A nerd pretending to be a software engineer.

Python Learning – Part 3

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Well, I was pretty busy this week, and I haven’t touch too much of Python these days, and even if if I done some exercise on Python, I was too lazy to write up a blog post for it, but I promised I will blog more to keep track of my progress, so here I am writing this.

Ok, so this time, I want to talk about the getting input from the user, and  flow control in Python, in other words, I am going to talk about conditional statement and loop.

Let’s start with the user input. The basic input function in python is raw_input(), which gets whatever the user entered, and process it as a string. You can pass an optional string argument that will be used as the prompt, here is an example code:

print "What is your name?"

name = raw_input()
print "Hello, %s"%name

#or you can pass the prompt as an argument

name = raw_input("What is your name?")
print "Hello, %s",%name

Another option is to use the input(), which is is similar to the raw_input, except that the user input isn’t processed as a string, but as an expression. So, if you entered a number, it will process the input as a number, or if you entered an variable name, it will be processed as an variable, and so on

# x is 5
x = input("Enter a number: ") #enter '5'

# x is 12
x = input("Enter another number: ") #enter '4*3'

# this will produce an error, because John is not defined as a variable

y = input("Enter a name: ") #enter 'John'
# y will be equal to x, because the input will be processed as avariable

y = input("Enter a letter: ") #enter 'x'

Ok, the next part is where things start to get fun, the basic flow control in python using if, elif, and else. Before I begin, let me talk a little bit about code blocks and boolean values. Code blocks are basically a group of code, which can consist of one or more statements and declarations. In addition, the variables inside a code block are isolated from the code outside of the block, so a variable declared inside a code block cant be used outside of the block. Code blocks are used in many places in programming, one of them is the conditional and looping statements. Code blocks are expressed in various ways in different programming languages, some of them uses the curly braces { } (C++, C#, Java), some of them uses the BEGIN and END (VB) keyword, and in Python, we uses indentation (4 spaces), so the codes that are indented  are considered as a block of code, and to nest several levels of code blocks, just add more indentation to the lines of code.

Boolean values are basically logical true or false values. Comparison operators will always return a boolean value of either true or false, even though comparison operators are not the only thing that can return a boolean value, it is the basics of it. Comparison operations in Python are pretty much self-explanatory, and it is similar to other programming language:

5 == 5 #True
5 != 5 #False

#Greater than/Equal
5 > 5 #False
5 >= 5 #True

#Less than/Equal
5 < 5 #False
5 <= 5 #True
3 < 5 #True
10 > 2 #True
5 != 10 #True

# there are also the 'and', 'or', and 'not' keyword
# and will evaluate to true if both are true
# or will evaluate to true if one of the is true
# and not will produce the reverse value

3 == 3 and 1 < 3 #True
3 != 3 and 1 < 3 #False
3 != 3 or 2 <5 #True
not 3 == 3 #False

#and so on

I am not sure how to explain code indentation in plain english, so let me just write some example if statement code,

x = input("Please enter a number: ")
if x > 10:
    print "You entered a number greater than 10"
elif x < 5:
    print "You entered a number that is less than 5"
    print "You entered a number between 5 and 10"

As you can see, the if statement will test if the statement is true or not, followed by a colon and an indented lines to specify which code to execute if the statement evaluates to true, the next elif statement will be executed only if the first if statement is false, and it will test it the statement again. You can put as many elif statement as you want, and finally, an else statement is put there to specify which code to execute if all of the tests before it evaluate to false. The elif and the else statement are optional, which means you can write a code to only run when a certain condition is met, or do nothing if that certain condition is not met.

x = raw_input("Enter your name: ")

#Only print these statements if the name is John
if x == "John":
    print "Hello, John! Good to see you again."
    print "What can I do for you today, %s"%x

The next thing I am going to talk about is looping. Similar to the conditional statement, looping uses code blocks to specify several statements of code to run as well. There are two ways to create a loop, the first one is the while loop, which will continue to run as long as the boolean statement remains true.

# This one will run the loop endlessly, because the boolean statement
# is hardcoded as True
while True:
    print "Never ending loop"
    print "Press ctrl-c or ctrl-z to terminate"

# You should add a statement to eventually cause the boolean statement
# turns False
x = 1 #initialize x to 1
while x < 10: #continue to run the loop as long as the value of x < 10
    print "Count no.%d"%x
    x += 1 #increase the value of x by one on each iterations
           #so that eventually the x < 10 statement will be false

The second type of loop is the for loop, which will loop through the content of a list one by one. (I will talk about list in another post, but basically, a list is just a sequence of values that is identified by a single variable).

# the basic syntax of a for loop is:
# for [variable name] in [list]:
# #codes for the loop
# for example:

#Defining a list, more on this later
nameList = ['John', 'Jack', 'Adam']

# running the loop, printing the content of the list
# one by one

for name in nameList:
    print "Hello,%s"%name

# The code above will print John,Jack and Adam on the console
# You can also generate a list of numbers by using the range function
# the range function takes two arguments, the beginning and the end of
the list

# Will print the numbers 2-10
for i in range(2,10):
    print i
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