This post will provide a brief introduction into the use of the elif clause and nested statements in Python.

elif Clause

The elif clause is used to add an additional set of conditions to an if statement. If you have ever used some sort of menu on a computer in which you had to make several choices it is possible that an elif clause was involved in the code.

The syntax for the elif clause is the same as for the if statement. Below is a simple example that employs the elif clause.

Here is what this code does.

1. In lines 1-3, I print three lines of code at the beginning. These are the choices available to the user.
2. In line 4, the “pick” variable stores whatever number the user inputs through the “input” function. It must be an integer which is why I used the “int” function
3. In line 5 we begin the if statement. If the “pick” variable is set to 1 you can see the printout in line 6.
4. In lines 7 and 8 we use the elif clause. The settings are mostly the same as in the if statement in line 5 except the “pick” variable is set to different numbers with different outputs.
5. Lastly, in line 11 we have the else clause. If for any reason the person picks something besides 1,2 or 3 this message will appear.

What this code means in simple English is this

• If the pick is 1 then print “dogs are your favorite animal”
• or else if the pick is 2 then print “cats are your favorite animal”
• or else if the pick is 3 then print “rabbits are your favorite animal”
• else print “I do not understand your choice”

Here is what the code looks like when you run it

As a note, if you type in a letter or string you will get an error message. This is because our code is not sophisticated enough to deal with non-integers at this point.

Nested Statements

Just like with functions which can be nested so can decision statements by nesting inside each other. To put this simply the conditions set by the first if statement can potentially affect the second condition. This is explained better with an example.

Here is what is happening in the code above.

1. In lines 1 and 2 the user has to pick numbers that are saved in the variables “num1” and “num2.”
2. Line 3 and 4 are the if statements. Line 3 and line 9 are the outer if statement and line 4-8 are the inner if statement.
3. Line 3 shares that “num1” must be between 1 and 10.
4. Line 4 shares that “num2” must be between 1 and 10.
5. Line 5 is the results of the inner if statement. The results are printed using the “.format” method where the {0} and {1} are the variables “num1” and “num2”. After the comma is what is done to the variables, they are simply added together.
6. Line 8 is the else clause. If someone types something different form a number between 1-10 for the second number they will see this message
7. Line 9 is the else clause for the outer if statement. This is only seen if a value different from 1-10 is inputted.

If you run the code here is what it should look like

Conclusion

The elif clause and nested decision statements are additional tools that can be used to empower your applications. This is some of the most basic ideas in using a language such as Python.

Applications often have to make decisions and to do this they need a set of conditions to him them decide what to do. Python, like all programming languages, has tools that allow the application to execute various actions based on conditions. In this post, we will look at the use of if statements.

If Statement Define

An if statement is a statement used in Python that determines when an action should happen. At a minimum, an if statement should have two parts to it. The first part is the condition and the second part is the action that Python performs if the condition is true. Below is what this looks like

if SOMETHING IS TRUE:
    DO THIS

This is not the most beautiful code but this is the minimum for a if statement in Python.

If Statement 

Below is the actual application of the use of an if statement

 number=5
if number ==5:
   print("Correct")
 
Correct

In the code above we created a variable called number and set its value to 5. Then we created the if statement by setting the condition as “if number equals 5” the action for this being true was prin the string “correct”, which Python does at the bottom.

Notice the double equal sign used in the if statement. The double equal sign is used for relational equality while the single equal sign is used for assigning values to variables. Also, notice the colon at the end of the if statement. This must be there in order to complete the code.

There is no limit to the number of tasks an if statement can perform. In to code above the, if statement only printed “Correct” however, we can put many more print functions or other actions as shown below.

number=5
if number ==5:
   print("Correct")
   print("Done")
 
Correct
Done

All that is new above is that we have two print statements that were executed because the if statement was true.

Multiple Comparisons

In the code above python only had to worry about whether number equaled 5. However, we can have Python make multiple comparisons as well below is an example.

number=5
if (number>0) and (number<10):
 print("Correct")
 print("Finished")
 
Correct
Finished

Now for the print functions to execute the number has to be greater than 0 but less than 10. If you change the number to something less than 0 or greater than 10 nothing will happen when you run the code because the conditions were not met. In addition, if you type in a letter like ‘a’ you will get an error message because Python is anticipating a number and not a string.

If Else Statements

The use of an else clause in an if statement allows for an alternative task to be executed if the first conditions in the if statement are not met. This allows the application to do something when the conditions are not met rather than leaving a blank screen. Below we modify our code in the previous example slightly so that something happens because of the else clause in the if statement.

number=15
if (number>0) and (number<10):
 print("Correct")
 print("Finished")
else:
 print("Number is out of range")
 
Number is out of range

In this code above we did the following

1. we set our number to 15
2. We created an if statement that searches for a number greater than 0 and less than 10.
3. If the conditions in step 2 are true we print the two statements
4. If the conditions in step 2 are not met we print the statement after the else clause
5. Our number was 15 so Python printed the statement after the else clause

It can get much more complicated and powerful than this but I think this is clear and enough for now.

Conclusion

This post provided an introduction to the if statement in Python. THe if statement will execute a command based on one or more conditions. You must be careful with the syntax. Lastly, you can also include an alternative task with the else clause if the conditions are not met.