Control Flow

Overview

Teaching: 10 min
Exercises: 20 min

Questions

• How do I repeat operations?

• How do I make decisions?

• How do I call built-in functions?

Objectives

• Write scripts that use for loops to iterate over lists and character strings.

• Write scripts that use if/elif/else to perform conditional operations.

• Call built-in functions.

• Call methods on strings and lists.

• Use online help to inspect functions’ documentation.

• Use range and for to iterate over a sequence of numbers.

• Correctly write programs that use if and else statements and simple Boolean expressions (without logical operators).

• Trace the execution of unnested conditionals and conditionals inside loops.

A for loop executes commands once for each value in a collection.

• for loops give items rather than indices.
  • Because that’s what people usually want.
• “for each thing in this group, do these operations”

for number in [2, 3, 5]:
    print(number)

2
3
5

The first line of the for loop must end with a colon, and the body must be indented.

• The colon at the end of the first line signals the start of a block.
• Python uses indentation rather than {} or begin/end to show nesting.
  • Any consistent indentation is legal, but almost everyone uses four spaces.

for number in [2, 3, 5]:
print(number)

IndentationError: expected an indented block

• Indentation is always meaningful in Python.

Use range to iterate over a sequence of numbers.

• The built-in function range produces a sequence of numbers.
  • Not a list: the numbers are produced on demand to make looping over large ranges more efficient.
• range(N) is the numbers 0..N-1

print('a range is not a list: range(0, 3)')
for number in range(0,3):
    print(number)

a range is not a list: range(0, 3)
0
1
2

Use if statements to control whether or not a block of code is executed.

• Structure is similar to a for statement.

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
    if mass > 3.0:
        print(mass, 'is large')

3.54 is large
9.22 is large

Use else to execute a block of code when an if condition is not true.

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
    if mass > 3.0:
        print(mass, 'is large')
    else:
        print(mass, 'is small')

3.54 is large
2.07 is small
9.22 is large
1.86 is small
1.71 is small

Use elif to specify additional tests.

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
    if mass > 9.0:
        print(mass, 'is HUGE')
    elif mass > 3.0:
        print(mass, 'is large')
    else:
        print(mass, 'is small')

3.54 is large
2.07 is small
9.22 is HUGE
1.86 is small
1.71 is small

Compound Relations Using and, or, and Parentheses

Often, you want some combination of things to be true. You can combine relations within a conditional using and and or. Continuing the example above, suppose you have

mass     = [ 3.54,  2.07,  9.22,  1.86,  1.71]
velocity = [10.00, 20.00, 30.00, 25.00, 20.00]

i = 0
for i in range(5):
    if mass[i] > 5 and velocity[i] > 20:
        print "Fast heavy object.  Duck!"
    elif mass[i] > 2 and mass[i] <= 5 and velocity[i] <= 20:
        print "Normal traffic"
    elif mass[i] <= 2 and velocity <= 20:
        print "Slow light object.  Ignore it"
    else:
        print "Whoa!  Something is up with the data.  Check it"

Just like with arithmetic, you can and should use parentheses whenever there is possible ambiguity. A good general rule is to always use parentheses when mixing and and or in the same condition. That is, instead of:

if mass[i] <= 2 or mass[i] >= 5 and velocity[i] > 20:

write one of these:

if (mass[i] <= 2 or mass[i] >= 5) and velocity[i] > 20:
if mass[i] <= 2 or (mass[i] >= 5 and velocity[i] > 20):

so it is perfectly clear to a reader (and to Python) what you really mean.

One more thing: in Python, comparisons can be chained; for example,

2 < mass[i] <= 5

is equivalent to

2 < mass[i] and mass[i] <= 5

Reversing a String

Fill in the blanks in the program below so that it prints “nit” (the reverse of the original character string “tin”).

original = "tin"
result = ____
for char in original:
    result = ____
print(result)

Accumulating

Fill in the blanks in each of the programs below to produce the indicated result.

# Total length of the strings in the list: ["red", "green", "blue"] => 12
total = 0
for word in ["red", "green", "blue"]:
    ____ = ____ + len(word)
print(total)

# List of word lengths: ["red", "green", "blue"] => [3, 5, 4]
lengths = ____
for word in ["red", "green", "blue"]:
    lengths = lengths.____(____)
print(lengths)

# Concatenate all words: ["red", "green", "blue"] => "redgreenblue"
words = ["red", "green", "blue"]
result = ____
for ____ in ____:
    ____
print(result)

# Create acronym: ["red", "green", "blue"] => "RGB"
# write the whole thing

Cumulative Sum

Reorder and properly indent the lines of code below so that they print an array with the cumulative sum of data. The result should be [1, 3, 5, 10].

cumulative += [sum]
for number in data:
cumulative = []
sum += number
print(cumulative)
data = [1,2,2,5]

Identifying Variable Name Errors

1. Read the code below and try to identify what the errors are without running it.
2. Run the code and read the error message. What type of NameError do you think this is? Is it a string with no quotes, a misspelled variable, or a variable that should have been defined but was not?
3. Fix the error.
4. Repeat steps 2 and 3, until you have fixed all the errors.

for number in range(10):
    # use a if the number is a multiple of 3, otherwise use b
    if (Number % 3) == 0:
        message = message + a
    else:
        message = message + "b"
print(message)

While Loops

Python also has a while loop that keeps going as long as some condition is true:

x = 15
while x > 0:
  print(x)
  x = x - 5

15
10
5

Use a while loop to print every second character in the string ‘fluorine’.

Trimming Values

Fill in the blanks so that this program creates a new list containing zeroes where the original list’s values were negative and ones where the origina list’s values were positive.

original = [-1.5, 0.2, 0.4, 0.0, -1.3, 0.4]
result = ____
for value in original:
    if ____:
        result.append(0)
    else:
        ____
print(result)

[0, 1, 1, 1, 0, 1]

Initializing

Modify this program so that it finds the largest and smallest values in the list no matter what the range of values originally is.

values = [...some test data...]
smallest, largest = None, None
for v in values:
    if ____:
        smallest, largest = v, v
    ____:
        smallest = min(____, v)
        largest = max(____, v)
print(smallest, largest)

What are the advantages and disadvantages of using this method to find the range of the data?

Key Points

• Repeat actions for each element in a collection with for loops.

• Use range to generate a list of numbers.

• Use if/elif/else to make choices.

• Use built-in functions like len and max to do calculations.

• Objects like strings and lists have methods that operate on them.

• Use if statements to control whether or not a block of code is executed.

• Conditionals are often used inside loops.

• Use else to execute a block of code when an if condition is not true.

• Use elif to specify additional tests.

• Conditions are tested once, in order.

• Create a table showing variables’ values to trace a program’s execution.