Decision Making in Python

In the last session, you understood about dictionaries; now, try to answer this simple question on dictionaries.

Dict = {1:'Asia',2:'Africa'}

From this dictionary, return the value when the key is 3, and if the key is missing, return ‘NA’. You cannot use Dict[3] because it throws an error, as 3 is not present in the dictionary. Instead, you can use the dictionary get method: Dict.get(3,'NA').

So, what exactly is happening using this method that is helping us avoid an error?

This method simply establishes a condition that specifies: if a key is absent, then print NA. This is nothing but an if-else construct. Before we look at the if-else construct, let’s quickly understand binary and relational operators. 

Having understood about relational operators, let’s now look at the if-else construct and the role of these relational operations in its implementation.

In the example discussed above: 

x = 10

if x%2  == 0:
    print(x, "is even number")
else:
  print(x, "is odd number")

x % 2 == 0 is a relational condition that would return 'true' or 'false' based on the value of x. If this condition is true, the set of statements under the if-block will be executed; otherwise, the statements under the else-block will be executed. 

Important things to note here are as follows:

The colon that indicates the start of block creation, The indentation here defines the scope of the if- or else- statement, Either code under the if-block is executed or under the else-block is executed, not both of them. 

So now that you have understood the basic if-else construct, can you try to write a code to return YES if x lies in the range of 1000 to 1100, else return NO?

If you break this down, you can get the following conditions:

Implementation:

if (X<1000):
    print('No')
else:
    if (X>1100 ):
        print('No')
    else:
        print('Yes')

You may think that this is not possible using a single if-else statement, but that is not true. You can use the logical operators in Python and combine two conditions. Let’s look at them in the following video:

In the video, you learnt that there are three types of logical operators: 

If you apply this concept to our earlier example for checking whether x lies in the range of 1000 to 1100, the code gets modified as shown below:

if (X>1000 & X<1100):
    print('YES')
else:
    print('No')

Isn’t this simple compared with the earlier code? That is how logical operators make our logic building easy.

Similar to an if-else construct, there is if-elif-else construct available in Python. Let’s understand it by looking at an example in our next video.

In the example shown in the video:

if shoppinng_total >= 5000:
    print("You won a discount voucher of flat 1000 on next purchase")
else:    
    if shoppinng_total >= 2500 :
        print("You won a discount voucher of flat 500 on next purchase")
    else:
        if shoppinng_total >= 1000:
            print("You won a discount voucher of flat 100 on next purchase")    
        else:
            print("OOPS!! no discount for you!!!")

Using an if-else construct would also give us an answer, but imagine that you have 10 conditions like this; wouldn’t it be clumsy? Instead, if you use something like an elif-construct, it would make the code more readable. 

In this segment, you learnt about relational operators, logical operators, if-else, if-elif- else and nested if-else constructs. You might be pretty confused about when to use each of them. Read the tips given below, which may help you to arrive at a conclusion faster.

TIPS:

When there is more than one condition to check and you need to:

1. Perform a different operation at each condition involving a single variable, you use an if-elif-else condition.
2. Just return a boolean functionality, i.e., Yes or No, you use logical operators in a single if-else construct.
3. Perform a different operation in each condition involving different variables, you use a nested if-else construct.

In the next segment, you will learn about the uses of looping constructs such as ‘for’ loops and ‘while’ loops.