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)

masses = [3.54, 2.07, 9.22, 1.86, 1.71]

for m in masses:
    if m > 3.0:
        print(m, 'is large')
        print(m, 'is small')

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.

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.

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')
        print(m, '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')
  • 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
        print('moving too slow')
        velocity = velocity + 10.0
print('final velocity:', velocity)
masses = [3.54, 2.07, 9.22, 1.86, 1.71]

def get_values(min,max):
    Gets the values inside an array in a given range
    for m in masses:
        # Here we can use multiple conditions to execute the code block
        if (m >= min and m <= max and m > 3.0):
            print(m, 'is large')
        elif(m >= min and m <= max):
            print(m, 'is small')