Operators are used to perform operations on variables and values. Python has many operators for different purposes. The following are the commonly used operators.
Assignment Operator
We have already used the assignment operator =, which assigns the value of the right side operand to the left side variable. E.g.,
name = Jack grade = 95
Arithmetic Operators
Arithmetic operators are used for performing mathematical operations. The following table shows the operators with some examples:
Name Operator Example
| Addition | + | >>> 10 + 212 |
| Subtraction | >>> 10 28 | |
| Multiplication | * | >>> 10 * 220 |
| Division | / | >>> 10 / 25.0 |
| Modulus | % | >>> 5 % 32 |
| Exponentiation | ** | >>> 10 ** 2100 |
| Floor division | // | >>> 5 // 31 |
Note: An arithmetic expression can be constructed by using multiple arithmetic operators and parentheses.
It is very common to assign an expression to a variable. E.g.,
PI = 3.14159
Radius = 6378
Perimeter = 2 * PI * Radius
print(The perimeter of earth is + str(Perimeter)) Volume = 4/3 * PI * Radius ** 3
print(The volume of earth is {}.format(Volume))
Remark: The expression 4/3 * PI * Radius ** 3 is equivalent to 4/3 * PI * (Radius ** 3) because ** has a higher priority (or precedence) than * and /. Now you can use Python as a calculator!
Comparison Operators (also called Relational Operators)
Comparison operators are used for comparing two values, and the value of a comparison expression is either True or False. The following table shows the operators with the examples:
Name Operator Example
| Equal | == | >>> x = 1>>> x == 1True>>> x == 2False |
>>> x = 1
>>> x != 1
| Not Equal | != | False>>> x != 2True |
| Greater than | > | >>> x = 1>>> x > 1False |
| Greater than or equal to | >= | >>> x = 1>>> x >= 1True |
| Less than | < | >>> x = 1>>> x < 1False |
>>> x = 1
Less than or equal to <= >>> x <= 1
True
Logical Operators
Logical operators are used for combining the conditional statements. The following table shows the operators and the examples:
Operator Description Example
| and | If both operands are True, then the result is True. Otherwise, the result if False. | >>> x = 5>>> x < 10 and x > 3True |
| or | If both operands are False, then the result if False. Otherwise, the result if True. | >>> x = 5>>> x == 1 or x == 5True |
| not | The result is the reverse of the operand. | >>> x = 5>>> not x > 10True |
Control Statements
The most direct way to affect the flow of control is with a conditional statement. In Python, the main types of control statements are if, while and for. if, ifelse, ifelifelse statements
The if statement performs an indicated action only when the condition is true; otherwise the action is skipped. The syntax is:
if expression:
statement
Remark: the colon : at the end of if statement in very important. It indicates that the following block of statements with the same amount of indentation will be executed if the expression is true.
Lets try the following example.
score = input(Please input the score: ) score = float(score)
if score >= 50:
print(Congratulations!)
print(You passed the course!)
With the else clause (ifelse statement), it allows you to specify that different actions are to be performed when the condition is false. The syntax is:
if expression:
statement(s) 1
else:
statement(s) 2
Try the following example.
| score = input(Please input the score: ) score = float(score)if score >= 50: print(PASS)print(Congratulations!) else:print(FAIL)print(Please work harder!) |
The ifelifelse statement is used for multiple cases by placing the elif with conditions. The syntax is:
if expression 1:
statement(s) 1 elif expression 2: statement(s) 2 elif
else
statement(s) N
For example, the following code will print different grades based on the score.
| score = input(Please input the score: ) score = float(score)if score >= 85: print(A) elif score >= 75: print(B) elif score >= 65: print(C) elif score >= 50: print(D) else:print(F) |
while statement
The while statement is used to execute a block of statements repeatedly while some condition remains true. The syntax is:
while expression:
statement(s)
Consider the following example code that prints the numbers from 1 to 10.
| i = 1 while i <= 10: print(i)i = i+1print(i) |
The initial value of the variable i is 1. In the body of the while statement, i is printed and increased by
- The loop runs total 10 times while i iterates from 1 to 10. Notice that after the while loop, i becomes 11.
Try another example that uses while to implement a guess game:
answer = Jack
print(======Can you guess my name?======) guess =
while guess != answer:
guess = input(Guess my name: ) print(Bingo!)
for statement
The for statement is used for iterating over a sequence. With the for statement, we can execute a block of statements, once for each item in the sequence (such as a list, tuple, range, and dictionary). The syntax is:
for iterating_var in sequence:
statement(s)
Consider the following example code that prints the day names. The items in the list will be assigned to the iterating variable d onebyone.
| days = [Monday, Tuesday, Wednesday, Thursday, Friday, Saturday,Sunday] for d in days: print(d) |
It is also very common to use the builtin function range() to generate an immutable sequence and then iterate through a sequence. For example:
| days = [Monday, Tuesday, Wednesday, Thursday, Friday, Saturday,Sunday] for i in range(7): print(days[i]) |
break and continue statements
The break and continue statements are used to alter the flow of control when executed in a while or for statement.
The break statement causes an immediate exit from that statement. Therefore, the break statement is used to escape early from a loop.
Try the following code:
| #%%1 for i in range(1, 10):2 if i > 3: 3 break4 print(i)5 print(Broke out at x = {}.format(i)) |
| 123Broke out at x = 4 |
Output:
Remark: In the previous example, lines 24 are the statements of the for loop and they are indented by four spaces. Line 3, the break statement, is under the if statement; so it has another level of indentation of four spaces.
