Conditionals

Overview

Teaching: 10 min
Exercises: 15 min

Questions

• How can programs do different things for different data?

Objectives

• 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.

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

• An if statement (more properly called a conditional statement) controls whether some block of code is executed or not.
• Structure is similar to a for statement:
  • First line opens with if and ends with a colon
  • Body containing one or more statements is indented (usually by 4 spaces)

mass = 3.54
if mass > 3.0:
    print(mass, 'is large')

mass = 2.07
if mass > 3.0:
    print (mass, 'is large')

3.54 is large

Conditionals are often used inside loops.

• Not much point using a conditional when we know the value (as above).
• But useful when we have a collection to process.

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

• else can be used following an if.
• Allows us to specify an alternative to execute when the if branch isn’t taken.

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

• May want to provide several alternative choices, each with its own test.
• Use elif (short for “else if”) and a condition to specify these.
• Always associated with an if.
• Must come before the else (which is the “catch all”).

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

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

Conditions are tested once, in order.

• Python steps through the branches of the conditional in order, testing each in turn.
• So ordering matters.

grade = 85
if grade >= 70:
    print('grade is C')
elif grade >= 80:
    print('grade is B')
elif grade >= 90:
    print('grade is A')

grade is C

• Does not automatically go back and re-evaluate if values change.

velocity = 10.0
if velocity > 20.0:
    print('moving too fast')
else:
    print('adjusting velocity')
    velocity = 50.0

adjusting velocity

• Often use conditionals in a loop to “evolve” the values of variables.

velocity = 10.0
for i in range(5): # execute the loop 5 times
    print(i, ':', velocity)
    if velocity > 20.0:
        print('moving too fast')
        velocity = velocity - 5.0
    else:
        print('moving too slow')
        velocity = velocity + 10.0
print('final velocity:', velocity)

0 : 10.0
moving too slow
1 : 20.0
moving too slow
2 : 30.0
moving too fast
3 : 25.0
moving too fast
4 : 20.0
moving too slow
final velocity: 30.0

• The program must have a print statement outside the body of the loop to show the final value of velocity, since its value is updated by the last iteration of the loop.

Find the First

Fill in the blanks to create a function that takes a list of numbers as an argument and returns the first negative value in the list. What does your function do if no negative number exists? What if the list is empty?

def first_negative(values):
    for v in ____:
        if ____:
            return ____

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.

Tracing Execution

What does this program print?

pressure = 71.9
if pressure > 50.0:
    pressure = 25.0
elif pressure <= 50.0:
    pressure = 0.0
print(pressure)

Hint

The variable ‘pressure’ is being updated. Is there any logical way that either the ‘if’ or ‘elif’ will not evaluate to True? Do you think Python would allow both ‘if’ and ‘elif’ to evaluate to True?

Solution

answer: 25.0

71.9 is greater than 50.0, so the ‘if’ statement evaluates to True. Once an ‘if’ or ‘elif’ evaluates to True the remaining conditionals are skipped, so even though pressure is now < 50.0, it is not reassigned to 0.0 in the ‘elif’ statement.

Trimming Values

Fill in the blanks.

The result should be a new list of 1’s and 0’s, where positive values are converted to 1’s, and negative values are converted to 0’s.

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)

# Predicted output
[0, 1, 1, 1, 0, 1]

Solution

original = [-1.5, 0.2, 0.4, 0.0, -1.3, 0.4]
result = []
for value in original:
    if value < 0:
        result.append(0)
    else:
        result.append(1)
print(result)

Initializing

Fill in the blanks.

This function should find the largest and smallest values in a list.

def min_max(values):
    smallest, largest = None, None
    for v in values:
        if ____:
            smallest, largest = v, v
        ____:
            smallest = min(____, v)
            largest = max(____, v)
    return ____

print(min_max([...some test data...]))

Hint

What does ‘None’ resolve to if evaluated in a conditional?

The ‘if’ statement should only execute on the first iteration of the for-loop

Don’t forget to add some actual numbers to the list passed to min_max()!

