Python Programming/Conditional Statements

A Decision is when a program has more than one choices of actions depending on a variables value. Think of a Traffic light when is green we continue our drive, when we see the light yellow we proceed to reduce our speed and when is red we stop. Does are logical decision that depend on the value of the traffic light. Luckily Python has a decision statement to help us when our application needs to make such decision for the user.

As always I believe I should start each chapter with a warm up typing exercise, so here is a short program to compute the absolute value of a number:

n = raw_input("Number? ")
n = int(n)
if n < 0:
 print "The absolute value of",n,"is",-n
else:
 print "The absolute value of",n,"is",n

Here is the output from the two times that I ran this program:

Number? -34
The absolute value of -34 is 34
Number? 1
The absolute value of 1 is 1

What does the computer do when it sees this piece of code? First it prompts the user for a number with the statement "n = raw_input("Number? ")". Next it reads the line "if n < 0:". If n is less than zero Python runs the line "print "The absolute value of",n,"is",-n". Otherwise python runs the line "print "The absolute value of",n,"is",n".

More formally Python looks at whether the expression n < 0 is true or false. An if statement is followed by an indented block of statements that are run when the expression is true. Optionally after the if statement is an else statement and another indented block of statements. This 2nd block of statements is run if the expression is false.

There are several different tests that an expression can have. Here is a table of all of them:

Another feature of the if command is the elif statement. It stands for "else if" and means if the original if statement is false and then the elif part is true do that part. Here's an example:

a = 0
while a < 10:
 a = a + 1
 if a > 5:
 print a," > ",5
 elif a <= 7:
 print a," <= ",7
 else:
 print "Neither test was true"

and the output:

1 <= 7
2 <= 7
3 <= 7
4 <= 7
5 <= 7
6 > 5
7 > 5
8 > 5
9 > 5
10 > 5

Notice how the elif a <= 7 is only tested when the if statement fail to be true. elif allows multiple tests to be done in a single if statement.

High_low.py

#Plays the guessing game higher or lower 
# (originally written by Josh Cogliati, improved by Quique)
#This should actually be something that is semi random like the
# last digits of the time or something else, but that will have to
# wait till a later chapter. (Extra Credit, modify it to be random
# after the Modules chapter)
number = 78
guess = 0
while guess != number : 
 guess = raw_input("Guess a number: ")
 guess = int(guess)
 if guess > number :
 print "Too high"
 elif guess < number :
 print "Too low"
 else:
 print "Just right"

Sample run:

Guess a number:100
Too high
Guess a number:50
Too low
Guess a number:75
Too low
Guess a number:87
Too high
Guess a number:81
Too high
Guess a number:78
Just right

even.py

#Asks for a number.
#Prints if it is even or odd
number = raw_input("Tell me a number: ")
number = float(number)
if number % 2 == 0:
 print number,"is even."
elif number % 2 == 1:
 print number,"is odd."
else:
 print number,"is very strange."

Sample runs.

Tell me a number: 3
3 is odd.
Tell me a number: 2
2 is even.
Tell me a number: 3.14159
3.14159 is very strange.

average1.py

#keeps asking for numbers until 0 is entered.
#Prints the average value.
count = 0
sum = 0.0
number = 1 #set this to something that will not exit
# the while loop immediately.
print "Enter 0 to exit the loop"
while number != 0:
 number = raw_input("Enter a number: ")
 number = int(number)
 if number != 0:
 count = count + 1
 sum = sum + number
print "The average was:",sum/count

Sample runs

Enter 0 to exit the loop
Enter a number:3
Enter a number:5
Enter a number:0
The average was: 4.0
Enter 0 to exit the loop
Enter a number:1
Enter a number:4
Enter a number:3
Enter a number:0
The average was: 2.66666666667

average2.py

#keeps asking for numbers until count have been entered.
#Prints the average value.
sum = 0.0
print "This program will take several numbers, then average them."
count = raw_input("How many numbers would you like to sum:")
count = int(count)
current_count = 0
while current_count < count:
 current_count = current_count + 1
 print "Number ",current_count
 number = input("Enter a number: ")
 sum = sum + number
print "The average was:",sum/count

Sample runs

This program will take several numbers, then average them.
How many numbers would you like to sum:2
Number 1
Enter a number:3
Number 2
Enter a number:5
The average was: 4.0
This program will take several numbers, then average them.
How many numbers would you like to sum:3
Number 1
Enter a number:1
Number 2
Enter a number:4
Number 3
Enter a number:3
The average was: 2.66666666667

1. Modify the password guessing program to keep track of how many times the user has entered the password wrong. If it is more than 3 times, print That must have been complicated.
2. Write a program that asks for two numbers. If the sum of the numbers is greater than 100, print That is a big number.
3. Write a program that asks the user their name, if they enter your name say "That is a nice name", if they enter "John Cleese" or "Michael Palin", tell them how you feel about them ;), otherwise tell them "You have a nice name".

A switch is a control statement present in most computer programming languages to minimize a bunch of If - elif statements. Sadly Python doesn't support this statement as official, but with the clever use of a array or dictionary we can recreate this Switch statement that depending on a value.

x = 1
defhello():
 print "Hello"
defbye():
 print "Bye"
defhola():
 print "Hola is Spanish for Hello"
defadios():
 print "Adios is Spanish for Bye"
# Our switch statement Notice is a regular variable only that we added the functions name inside and there is no quote
menu = [hello,bye,hola,adios]
# To call our Switch statement we simple make reference to the array with a par of parenthesis at the end to call the function
menu[3]() # calls the adios function since is number 3 in our array.
menu[0]() # Calls the hello function being our first element in our array.
menu[x]() # Calls the bye function as is the second element on the array x = 1

This is done cause Python storing a reference of the function in the array in its particular index and by adding a par of parenthesis we are actually calling the function.

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