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Unit 5 - Loops Cheat Sheet by

Unit 5 - Loops
python     computer     computerlanguage

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: ")
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)
TestBr­eak.py

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)
TestCo­nti­nue.py

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