# Non-Programmer's Tutorial for Python 3/Decisions

### If statement[edit]

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 an integer:

```
n = int(input("Number? "))
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?-34The absolute value of -34 is 34

Number?1The absolute value of 1 is 1

So what does the computer do when it sees this piece of code? First it prompts the user for a number with the statement "`n = int(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 it 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 second block of statements is run if the expression is false.

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

operator | function |
---|---|

`<` |
less than |

`<=` |
less than or equal to |

`>` |
greater than |

`>=` |
greater than or equal to |

`==` |
equal |

`!=` |
not equal |

Another feature of the `if`

command is the `elif`

statement. It stands for else if and means if the original `if`

statement is false but the `elif`

part is true, then do the `elif`

part. And if neither the `if`

or `elif`

expressions are true, then do what's in the `else`

block. Here's an example:

```
a = 0
while a < 10:
a = a + 1
if a > 5:
print(a, ">", 5)
elif a <= 3:
print(a, "<=", 3)
else:
print("Neither test was true")
```

and the output:

1 <= 3 2 <= 3 3 <= 3 Neither test was true Neither test was true 6 > 5 7 > 5 8 > 5 9 > 5 10 > 5

Notice how the `elif a <= 3`

is only tested when the `if`

statement fails to be true. There can be more than one `elif`

expression, allowing multiple tests to be done in a single `if`

statement.

### Examples[edit]

```
# This Program Demonstrates the use of the == operator
# using numbers
print(5 == 6)
# Using variables
x = 5
y = 8
print(x == y)
```

And the output

False False

**high_low.py**

```
# Plays the guessing game higher or lower
# 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 = 7
guess = -1
print("Guess the number!")
while guess != number:
guess = int(input("Is it... "))
if guess == number:
print("Hooray! You guessed it right!")
elif guess < number:
print("It's bigger...")
elif guess > number:
print("It's not so big.")
```

Sample run:

Guess the number! Is it...2It's bigger... Is it...5It's bigger... Is it...10It's not so big. Is it...7Hooray! You guessed it right!

**even.py**

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

Sample runs:

Tell me a number:33 is odd.

Tell me a number:22 is even.

Tell me a number:3.48953.4895 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 to something that will not exit the while loop immediately.
print("Enter 0 to exit the loop")
while number != 0:
number = float(input("Enter a number: "))
if number != 0:
count = count + 1
sum = sum + number
if number == 0:
print("The average was:", sum / count)
```

Sample runs:

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

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

**average2.py**

```
# keeps asking for numbers until count numbers have been entered.
# Prints the average value.
#Notice that we use an integer to keep track of how many numbers,
# but floating point numbers for the input of each number
sum = 0.0
print("This program will take several numbers then average them")
count = int(input("How many numbers would you like to average: "))
current_count = 0
while current_count < count:
current_count = current_count + 1
print("Number", current_count)
number = float(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 average:2Number 1 Enter a number:3Number 2 Enter a number:5The average was: 4.0

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

### Exercises[edit]

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."

```
name = input('Your name: ')
if name == 'Bryn':
print('That is a nice name.')
elif name == 'John Cleese':
print('... some funny text.')
elif name == 'Michael Palin':
print('... some funny text.')
else:
print('You have a nice name.')
```

Modify the higher or lower program from this section to keep track of how many times the user has entered the wrong number. If it is more than 3 times, print "That must have been complicated." at the end, otherwise print "Good job!"

```
number = 7
guess = -1
count = 0
print("Guess the number!")
while guess != number:
guess = int(input("Is it... "))
count = count + 1
if guess == number:
print("Hooray! You guessed it right!")
elif guess < number:
print("It's bigger...")
elif guess > number:
print("It's not so big.")
if count > 3:
print("That must have been complicated.")
else:
print("Good job!")
```

Write a program that asks for two numbers. If the sum of the numbers is greater than 100, print "That is a big number."

```
number1 = float(input('1st number: '))
number2 = float(input('2nd number: '))
if number1 + number2 > 100:
print('That is a big number.')
```