Add Flow Control to your Python application

The if statement is probably the most well-known flow control statement in Python. To show you how that works, let's create a variable called X, and X is going to convert to an integer, the input it receives from a console prompt.

We'll evaluate X with an if statement and say if X is less than zero, we'll set the value to zero and then print negative number change to zero. We can run that with the Python 3 command and then provide the name of the file.

I'm prompted for the integer, so if I enter the value negative 1, it evaluates to true in the if statement, sets the value to zero and then prints out our statement. If we run it again and I give it something that won't evaluate to true such as the number 2, whenever it runs, the if statement doesn't evaluate to true and there's nothing left in the program to do, so the program exits.

Let's expand on it a little bit and give it something else to do. If statements can have optional elif and else statements, so let's give it an elif which stands for else if, and we'll say if X is equal to zero, then we'll just print the word zero.

Notice that the if and elif conditional statements aren't indented, but the code blocks that execute when they evaluate to true are indented. This is important because in Python everything is space sensitive so you have to get it right or your program won't execute.

Next, we'll say elif X equal to 1, then we're going print out the word 1 and we'll give it a final else clause that's going to execute if none of the other statements evaluate to true. Let's run it again and if I give it a negative 1, that's the condition, we originally had, so that still works, and run it one more time.

If I give it the integer of 1, the 1 is printed out, and if I give it a number that we didn't code for such as 3, that triggers the else statement which prints out some other number and then exits.

An if statement constructed like this with multiple elifs is a substitute in Python for a switch or case statements that you'll find in other languages. A for statement can be used to iterate over any sequence which can be a list or a string.

To show you how those work, I'm just going to go into the Python console. I'm going to create a list called pets and inside of the list we'll have cat, dog and elephant. I'm going to start our for loop by typing for pet in pets.

We've got the keyword for and it's going to iterate over the list object pets and for each item in the pets list, it's going to create a temporary variable called pet with that value. Inside the for block, we're going to print out I have a, and then use the format method to and the positional operator to print out the name of the pet.

As the for loop iterates through the list, the variable pet is set to the list element from that iteration. We can also create a for loop in combination with the range operators. We can say for i in range, and then specify the end of the range. It'll increment up to but not including the endpoint number specified in the range.

You can also specify the starting point of the range in the range method if you supply two arguments. The first one is the starting point, the second one is the endpoint and as that runs, it starts as 10 and then goes up to but doesn't include the endpoint of 15.

You can specify the increment or the step for it as well. The increment or step is provided as the third argument to the range method, and as this executes it starts at zero, goes up to but doesn't include 10 counting by two's.

You can increment with negative numbers as well. This time if we start the range at 10 and then go down to zero with a step of negative 1, when it runs, it counts from 10 all the way down to 1. You can use a while statement to keep a loop that continues running as long as a given condition is true.

I've got a variable of X set to 1 and I can say while X is less than 10, print X and then increment X by 1. Remember that the code inside of your loop block has to be indented, and when this runs, it continues to print in an increment the value of X until the statement X less than 10 no longer evaluates to true.

All of the loop statements with the exception of while will continue to iterate and loop until they've reached the end of their given sequence. Our original for statement iterated over the list of pets, cat, dog and elephant, and once it started, it continued iterating until it got to elephant, the last item in the list.

You can terminate a loop with a break statement. If we do for pet in pets, and then evaluate if the pet is equal to the word dog, we'll print no dogs allowed and then break. Otherwise or in Python lingo else, we'll print out we love and use our positional operator with the format method to print out the name of the pet.

If we look at what happened here, the first item in the list, cat, doesn't evaluate to true in the if statement, so it triggered the else and printed we love cat. The next item dog does evaluate to true, so it printed no dogs allowed and then broke the loop and the word elephant was never reached in iteration.