Built-in Functions Part 01¶

What are built-in functions?¶

A function is a reusable block of code that performs a specific task. You give it some input (sometimes), it does something with it, and it gives you back a result. Python has many built-in functions which you can use to make your life easier. In this section, we will take a look at print, input and some type-casting functions in detail for this chapter.

print() function¶

The print function is used to show text on the screen (also known as the terminal or standard output). With this, you can write your first program that every programmer writes when they learn a new language:

print("Hello World!")

When you run this program, it should show Hello World! in your terminal. We are not limited to only strings, we can even print out other kinds of data types!

print("My name is Alex")
print(56)
print(3.14)
print(False)

We can also print variables, this prints out the data present within them.

age = 43
print(age)

language = "python"
print(language)

The output of the code above will be:

43
python

Each print statement prints on a new line. You can print multiple things using a single print statement by separating them using commas (,), which will result in all the values being printed on the same line separated by spaces.

print("Value of pi is", 3.14)

This allows us to also print out variables in the same line too.

name = "Radagon"
money = 30
print(name, "has", money, "dollars to his name.")

This will print out Radagon has 30 dollars to his name.

input() function¶

The input function is used to take input from the user through the terminal. It returns everything the user types until the user hits the Enter key. You can also pass in a string to the function which will appear as a prompt before asking for input, like so:

input("Enter your name: ")

The above code will print Enter your name: followed by a blinking cursor where you can type the actual input. Now, to actually use the value entered by the user, you need to assign the value returned by the input function to a variable.

name = input("Enter your name: ")
print("Hello", name, "! Hope you have a wonderful day.")

The output of this will be:

Enter your name: Krish
Hello Krish ! Hope you have a wonderful day.

Type Casting¶

Type casting is the process of converting one data type to another. Remember the note from above?

You must keep in mind that the input function will always return the user input as a string.

There will be scenarios where you need the input function to return a data type other than a string. One such place could be when you want to make a calculator that adds two numbers. Let's take a look at how we would do it without any type casting.

num1 = input("Enter the first number: ")
num2 = input("Enter the second number: ")
result = num1 + num2

print("The sum of", num1, "and", num2, "is", result)

If we run this program and enter two numbers, this is the result we would get:

Enter the first number: 23
Enter the second number: 12
The sum of 23 and 12 is 2312

As we can see, 23 + 12 is clearly not supposed to be 2312. So what's happening here?

The variables num1 and num2 hold a string data type of the user input. When you perform an addition operation on two strings (result = num1 + num2) you are simply joining two strings together, side-by-side. This is also called string concatenation which we will learn more about in the next chapter.

This is where type casting comes to fix this problem. We need to convert the string data type to integer so the computer can follow regular arithmetic for our use case.

num1 = input("Enter the first number: ")
num2 = input("Enter the second number: ")

int_num1 = int(num1)
int_num2 = int(num2)

result = int_num1 + int_num2

print("The sum of", num1, "and", num2, "is", result)

Running it now gives the correct result:

Enter the first number: 23
Enter the second number: 12
The sum of 23 and 12 is 35

int_num1 = int(num1) and int_num2 = int(num2) is what we call type casting. We're casting the input string to an integer, then assigning it to a variable. (Assigning the cast value to a separate variable is optional and can be omitted.)

Another common way to write the code above is:

num1 = int(input("Enter the first number: "))
num2 = int(input("Enter the second number: "))

result = num1 + num2

print("The sum of", num1, "and", num2, "is", result)

Both the programs perform the same way, with the only difference being that we aren't creating an extra variable in the latter. Whenever you see functions being executed within another function:

num1 = int(input("Enter the first number: "))

Remember that the innermost function is the one that always executes first. In this case, the input function is the one that fires first. It waits for the user to give an input. After an input is given, the data transfers to the int() function - converting the string data to an integer (if the input is valid). Finally, the data gets assigned to the variables.

The example above shows you how to convert user input to an integer. In case you want to use decimals instead of integers, you can type cast it to float.

num1 = float(input("Enter the first number: "))
num2 = float(input("Enter the second number: "))

result = num1 + num2

print("The sum of", num1, "and", num2, "is", result)

Similarly, we can cast data to other types, such as strings and booleans, by calling their respective functions.