Pseudocode vs Python cheatsheet

General comments

• The idea of having "standard" pseudocode is an oxymoron, so I'm giving you what the textbook seems to prefer
• There are other attempts at standards. For instance this LaTeX package.

Variables

JOES_VARIABLE <- 35

• Declare constants at start
• Note we use '<-' rather than '='
• Standard style seems to be to use capitals and '_' not CamelCase

Python:

joes_variable = 35

• Use '=' for assignment and '==' for comparison
• Python style is to use lower case for Variables
• Objects get an upper case first letter in the manner of a proper noun
• CamelCase is a Java thing

Output

OUTPUT "Something to display on screen"

• Use double quotes
• "PRINT" is a BASIC command, so is probably acceptable too

Python:

print( 'Something to display on screen' )

• Python style is to use 'single quotes' but "double quotes work fine too"
• Old-style Python didn't treat print as a function so brackets weren't necessary
• You can format output nicely using "{0.%5f}" style notation. Take a look at this.

Input

// First a prompt //

OUTPUT "Say Something"

// Then the actual input //

INPUT CHARLIES_VARIABLE

• Probably best separate the prompt (which is output) from the input itself
• There isn't a standard way to write a comment in pseudocode

Python:

charlies_variable = input( 'Say something: ' )

• Note that the input function takes a prompt as input.
• It returns a string as output.

Loops

FOR ... NEXT

FOR INDEX = 1 TO 20
	OUTPUT INDEX
NEXT INDEX

• You can have a STEP clause at the end of the FOR ... TO line to have jumps of 5 at a time or whatever.
• This is a standard "BASIC" loop.

Python:

for i in range( 1 , 21 ):
	print( i )

• No NEXT because of indentation.
• range( 1 , 20 , 5 ) will give you jumps of size 5.

WHILE ... END WHILE

i <- 1
WHILE i < 21
	OUTPUT i
	i <- i + 1
END WHILE

Python:

i = 1
while i < 20 :
	print ( i )
	i = i + 1

• Indentation tells us the end of the loop so no ENDWHILE.
• You can leave the loop at any time using a BREAK. See below...

REPEAT ... UNTIL

i <- 1
REPEAT
	OUTPUT i
	i <- i + 1
UNTIL i = 20

Python:

• Doesn't exist.
• Use while instead and consider using a break if it helps with structure:

i = 1
## Standard trick for an infinite loop in Python:
while True:
	print( i )
	i = i + 1
	if i == 20:
		## Next line will leave the loop
		break

• Using too many breaks can be messy so use sparingly.

Decision/branching

INPUT SOMETHING

IF SOMETHING = 20 THEN OUTPUT "You typed 20"

• Since "<-" is used for assignment, "=" is not ambiguous in pseudocode.
• If the THEN block is long then use multiple lines and finish with an END IF.
• ELSE blocks can be added as required (and finish with an END IF).

Python:

something = input()

if something == 20:
	print( 'You typed 20' )

• No THEN because of the colon.
• No END IF because of indentation.
• Additional elif ...: and else: clauses can be added.

Arrays/lists

In Python we use lists most of the time.

• A list's size does not need to be declared but can be intialized as empty.
• Lists are specifically 1d, if you want more then either a list of lists or use something like import numpy.

# Empty list
my_list = []

# Adding to end
my_list.append( 'a thing' )

# Defining list with entries
another_list = [ 'Apple' , 'Orange' ]

# Combining lists
my_list = my_list + another_list

# First item in list
print( my_list[ 0 ] )

# Last item in list
print( my_list[ -1 ] )

# Delete item from list
del my_list[ 0 ]

In Pseudocode we tend to talk about arrays rather than lists

// DECLARE ARRAYS
DECLARE days : ARRAY [1:20] OF STRING
DECLARE bus_names: ARRAY [1:6] OF STRING

// DECLARE AN ARRAY OF ARRAYS
DECLARE bus_times: ARRAY[1:6] OF ARRAYS [1:20] OF INTEGER

days <- [“Mon1”, “Tues1”, … ]
bus_names <- [“Bus A”, “Bus B”, …]

bus_times[i][j] = late_val