Python - Loops


Let's get to some of the more exciting stuff in programs. One of the main reasons we write programs is to automate tasks. Let's say we have a list of salaries we want to count. This is a pretty simple example that can be solved in Excel, but can serve as a good introduction to loops in python. We will discuss two types of loops: While and For.

While loops

salaires = [100, 200, 200]
totalSalaries  = 0

index = 0
while index < len(salaries):
    totalSalaries += salareis[index]
    index += 1

This first example show the use of a while loop. What we are saying here is "while the variable called index is less than the size of our salaries list, keep looping".

We are also accessing the list of salaried with the index. For example salaries[index] will start by being salaries[0] which equals 100, the first item in our list.

A key thing to remember is to increment your index (or a different variable that you may use). For example, we have a line index += 1 to keep the index increasing. If we forgot this, our program would run forever (or at least until we stop the program :P).

Because of this issue, it is my recommendation that you try to use the for loop for most cases. Let's take a look at that.

For Loops

salaires = [100, 200, 200]
totalSalaires = 0

for salary in salaires:
    totalSalaries += salary

We can see that this is a little more concise than the while loop. Also, we can use the in keyword to automatically grab the element in our list. So here we are saying, "for each salary in our salaries, do the following."

Python will give us each value of the list 100, 200, 200 in that order. We then add them each to our total.

Notice there is not need to worry about incrementing an index. So, this keep our programs safe and prevents us from getting into an infinite loop. While this is a benefit, sometimes you will need to use a while loop. Be sure to practice both :D.

Further Reading