If condition

If conditions are the most basic tool in programming, it allows computer to decide which code gets executed and which blocks to skip.

number_of_apples = 100

if number_of_apples > 5:
    # lets call this block 1
    print("Sir, you have an obscure amount of apples. Are you ok?")
else:
    # lets call this block 2
    print("The number of apples you have is ok. Eat some.")

this code decides if block 1 or 2 gets executed based on number_of_apples > 5 condition. If it evaluates to True block 1 it is. Otherwise block 2.

You can also ommit the else statement.

A little bit of input

So far we have only dealt with hard-coded values which makes if conditions kind of boring - i mean you already knew what block will get executed. The fun part comes when you plug in the input that you can’t predict. This is where the input function comes in.

number_of_apples_string = input("Yo, how many apples do you have??? ")
number_of_apples = int(number_of_apples_string)

if number_of_apples > 5:
    print("Sir, you have an obscure amount of apples. Are you ok?")
else:
    print("The number of apples you have is ok. Eat some.")

The input function takes input from user but it does not not know what kind of input you want - it can be any text. But we need a number to compare how many apples we have! Thats where the int function comes.

It has its drawbacks: if the user does not provide number but for example string none your program will crash.

Yo, how many apples do you have??? I have no fricking apples

Traceback (most recent call last):
  File "apples.py", line 2, in <module>
    number_of_apples = int(number_of_apples_string)
ValueError: invalid literal for int() with base 10: 'I have no fricking apples'

This is Python telling you where it crashed and why. Lets go line by line:

Traceback (most recent call last):

traceback means something like a path that Python followed in your code.

File "apples.py", line 2, in <module>

This says where exactly did the crash occured. It was in file called apples.py at the second line.

number_of_apples = int(number_of_apples_string)

this is a line that is responsible for the crash.

ValueError: invalid literal for int() with base 10: 'I have no fricking apples'

There was an error with value int requires numbers and you provided I have no fricking apples a text - not a number. Therefore its ValueError.

More ifs

Sometimes you want more if branches. We call these elif.

number_of_apples_string = input("Yo, how many apples do you have??? ")
number_of_apples = int(number_of_apples_string)

if number_of_apples < 5:
    print("The number of apples you have is ok. Eat some.")
elif number_of_apples == 11:
	print("What a lovely number of apples!")
elif number_of_apples > 30 and number_of_apples < 40:
	print("You have something like 35 apples. Ok.") 
else:
    print("Sir, you have an obscure amount of apples. Are you ok?")

Practise / homework

Copy the following Python code that calculates the currency conversion and fix it.

USD_PER_GBP = 0.8

money_in_usd = int(input("How many dollars do you have? "))

money_in_gbp = 0 # calculate the correct amount

print('You have {0} brittish pounds'.format(money_in_gbp))


# now fix this condition

if False: # <- fix this condition
    print("You are GBP milionare.")
else:
    print("poor you")

Have the user input two numbers and one of +, -, * or /. Proceed to perform the appropriate operation on the two numbers. Be sure to handle the case where the user supplies an invalid operator.
Create an empty list, ask the user for several strings and add them all to the list (recall: that’s what the append method does). Then ask the user for one extra string and then check whether that string is on the list. If it is, print a message, otherwise print a different one.

Reading

Digital ocean tutorial about IFs
Official docs, might be harder to read but goes further.