Variables and Datatypes¶
Every programming language has its own grammar rules just like the languages we speak.
Keywords and Identifiers¶
The following identifiers are used as reserved words, or keywords of the language, and cannot be used as ordinary identifiers. They must be typed exactly as written here:
False class finally is return None continue for lambda try True def from nonlocal while and del global not with as elif if or yield assert else import pass break except in raise
In Python we don’t specify what kind of data we are going to put in a variable. So you can directly write abc = 1 and abc will become an integer datatype. If you write abc = 1.0 abc will become of floating type. Here is a small program to add two given numbers
>>> a = 13 >>> b = 23 >>> a + b 36
From the above example you can understand that to declare a variable in Python , what you need is just to type the name and the value. Python can also manipulate strings They can be enclosed in single quotes or double quotes like
>>> 'India' 'India' >>> 'India\'s best' "India's best" >>> "Hello World!" 'Hello World!'
Reading input from the Keyboard¶
Generally the real life Python codes do not need to read input from the keyboard. In Python we use input function to do input. input(“String to show”) , this will return a string as output. Let us write a program to read a number from the keyboard and check if it is less than 100 or not. Name of the program is testhundred.py
#!/usr/bin/env python3 number = int(input("Enter an integer: ")) if number < 100: print("Your number is smaller than 100") else: print("Your number is greater than 100")
$ ./testhundred.py Enter an integer: 13 Your number is smaller than 100 $ ./testhundred.py Enter an integer: 123 Your number is greater than 100
In the next program we are going to calculate investments.
#!/usr/bin/env python3 amount = float(input("Enter amount: ")) inrate = float(input("Enter Interest rate: ")) period = int(input("Enter period: ")) value = 0 year = 1 while year <= period: value = amount + (inrate * amount) print("Year %d Rs. %.2f" % (year, value)) amount = value year = year + 1
$ ./investment.py Enter amount: 10000 Enter Interest rate: 0.14 Enter period: 5 Year 1 Rs. 11400.00 Year 2 Rs. 12996.00 Year 3 Rs. 14815.44 Year 4 Rs. 16889.60 Year 5 Rs. 19254.15
Some examples of variables and datatypes:
Average of N numbers¶
In the next program we will do an average of N numbers.
#!/usr/bin/env python3 N = 10 sum = 0 count = 0 while count < N: number = float(input("")) sum = sum + number count = count + 1 average = float(sum)/N print("N = %d , Sum = %f" % (N, sum)) print("Average = %f" % average)
$ ./averagen.py 1 2.3 4.67 1.42 7 3.67 4.08 2.2 4.25 8.21 N = 10 , Sum = 38.800000 Average = 3.880000
In this program we will convert the given temperature to Celsius from Fahrenheit by using the formula C=(F-32)/1.8
#!/usr/bin/env python3 fahrenheit = 0.0 print("Fahrenheit Celsius") while fahrenheit <= 250: celsius = ( fahrenheit - 32.0 ) / 1.8 # Here we calculate the Celsius value print("%5.1f %7.2f" % (fahrenheit , celsius)) fahrenheit = fahrenheit + 25
$ ./temperature.py Fahrenheit Celsius 0.0 -17.78 25.0 -3.89 50.0 10.00 75.0 23.89 100.0 37.78 125.0 51.67 150.0 65.56 175.0 79.44 200.0 93.33 225.0 107.22 250.0 121.11
Multiple assignments in a single line¶
You can even assign values to multiple variables in a single line, like
>>> a , b = 45, 54 >>> a 45 >>> b 54
Using this swapping two numbers becomes very easy
>>> a, b = b , a >>> a 54 >>> b 45
To understand how this works, you will have to learn about a data type called tuple. We use comma to create tuple. In the right hand side we create the tuple (we call this as tuple packing) and in the left hand side we do tuple unpacking into a new tuple.
Below we have another example of tuple unpacking.
>>> data = ("Kushal Das", "India", "Python") >>> name, country, language = data >>> name 'Kushal Das' >>> country 'India' >>> language 'Python'