Comp Sci Exam #1 review

Programming

The act of designing, implementing, and testing computer programs

Hardware

consists of the physical elements in a computer system.

• ex: monitor, the mouse, external storage, and the keyboard.

what does the central processing unit (CPU) do?

performs program control and data processing

The CPU has what two components?

• control unit

• arithmetic logic unit

Control unit

directs the operation of the processor

• communication and coordination between input/output devices.

• reads and interprets instructions

• determines the sequence for processing the data

• provides timing and control signal

arithmetic logic unit

contains the circuitry to perform calculations and do comparisons

Primary storage

composed of memory chips: electronic circuits that can store data as long as it is provided electric power

Secondary storage

provides a slower, less expensive storage that is persistent

• the data persists without electric power

Algorithm

describes a sequence of steps that is:

1. Unambiguous

• No “assumptions” are required to execute the algorithm

• The algorithm uses precise instructions

2. Executable

• The algorithm can be carried out in practice

3. Terminating

• The algorithm will eventually come to an end, or halt

variable

a named storage location in a computer program

example: total = 0

assignment statement

“=” is used to place a new value into a variable

what is an integer (int)?

A whole number (no fractional part)

• ex: 7

what is a float?

A number with a fraction part

• ex: 8.88

what is a string (str)?

A sequence of characters

• ex: "Bob”

The data type is associated with the____, not the ____

value, variable

• ex: cansPerPack = 6 #int

• ex: canVolume = 12.0 #float

warning about variables and assigning different values at different places

If you mix stings with integer or floating point values the result is an error

naming variables

names must start with a letter or an underscore

continue with letters (upper or lower case), digits or the underscore

• You cannot use other symbols (? or %...) and spaces are not permitted

A constant is a variable….

whose value should not be changed after it’s assigned an initial value

naming a constant

use this format: UPPER_CASE

Mixing numeric types

If you mix integer and floating-point values in an arithmetic expression, the result is a floating- point value.

• ex: 7 + 4.0 # Yields the floating value 11.0

double stars ** are used to…

calculate an exponent

floor division using the // operator

The “//” operator computes the quotient and discards the fractional part

what is % used as

to find a remainder

what is a library?

collection of code, written and compiled by someone else, that is ready for you to use in your program

what is a standard library?

a library that is considered part of the language and must be included

with any Python system

Floating-point to integer conversion

You can use the function int() and float() to convert between integer and floating point values:

• balance = total + tax # balance: float

• dollars = int (balance) # dollars: integer

Text, Characters, and strings

Text consists of characters

Characters are letters, numbers, punctuation marks, spaces, ….

A string is a sequence of characters

String Length

The number of characters in a string

• (For example, the length of "Harry" is 5).

You can compute the length of a string using Python’s len() function:

• length = len("World!") # length is 6

What will be the values of the variables num1 and num2 after the given set of assignments?

num1 = 20
num2 = 10
num1 = num1 + num2 / 2
num2 = num1

num1 = 25.0, num2 = 25.0

What must be done first before you can use a function from the standard library?

the function must be imported

what is wrong with this?

result = num1 // num2 / num3 num1 = 20 num2 = 10 num3 = 2 print(result)

A variable is used before it is assigned a value.

What is wrong with the following code snippet?

((num1 + num2) * num3 / 2 * (1 - num4)

there is an extra parenthesis

Which statement correctly creates a new variable by combining the two string variables: firstName and lastName?

name = firstName + lastName

What is printed from the following code snippet:

message = "ho.."
print(message * 3)

ho..ho..ho..

What is printed by the following code snippet?

num = int("45") * float("1.5")
print(num)

67.5

What is the index value of the letter 'h' in the string below ?

message = "hello"

0

Assignment vs. Equality Testing

Assignment: makes something true

• floor = 13

Equality testing: checks if something is true

• if floor == 13

Which statement finds the last letter of the string variable name?

last = name[len(name)]

last = len(name) - 1

last = len(name)

last = name[len(name) - 1]

last = name[len(name) - 1]

Which username would be accepted as valid?

 

username = input("Enter username: ")

if " " in username or not username.endswith("3"):
    print("Invalid")
else:
    print("Valid")

 

A. truman 1839

 B. truman18393

 C. 1839Truman

 D. Tru18man39

 B. truman18393

What is printed by the following code snippet?

cost = 25.45378
print("%.2f" % cost)

 

A.) 25.45378

B.) %25.45

C.) 25.45

 D.) nothing, there is an error

C.) 25.45

What is the data type of the value returned by the input function?

string

What is the output of the following code snippet if count contains 56?

if count % 2 == 0 :
   print("Count is an even number")
else :
print("Count is an odd number")

A.) Count is an even number

B.) Count is an odd number

C.) Nothing, there is a syntax error

D.) Nothing, the program runs but does not print any output

C.) Nothing, there is a syntax error

What value causes the following logical expression to 'short-circuit'?

if temp >= 32 and temp <= 100

temp = 0

What string method can be used to determine if the string contained in the variable text only consists of numbers?

text.isalnum()

text.isalpha()

text.isdigit()

text.islower()

text.isdigit()

Using De Morgan's law, what is the equivalent to this statement?

if not (state == "PA" or state == "OH")

if state != "PA" and state != "OH"

Boolean Variables

it is often called a flag because it can be either up (true) or down (false)

there are two operators: and, or

• they are used to combine multiple conditions

Combined Conditions: and

Both sides of the and must be true for the result to be true

ex:

if temp > 0 and temp < 100 :

print("Liquid")

Combined Conditions: or

We use or if only one of two conditions need to be true

ex:

if temp <= 0 or temp >= 100:

print("Not liquid")

The not operator: not

If you need to invert a boolean variable or comparison, precede it with not

ex:

if not attending or grade < 60 :

print("Drop?")

The not operator: inequality !

sed for the not when checking for inequality rather than negation.

Example inequality:

• The password that the user entered is not equal to the password on file.

• if userPassword != filePassword :

De Morgan’s law

tells you how to negate and and or conditions

A while loop that is controlled by a ____

counter

ex:

counter = 1 # Initialize the counter

while counter <= 10 : # Check the counter

print(counter)

counter = counter + 1 # Update the loop variable

Infinite Loops

The loop body will execute until the test condition becomes False

sentinel value

denotes the end of a data set, but it is not part of the data

Common Loop Algorithms - Average Example

total = 0.0

count = 0

inputStr = input("Enter value: ")

while inputStr != "" :

value = float(inputStr)

total = total + value

count = count + 1

inputStr = input("Enter value: ")

if count > 0 :

average = total / count

else :

average = 0.0

Common Loop Algorithms - Sum Example

total = 0.0

inputStr = input("Enter value: ")

while inputStr != "" :

value = float(inputStr)

total = total + value

inputStr = input("Enter value: ")

Uses of a for loop:

• The for loop can be used to iterate over the contents of any container.

• A container is is an object (Like a string) that contains or stores a collection of elements

• A string is a container that stores the collection of characters in the string

For Loop - while version

stateName = "Virginia"

i = 0

while i < len(stateName) :

letter = stateName[i]

print(letter)

i = i + 1

For Loop - for version

stateName = "Virginia"

for letter in stateName :

print(letter)