Solution

def min_max(values):
    smallest, largest = None, None  # Initialize some variables but don't set any actual values (you don't know what the `values` list will contain a priori)
    for v in values:
        if not smallest:  # `None` evaluates to `False`, so the 'double negative' evalutes to `True` on the first pass through the for-loop
            smallest, largest = v, v  # Now it's time to set actual values to `smallest` and `largest`
        else:
            # After the first pass through the loop, every `v` will be checked against the current min and max
            smallest = min(smallest, v)
            largest = max(largest, v)
    return smallest, largest

print(min_max([...some test data...]))

Bring it all together by processing a FASTA file

The FASTA format specification consists of a header, prefixed with a greater-than symbol (>), that contains the sequence IDs and space-separated metadata, followed by the sequence data on one or more subsequent lines.

Use nano to open up the FASTA file in the ‘sequences’ directory.

$: nano swc-data/sequences/pannexins.fa

 GNU nano 2.2.6          File: swc-data/sequence/pannexins.fa

>Dme-Panx1 FBgn0004646 dmInx1.
MYKLLGSLKSYLKWQDIQTDNAVFRLHNSFTTVLLLTCSLIITATQYVGQPISCIVNGVP
PHVVNTFCWIHSTFTMPDAFRRQVGREVAHPGVANDFGDEDAKKYYTYYQWVCFVLFFQA
MACYTPKFLWNKFEGGLMRMIVMGLNITICTREEKEAKRDALLDYLIKHVKRHKLYAIRY
WACEFLCCINIIVQMYLMNRFFDGEFLSYGTNIMKLSDVPQEQRVDPMVYVFPRVTKCTF
HKYGPSGSLQKHDSLCILPLNIVNEKTYVFIWFWFWILLVLLIGLIVFRGCIIFMPKFRP
RLLNASNRMIPMEICRSLSRKLDIGDWWLIYMLGRNLDPVIYKDVMSEFAKQVEPSKHDR
AK
>Dme-Panx2 FBgn0027108 dmInx2.
MFDVFGSVKGLLKIDQVCIDNNVFRMHYKATVIILIAFSLLVTSRQYIGDPIDCIVDEIP
LGVMDTYCWIYSTFTVPERLTGITGRDVVQPGVGSHVEGEDEVKYHKYYQWVCFVLFFQA
ILFYVPRYLWKSWEGGRLKMLVMDLNSPIVNDECKNDRKKILVDYFIGNLNRHNFYAFRF
FVCEALNFVNVIGQIYFVDFFLDGEFSTYGSDVLKFTELEPDERIDPMARVFPKVTKCTF
HKYGPSGSVQTHDGLCVLPLNIVNEKIYVFLWFWFIILSIMSGISLIYRIAVVAGPKLRH

^G Get Help    ^O WriteOut    ^R Read File   ^Y Prev Page   ^K Cut Text    ^C Cur Pos
^X Exit        ^J Justify     ^W Where Is    ^V Next Page   ^U UnCut Text  ^T To Spell

For the following exercise write a program that will print only the IDs from the Drosophila melanogaster (Dme) sequences, like this:

Dme-Panx1
Dme-Panx2
Dme-Panx3
Dme-Panx4
Dme-Panx5
Dme-Panx6
Dme-Panx7
Dme-Panx8

To read the file into your program, use the following code snippet:

with open("pannexins.fa", "r") as ifile:
   lines = ifile.readlines()

The ‘readlines()’ function will create a new list and then append each line in the file to that list.

NOTE: This is NOT the recommended solution for very large files, because readlines() stores everything in the file in memory.

Solution

# Read the file into a list
with open("pannexins.fa", "r") as ifile:
    lines = ifile.readlines()

# Loop over every line, searching for the header lines
for line in lines:
    # Confirm that the line is a header, not sequence
    if line[0] == ">":
        # 'split' the string into a list on the space (" ") character
        line_split = line.split(" ")

        # The sequence ID is the first index of the header list
        seq_id = line_split[0]

        # Remove the leading > symbol from the ID
        seq_id = seq_id[1:]

        # Test if the first three characters of the ID are "Dme"
        if seq_id[:3] == "Dme":
            print(seq_id)

Key Points

• 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.