Think Twice
Code Once
The Islamic University of Gaza
Engineering Faculty
Department of Computer Engineering
Fall 2017
LNGG 1003
Khaleel I. Shaheen
Introduction to Computers
Laboratory Manual
Experiment #5
Loops
Experiment #5: Loops
What is Loop?
A loop can be used to tell a program to execute statements repeatedly. In other words, A loop
statement allows us to execute a statement or block of statements multiple times.
Python provides two types of loop statements: while loops and for loops.
The while Loop
A while loop executes statements repeatedly as long as a condition remains true. while loop is
a condition-controlled loop, it is controlled by a true/false condition. It tests the condition before
executing the loop body
The syntax for the while loop is:
while loop-continuation-condition:
# Loop body
Statement(s)
Remember: All the statements indented by the same number of character spaces after a
programming construct are considered to be part of a single block of code.
Here is the flowchart for while loop:
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Experiment #5: Loops
The following example prints "Hi Python!" 100 times to the console.
count = 0
while count < 100:
print("Hi Python!")
count = count + 1
print("Done!")
When the condition is tested and the result is false, the loop body will be skipped and the first
statement after the while loop will be executed.
Ex: Write a Python program that prints the numbers from 0 to 9.
count = 0
while count < 10:
print(count)
count = count + 1
print("Done!")
print(count) # prints 10
Ex: Write a Python program that prints the sum of numbers from 1 to 20.
sum = 0
i = 1
while i <= 20:
sum = sum + i
print i
i = i + 1
print("sum is", sum) # sum is 210
Suppose the loop is mistakenly written as follows:
sum = 0
i = 1
while i < 10:
sum = sum + i
i = i + 1
Here the statement i = i + 1 is not in the loop body. This loop is infinite, because i is always
1 and i < 10 will always be true.
Note that the entire loop body must be indented inside the loop.
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Experiment #5: Loops
The for Loop
A for loop iterates through each value in a sequence. for loop is a counter-controlled loop.
A while loop can be written as follows:
i = initialValue # Initialize loop-control variable
while i < endValue:
# Loop body
...
i += 1 # Adjust loop-control variable
A for loop can be used to simplify the preceding loop:
for i in range(initialValue, endValue):
# Loop body
Here is the example that prints "Hi Python!" 100 times, but written using for loop this time.
for i in range(0, 100):
print("Hi Python!")
There are three versions of range function:
1. The function range(a, b) returns the sequence of integers from a to b - 1.
2. The function range(b) returns the sequence of integers from 0 to b - 1. It is the same as
range(0, b).
3. The function range(a, b, k) returns the sequence of integers from a to b – 1 using the
step value k (Difference between each number in the sequence).
for v in range(3, 9, 2):
print(v)
The output:
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Experiment #5: Loops
Ex: Suppose the input is 2 3 4 5 0 (one number per line). What is the output of the following
code?
number = 0
sum = 0
for count in range(5):
number = input("Enter an integer: ")
sum += number
print("sum is", sum)
print("count is", count)
break Statement
You can use the keyword break in a loop to immediately terminate a loop. break could be used
in a while or for loop.
The following example illustrates the using of break keyword, if the sum of numbers is greater
than or equal to 100 then break the loop and print the result:
sum = 0
number = 0
while number < 20:
number += 1
sum += number
if sum >= 100:
break
print("The number is", number)
print("The sum is", sum)
The output is:
The number is 14
The sum is 105
continue Statement
You can use the continue keyword in a loop skip all the remaining statements in the current
iteration. When it is encountered, it ends the current iteration and program control goes to the
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Experiment #5: Loops
end of the loop body. In other words, continue keyword breaks out of an iteration, while the
break keyword breaks out of a loop. continue could be used in a while or for loop.
The following example prints the numbers from 0 to 100 except those numbers that divide by
7.
for i in range(100):
if i % 7 == 0:
continue
print(i)
The Infinite Loop
A loop becomes infinite loop if a condition never becomes False. You must pay attention when
using while loops because a loop may never end. Such a loop is called an infinite loop.
count = 0
while count < 5:
print 'The count is:', count
In this example, the condition is always True, so the loop never ends. To solve this problem, we
need to increment count variable every iteration, to make the condition False eventually.
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Experiment #5: Loops
Lab Work
Ex1: Write a program that prints the sum of all positive integers less than 50.
Solution:
sum = 0
for i in range(1, 50):
sum += i
print("Sum numbers (1 to 49) =", sum)
Ex2: Write a program that sums the integer numbers entered by the user. The program requests
from user to enter an integer until 0 is entered, if so, the sum is displayed.
Solution:
a = input("Enter numbers to sum, Zero number ends list : ")
sum = 0
while a != 0:
sum += a
a = input("Enter number : ")
print("Sum numbers = ", sum)
Ex3: Write a program that asks the user to type 10 integers, then prints the smallest value of
the entered integers.
Solution:
a = input("Enter a number : ")
min = a
for i in range(9):
a = input("Enter a number : ")
if a < min:
min = a
print("The minimum number = ", min)
Ex4: Write a Python program that displays the sum of even numbers form 0 – 100, and the
sum of odd numbers from 0 – 100, and the sum of all numbers from 0 – 100.
Solution:
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Experiment #5: Loops
count = 0
sumOdd = 0
sumEven = 0
sumAll = 0
while count <= 100:
sumAll += count
if count % 2 == 0:
sumEven += count
else:
sumOdd += count
count += 1
print "Sum even numbers =", sumEven
print "Sum odd numbers =", sumOdd
print "Sum all numbers =", sumAll
Homework
1. Write a program that computes the factorial of an integer entered by user.
2. Write a program that use for statement to compute the following sums and products.
• 1 + 2 + 3 + … + 100
• 5 + 10 + 15 + … + 50
• 1 + 3 + 7 + 15 + 31 + … + (220 - 1)
• 1 * 2 * 4 * 8 * … * 220
3. Write a program that reads an unspecified number of integers, determines how many
positive and negative values have been read, and computes the average of the input
values. Your program ends with the input 0. Print the average in float format.
Good Luck
