Compute Loops

Now that we know how a few collection/sequence style objects work we can dive into some more powerful ways to utilize them through looping.

Looping is a relatively simple concept. It provides a means of telling the program to run the same bit of code over and over a number of times. This is useful because it prevents the programmer from having to rewrite the same code over and over. There a few different types of loops. The first one we are going to look at is the “while” loop.

The ‘while’ loop

The while loop is pretty simple. It tells the computer that while a certain condition is true, keep doing a certain type of code. The classic example of the while loop involves printing a series of numbers to the console using three lines of code. It looks like this:

a = 0
while a < 10:
    print a
    a = a + 1

Notice, at the end of the “while” line I added a colon “:”. The colon lets the interpreter know that the indented code on the following lines is what should be run until the “while” condition is met. In this case, as you can probably guess, we are instructing the computer to to print the variable a as long as it has a value that is less than ten. One thing that can be annoying about Python is the indentation. Your indentations can be spaces or tabs but not both. Meaning if your first indentation is done with spaces, all of the following indentations must also be spaces. If you use tabs, then you have to continue using tabs. The number of spaces doesn’t matter, so long as it is consistent. The general rule of thumb is 4 spaces for each indentation. In Notepad++ there is an option in the “Settings > Preferences” dialog for “tab settings” you can set it to replace your tabs with spaces. I would recommend using that option if you use Notepad++.

While loops can be very useful, but there is also a danger in using them. Imagine what might happen if we didn’t include the last line in the code above, a = a + 1. Without that line, a would always be equal to zero, meaning that the “while” condition would never be met and the computer would continue printing a indefinitely until the end of time. (If this happens press “ctrl + c” to terminate your program.) This is known as an “infinite loop” and it is to be avoided. As a safety measure, when writing a while loop, it is a good idea to put the the code in that will complete the loop first and then insert the code that needs to run during the loop above it. There have been many times that I have written while loops that have some complicated code to run and by the time I finished writing the code I forgot to add the line that would fulfill the while condition.

Here is an example of a while loop that prints all of the numbers in the fibonacci sequence up to 1000.

a = 0
b = 1
while b < 1000:
    print b
    x = a
    a = b
    b = x + b

Try running the code again, but add a comma after the “print b” line.

The ‘for’ loop

The ‘for’ loop is a wonderful little tool for accessing all of the elements in a sequence/collection style object. That means that for loops can be used with strings, lists, and tuples (as well as a few other objects). It works like this:

for <variable> in <object>:
    do something

Let’s learn through practice. Make a string object called myString and set it equal to some word. Then use a for loop to print out each letter individually like so:

for letter in myString:
    print letter

The ‘letter’ in the above example doesn’t have to be ‘letter’ it could be any variable name. The word that comes after ‘for’ is a variable name. The for loop then loops through the sequence object and assigns each item in the object to the variable name specified. This looping is known as “iterating” in programmer jargon and so the act of looping is known as “iteration”. So, if you are describing the above program you can say that “I am iterating through my string and printing out each character.” In the above example we are telling the interpreter: for each character in myString, assign the character to a variable called letter, then do whatever code is below.

So in my example myString = “cow” and when I ran the code it printed the letters c o w. I could also have told it to print myString and it would have printed “cow” three times. Why? Because the for loop performs some code for each item in the object. “cow” has three characters so for each character it will run the code “print myString”.

So you see, you don’t only have to use for loops to do things with the items in a sequence or collection object. For instance, if there wasn’t a len() function available to us, how might we write one using a for loop? Observe:

length = 0
for x in myString:
    length = length + 1

print length

Here we ignore the value of x, we simply add 1 to the value of our length integer object each time we encounter an item in the myString object. Notice that when I added the print length line I didn’t add any indentation. This lets the interpreter know that it shouldn’t run the print line until it is finished with the for loop. So the interpreter knows that the any code that is indented under the for loop should be run in the loop, when it finds a line that is not indented it will not run it until the for loop is complete. See what happens when I don’t indent the print statement.

Loops can also be used with lists and tuples in the same way that they are used above. For example:

myList = [‘camera’, ‘steve’, 4, ‘cheesesteak’]
myTuple = (‘flippy’, 912, 0.0074, myList, 99, 42, ‘bilbo’)
for item in myTuple:
    print type(item)

That’s it for loops for now. Next post we will discuss the if / else statement and boolean objects and I’ll probably throw in some comparison operators…anyway, more on that next time.