0:
    print('x is not positive')
if x > 0: # this condition is NOT of the above
    print('x is positive')

x = int(input("x? "))
if x <= 0:
    print('x is not positive')
else: # NOT x <= 0
    print('x is positive')

1.4   elif

The elif statement is a shorthand for an if statement inside an else block. You just need one less block. Compare these two.

x = int(input("x? "))
if x <= 0:
    print('x is not positive')
else:
    if x > 5:
        print('x > 5')

x = int(input("x? "))
if x <= 0:
    print('x is not positive')
elif x > 5:
    print('x > 5')

A series of if, elif, and else is possible, but there should be only one else at the end logically.

x = int(input("x? "))
if x <= 0:
    print('x is not positive')
elif x > 5:
    print('x > 5')
else:
    print('0 < x <= 5')

1.5   Exercise: Branch

Try this code. Can you control the branching (if-elif-else) algorithm by changing your input to print a certain string (one, two, or other)?

x = input("Enter data: ")
if x == 1:
    print("one")
elif x == 2:
    print("two")
else:
    print("other")

1.6   Exercise: Debug branching

Did the branching exercise work for you? Why or why not? Can you debug it?

1.7   Exercise: Print the range of an integer using if

You need to test if x is between $(0,2]$, $(2,4]$, $(4,6]$, $(6,8]$, or $(8,10]$. What is your algorithm using if statements? Here, $(a, b]$ means that a number is greater than $a$, but less than or equal to $b$. That is $a<\text{number}\leq b$.

Does this code work? Why or why not? How would you fix it?

x = int(input("x? "))
if x <= 0:
    print('x <= 0')
if x > 0:
    print('x in (0,2]')
if x > 2:
    print('x in (2,4]')
if x > 4:
    print('x in (4,6]')
if x > 6:
    print('x in (6,8]')
if x > 8:
    print('x in (8,10]')
if x > 10:
    print('x > 10')

x = int(input("x? "))
if x <= 0:
    print('x <= 0')
if x > 0 and x <= 2:
    print('x in (0,2]')
if x > 2 and x <= 4:
    print('x in (2,4]')
if x > 4 and x <= 6:
    print('x in (4,6]')
if x > 6 and x <= 8:
    print('x in (6,8]')
if x > 8 and x <= 10:
    print('x in (8,10]')
if x > 10:
    print('x > 10')

x = int(input("x? "))
if x <= 0:
    print('x <= 0')
if 0 < x <= 2: # chain and's
    print('x in (0,2]')
if 2 < x <= 4:
    print('x in (2,4]')
if 4 < x <= 6:
    print('x in (4,6]')
if 6 < x <= 8:
    print('x in (6,8]')
if 8 < x <= 10:
    print('x in (8,10]')
if x > 10:
    print('x > 10')

1.8   Exercise: Print the range of an integer using if, elif, and else

Let’s try the same algorithm again, but you’re a little smarter now. You need to test if x is between $(0,2]$, $(2,4]$, $(4,6]$, $(6,8]$, or $(8,10]$.

x = int(input("x? "))
if x <= 0:
    print('x <= 0')
elif x <= 2:
    print('x in (0,2]')
elif x <= 4:
    print('x in (2,4]')
elif x <= 6:
    print('x in (4,6]')
elif x <= 8:
    print('x in (6,8]')
elif x <= 10:
    print('x in (8,10]')
else:
    print('x > 10')

2.1   for

A for loop iterates each item in an array-like object and runs the block.

# tuple
for i in (1, 2, 3):
    print(i)

# list
for i in [1, 2, 3]:
    print(i)

# range
for i in range(1, 4):
    print(i)

2.2   while

A while loop tests a given condition and loops the block as long as the condition is truthy.

What does this code print?

x = 1
while x <= 10:
    print(x)
    x = x + 1

2.3   continue

While inside a for or while loop, you may want to skip the rest of the block and move to the next iteration. In this case, use the continue statement.

What is this code supposed to print? Does it even work as intended? Why not?

x = 1
while x <= 10:
    if x % 2:
        continue
    print(x)
    x = x + 1

x = 1
while x <= 10:
    if x % 2:
        x = x + 1
        continue
    print(x)
    x = x + 1

2.4   break

Use the break statement to get out a for or while loop without looping it further. You only exist one level of a loop.

What does this code do?

for x in range(1, 10):
    for y in range(1, 10):
        if y < 3:
            continue
        if y > 6:
            break # exit the y loop only
        print(x+y)

2.5   Exercise: for and list

Iterate over each element in a list and print it.

A = [12, 34, 45, 67]
for a in A:
    print(a)

2.6   Exercise: for and forward indexing

Let’s iterate over a list and print its elements using indexing.

A = [12, 34, 45, 67]
n = len(A)
for i in range(n):
    a = A[i]
    print(a)

2.7   Exercise: for and reverse indexing

Let’s iterate over a list and print its elements using reverse indexing.

A = [12, 34, 45, 67]
n = len(A)
for i in range(n):
    a = A[n-1-i]
    print(a)

2.8   Exercise: Summation using a for loop

Summation from $1$ to $n$ i expressed as $x=\sum_{i=1}^n i$. What this equation means is that we add all $i$'s from $1$ up to $n$ and assign that result to $x$. Let’s try $n=2$ first. That’s $x=\sum_{i=1}^2 i=1+2=3$. If $n=3$, $x=\sum_{i=1}^3 i=1+2+3=6$. If $n=4$, $x=\sum_{i=1}^4 i=1+2+3+4=10$. Let’s write the $n=2$ case in Python.

x = 0 # we need this variable x called an accumulator because
      # we're going to accumulate the values of i in this variable
for i in range(1, 3): # we start from 1 and stop at 2;
                        # guess why you need 3, not 2
    x = x + i
print(x)

2.9   Exercise: Summation using a while loop

What about the while loop version of summation for $n=2$? Let’s try.

i = 1
x = 0
while i <= 2:
    x = x + i
    i = i + 1 # you have to advance i by i yourself because while doesn't do this for you
print(x)