Loops do the heavy lifting in a program, allowing it to do things that humans cannot. For example, a security system might have a loop integrated into its program to check a camera once every second. Computers are always vigilant, unlike some security guards, and you don’t have to pay them! Loops simply execute a block of code over and over again until a stopping point.
There are two types of loops in Python:
for loop - interates through an iterable itemwhile loop - runs while an expression is trueDon’t worry if those definitions are confusing, because I’ll be explaining them. A for loop iterates (or cycles) through an iterable variable, like a list. On each cycle, the loop variable is set to a different value. This is the format for a for loop (remember this!):
for loop_variable in iterable_variable:
do
something
here
A specific example:
toDoList = ["eat brick", "fly sideways", "photograph the moon"] #initialize todo list
for thingToDo in toDoList: #loop through each item
print thingToDo #print each item
This program simply prints my todo list item by item. What Python is doing is initializing thingToDo with every item in toDoList, then running the indented code section underneath it (called the loop block). When the indented section is done, it cycles back around to the top of the indented section until it has cycled through the entire list. With this loop you can perform an operation given each item in a list.
Another example is multiplying the numbers from 1 to 100. (I don’t know why any sane individual would want to do that, but let’s do it anyway!) Here is the code:
n = 1 #this is the variable we will be multiplying by each number
for i in range(1, 100): #this is the top of the loop
n = n * i #with each loop, n's value is increasing
print n #display the final value of n
Let’s explain what’s going on here. On the first line, n is set to the value of 1. On the next line, the loop is setup: i is our loop variable and range(1, 100) is our iterable variable. This means that Python will cycle through the contents of range(1, 100) and for each item in that variable, i will be set to that variable. So on the first loop i is equal to 1. On the next loop i is equal to 2, and so on. The third line is the indented, so this will be the code that we will run for each item in our iterable. In this case, n is being set to its current value times the value of i. This means that n is always getting larger in value, because i is growing in value as the program runs. At the end of the program, the value of n will be the product of all the numbers from 1 to 100!
Here is an example of a more difficult-to-understand program, but I’m sure you guys will be able to figure it out!
xValues = range(1, 100) #numbers from 1 to 100
yValues = [] #this is an empty list
for x in xValues: #loop start
y = x**2 + 5*x + 8 #this is the most important line
yValues.append(y) #add y value to the list
print yValues
This program fills the list yValues with 100 numbers. Each number in the list is the output of x**2 + 5*x + 8 given a certain x value. Let’s look at the start of the loop. On the first cycle of the loop, what is the value of x? Well, it is just the first item in the list xValues. We know that xValues is just a list of all the numbers from 1 to 100, so the first item in the list is just 1. That means on the first iteration of our loop, x is 1. On the next iteration, it is the next item in the list xValues, which is 2. On each iteration, there is a value added to the yValues list. This value is dependent on the value of x, because y = x**2 + 5*x + 8. I hope you can visualize what is going on. The output of this program is always:
[14, 22, 32, 44, 58, 74, 92, 112, 134, 158, 184, 212, 242, 274, 308, 344, 382, 422, 464, 508, 554, 602, 652, 704, 758, 814, 872, 932, 994, 1058, 1124, 1192, 1262, 1334, 1408, 1484, 1562, 1642, 1724, 1808, 1894, 1982, 2072, 2164, 2258, 2354, 2452, 2552, 2654, 2758, 2864, 2972, 3082, 3194, 3308, 3424, 3542, 3662, 3784, 3908, 4034, 4162, 4292, 4424, 4558, 4694, 4832, 4972, 5114, 5258, 5404, 5552, 5702, 5854, 6008, 6164, 6322, 6482, 6644, 6808, 6974, 7142, 7312, 7484, 7658, 7834, 8012, 8192, 8374, 8558, 8744, 8932, 9122, 9314, 9508, 9704, 9902, 10102, 10304]
I hope you can see from this example that computers are VERY powerful math machines. What a computer can do in milliseconds would take you many excruciating hours of back-breaking, brain-hurting, miserable calculations.
This loop executes while an expression is true. This is the format:
while expression:
do
something
here
This is like a repeated conditional. Python runs the code inside the loop over and over again until the while condition becomes False. When that happens, Python moves on to the code underneath it. This is like a for loop except that this loop runs as long as its condition is True. A for loop only runs as many times as there are items in the iterable variable.
#Simple Password Program
passwordWrong = True
while passwordWrong:
password = raw_input("Password:") #this is an operation called a function that retrieves user input and puts it in a string
if password == "1L0V3CSCLUB":
passwordWrong = False #this will make the while condition False
else:
print "That password it wrong! Try again!"
print "That password is correct!"
This is a real-world example of some code I wrote to automatically download NY Times article data from their database. I didn’t do this because I love their articles, but because I wanted a computer to replicate their headlines. I don’t expect you to understand much of it:
for year in years:
data = collectInfoByYear(year, keys)
print("Writing %d year info to %s..." % (year, filename))
fp.write(data)
This code block writes NY Times article properties to a file given a range of years. I chose to grab all the article data from 1852 to 2017. I’m so glad I don’t pay for internet by the byte! Oh, and by the way this took under a minute to grab and write to a file!
for i in range(int(duration*self.SAMPLE_RATE)):
if self.channels == 1:
self.data += [[(random.random() * 2) - 1]]
elif self.channels == 2:
self.data += [[random.uniform(-1, 1), random.uniform(-1, 1)]]
This code block is a part of a function that generates static that you can use for various audio manipulations. duration*self.SAMPLE_RATE is how many data points to write to the sound file. self.data is the variable that holds all the audio data. random is a Python package that is used to generate random numbers, so I used those numbers to make static.