The continue statement skips the remaining statements in the body of that control statement and evaluate the condition immediately (while loop) or performs the next iteration (for loop). Try the following code:
| #%% i = 0 while i < 6: i = i+1if i == 3:continueprint(i) |
| 12456 |
Output:
Case Studies
Case Study 1 Class Average Program 1
Imagine that a class of ten students took a quiz. The scores, integers in the range of [0, 100], for this quiz are available to us. We need to determine the grades and the class average on the quiz.
Assume that the scores are 50, 100, 75, 66, 88, 85, 46, 91, 0, and 23.
The grade assignment is based on the table below:
| Score range | Grade |
| 85 or above | A |
| 75 to 84 | B |
| 65 to 74 | C |
| 50 to 64 | D |
| Below 50 | F |
Lets try the following code:
| scores = [50, 100, 75, 66, 88, 85, 46, 91, 0, 23] total = 0;for s in scores:total += s if s >= 85:grade = Aelif s >= 75:grade = Belif s >= 65:grade = Celif s >= 50:grade = Delse:grade = F print(i, grade, sep=t) print(Class average is , total / len(scores)) |
| 50 D100 A75 B66 C88 A85 A46 F91 A0 F23 FClass average is 62.4 |
Output:
In this example, we use a for statement to get each score from the list and assign it to the variable s.
In the for loop, we use the operator += to increase the value of total by s. It is exactly the same as total = total + s. So += is also an assignment operator.
An ifelifelse statement is used for comparing the value of s and assign the grade.
The print(i, grade, sep=t) statement outputs two values, i and grade, separated by a tab (i.e., t).
Case Study 2 Class Average Program 2
We need to implement a program for determining the grades and the class average on the quiz. Our program will prompt the user to input the scores until the input equals 1. The grade assignment is based on the same criteria mentioned in Case Study 1.
Lets try the following code:
| #%%total = 0 # total markscount = 0 # to count the number of students while True:s = input(Enter score, 1 to end: )s = int(s) if s < 0: break total += scount += 1 |
Output:
|
|
As we do not know how many scores will be inputted, we use a while statement with True. It is an infinite loop that will not stop because the condition always is true.
In addition, we check whether the user input equals 1 or not. If yes, we use a break statement to exit the loop.
We use the variable count to count the number of scores inputted.
One more thing we need to pay attention to is that the user may input 1 immediately after starting the program. The value of count will be zero. There is a zero division program when we calculate the average using the statement average = total / count. Therefore, we need to check whether count is greater than zero or not.
Finally, to only print 2 factional digits of variable average, we use a placeholder {0:.2f} in the last print statement. The {0} indicates that the placeholder {} will be replaced by the 1st argument in the format(). The .2f indicates that the argument is a floating number with 2 digits after the period.
Case Study 3 Shapes Printing Program
We are going to use Python program to print shapes with star symbols. The expected outputs are as follows.
*
**
***
****
*****
******
*******
********
*********
Lets try the following code:
for i in range(10):
for j in range(i):
print(*, end=) print()
In the above example, we nest for statements inside another for statements. The outer for statement is used for printing ten rows. The inner for statement is used for printing a single row. Note that the print() function with end= (empty string instead of the default
newline character) will print the value without a newline. And, the print() function without parameter will print a newline.
A more interesting example: In the following example, the program will output the same shape as an animation by sleeping for 0.5 second after displaying a *. Python has a time module that offers many useful functions related to time and date. To access this time module, we need to use import time first. The function time.sleep(sec) will suspend the execution of the program for the given number of seconds. We will learn more Python modules in this course.
| 1234567 | import time # import the time module of Python for i in range(10):for j in range(i):print(*, end=)time.sleep(0.5) # pause for 0.5 second print() |
Exercises
Exercise 1 Greeting
Write a program to accept a user input and then print the greeting based on the following rules:
| Rule | Input | Reply |
| 1 | How are you? | Fine, and you? |
| 2 | How do you do. | How do you do. |
| 3 | Good to see you. | Me too. |
| 4 | others |
Hint:
- Use the function input() to get the user input.
- Use .then..else statement to check the inputted text.
- Use the function print() to print the reply.
- The following is the sample output of the program:
Computer: Hi!
You: Good to see you.
Computer: Me too.
Exercise 2 Summation
Write a program to accept integer inputs until the user inputs a noninteger. Then, the program prints the summation of the inputted numbers.
Hint:
- Use the function input() to get the user input.
- Use a variable total for calculating the summation.
- Need to use while loop for the repeated inputs.
- Use if statement with isdigit() function to check whether the user input is an integer or not.
if (n.isdigit() == False):
break
- If the user input is an integer, use the function int() to convert the user input to an integer and add it to the variable total.
a = int(n)
- If the user input is not an integer, use the break statement to exit the loop immediately.
- The following is the sample output of the program:
Input an integer: 5
Input an integer: 10
Input an integer: 2
Input an integer: x total = 17
Exercise 3 Maximum Value
Write a program to get 5 integer inputs and then print the maximum one. Assume that the user will input integers only.
Hint:
- Use for statement to get user inputs 5 times.
- Use if statement to compare the integers.
- The following is the sample output of the program:
Input a number: 15
Input a number: 3
Input a number: 21
Input a number: 9
Input a number: 18
Maximum is 21
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