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What is LOOP
A loop can be used to tell a program to execute statements repeatedly.
Suppose that you need to display a string (e.g., Programming is fun!) 100 times. It would
be tedious to type the statement 100 times:
print("Programming is fun!")
print("Programming is fun!")
...
print("Programming is fun!") |
count = 0
while count < 100:
print("Programming is fun!")
count = count + 1
The for Loop
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
In general, the syntax of a for loop is:
for var in sequence:
# Loop body
|
A Python for loop iterates through each value in a sequence.
Loop Design Strategies
Step 1: Identify the statements that need to be repeated.
Step 2: Wrap these statements in a loop like this:
while True:
Statements
Step 3: Code the loop-continuation-condition and add appropriate statements for controlling the loop.
while loop-continuation-condition:
Statements
Additional statements for controlling the loop |
Controlling a Loop
|
With user Confirmation continueLoop == 'Y':
while continueLoop == 'Y' :
# Execute the loop body once
...
# Prompt the user for confirmation
continueLoop = input("Enter Y to continue and N to quit: ")
|
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Sentinel Value Another common technique for controlling a loop is to designate a special input value, known as a sentinel value, which signifies the end of the input. A loop that uses a sentinel value in this way is called a sentinel-controlled loop.
data = eval(input("Enter an integer (the input ends " + "if it is 0): "))
# Keep reading data until the input is 0
sum = 0
sum += data
data = eval(input("Enter an integer (the input ends " + "if it is 0): "))
print("The sum is", sum)
|
Input-Output Redirection
number = eval(input("Enter an integer: "))
max = number
while number != 0:
number = eval(input("Enter an integer: "))
if number > max:
max = number
print("max is", max)
print("number", number)
|
The while Loop
A while loop executes statements repeatedly as long as a condition remains true.
while loop-continuation-condition:
# Loop body
Statement(s) |
sum = 0
i = 1
while i < 10:
sum = sum + i
i = i + 1
print("sum is", sum)
|
|
Nested Loop
print(" Multiplication Table")
# Display the number title
print(" |", end = ‘’)
print(" ", j, end = '')
print() # Jump to the new line
print("——————————————————————————————————————————")
# Display table body
print(i, "|", end = ‘’)
# Display the product and align properly
print(format(i * j, "4d"), end = '')
print() # Jump to the new line
|
A loop can be nested inside another loop.
Nested loops consist of an outer loop and one or more inner loops.
Each time the outer loop is repeated, the inner loops are reentered and started anew.
Keyword break
1 sum = 0
2 number = 0
3
4 while number < 20:
5 number += 1
6 sum += number
7 if sum >= 100:
8 break
9
10 print("The number is", number)
11 print("The sum is", sum)
|
Keyword continue
1 sum = 0
2 number = 0
3
4 while number < 20:
5 number += 1
6 if number == 10 or number == 11:
7 continue
8 sum += number
9
10 print("The sum is", sum)
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