Loops and if-tests

Important

python uses whitespace to denote a code block in if-statements, loops, functions, and more.

Loops

The python while loop behaves just like in C++:

n = 0
while n < 10:
    print(n)
    n += 1

Notice that the condition ends with a : and then the body of the loop is indented.

For the loop above, we would usually use the range function (this is analogous to C++ std::ranges::iota that we looked at iota):

for n in range(10):
    print(n)

Looping over containers

We already saw that we can use for to loop over elements of a list. This also works for characters in a string, and pretty much any other container in python.

For example, we can loop over characters in a string:

for c in "the quick brown fox jumps over the lazy dog":
    print(c)

but strings have lots of methods, and we can use the .split() method to split based on spaces and loop over words:

for w in "the quick brown fox jumps over the lazy dog".split():
    print(c)

Conditionals

python if-statements work the same as in C++. The C++ combination else if is contracted to elif in python, and whitespace is used to denote the block that is executed for each branch:

x = 0

if x < 0:
    print("negative")
elif x == 0:
    print("zero")
else:
    print("positive")

Examples

Machine epsilon

In Homework #5, we found machine \(\epsilon\) by looping until \(1 + \epsilon == 1\). Let’s rewrite that in python:

Listing 136 epsilon.py

epsilon = 1.0
while 1.0 + epsilon != 1:
    epsilon /= 2

print(f"epsilon = {epsilon}")

Notice how similar this code is to the C++ version.

Fibonacci sequence

Let’s rewrite our Fibonacci sequence example in python:

Listing 137 fibonacci.py

import sys

# initialize a list with 2 elements

fib = [0, 1]

N = int(input("How many terms of the Fibonacci sequence to compute? (N > 2) "))

if N <= 2:
    # we've already got 2
    sys.exit("Error: must enter N > 2")

# add to the sequence
while len(fib) < N:
    fib.append(fib[-1] + fib[-2])

# now output -- we can just print the list
print(fib)

A few notes:

• We use input() to read from the terminal. This will return a string, so we cast it to an int using int().

• We can easily access the last 2 elements of the list using fib[-1] and fib[-2].

• Unlike the C++ version (which used long), we don’t overflow even if we ask for 100 terms, because python integers grow as needed.