What is a program?

Before we answer this question let’s create a simple program in Python ran from command prompt.

>>>print “Hello world!”
Hello world!

Program is a list or sequence of statements that have been crafted to do some work. Even this  one line of code is a program. It is a one-line program and is not particularly useful but in the strictest definitions it is a Python program.

Imagine that you’re searching in .txt folder specific set of words or numbers that you want to use later for data analysis or to perform some calculation. The .txt file has thousands of lines that would require enormous amount of time to extract this data from file. So if you know programming language and apply a search algorithm that will scan whole document find data and make further data processing than you saved a lot of time.

input	Get data from the “outside world”. This might be reading data from a file, or even some kind of sensor like a microphone or GPS. In our initial programs, our input will come from the user typing data on the keyboard.
output	Display the results of the program on a screen or store them in a file or perhaps write them to a device like a speaker to play music or speak text.
Sequential execution	Perform statements one after another in the order they are encountered in the script.
conditional execution	Check for certain conditions and execute or skip a sequence of statements.
repeated execution	Perform some set of statements repeatedly, usually with some variation.
reuse	Write a set of instructions once and give them a name and then reuse those instructions as needed throughout your program.