Answer to Question #145623 in Python for Lee

Question

Give an example of a while loop, then provide the equivalent do-while loop and for loop. Then give a different example of a do-while loop along with the equivalent while loop and for loop. Finally give an example of a for loop along with the equivalent while loop and do-while loop. Use your examples to illustrate the advantages and disadvantages of each looping structure and describe those disadvantages and advantages.

Answer 1

# FOR (array sum)

a = [4, 3, 5, -1, 7, 0, 2]
print("sum of array:", a)

# for
s = 0
for e in a:
    s += e
print(s)

# while
s = 0
i = 0
while i < len(a):
    s += a[i]
    i += 1
print(s)

# do_while
s = 0
i = 0
while True:
    s += a[i]
    i += 1
    if i == len(a):
        break
print(s)

print()



# WHILE (counting occuring value in array)

val = 9
b = [val, 2, 5, val, 0, val, 3, 1, val, 5, 8, 2, val]
print("counting occuring value", val, "in array:", b)

# while
ind = -1
count = 0
while val in b[ind+1:]:
    count += 1
    ind += b[ind+1:].index(val) + 1
print(count)

# do_while
ind = -1
if val in b[ind+1:]:
    count = 1
    while True:
        ind += b[ind+1:].index(val) + 1
        if val not in b[ind+1:]:
            break
        count += 1
else:
    count = 0
print(count)

# for
count = 0
for e in b:
    if e == val:
        count += 1
print(count)

print()



# DO_WHILE (product of (firsts) positive inputs)

print("product of firsts positive inputs")

# do_while
prod = 1
while True:
    e = int(input("Enter positive integer: "))
    if e <= 0:
        break
    prod *= e
print(prod)

# for
prod = 1
for i in range(1000000000):
    e = int(input("Enter positive integer: "))
    if e <= 0:
        break
    prod *= e
print(prod)

# while
prod = 1
e = int(input("Enter positive integer: "))
while e > 0:
    prod *= e
    e = int(input("Enter positive integer: "))
print(prod)

print()

Firstly it should be noted that in python there are no do_while-loop, and

it have to be coded with while-loop by using if-break statement.

Secondly, we have to say, where we use each loop:

   for - when we have to go through the iterated object

   while and do_while - when we have to use check for some condition

   (the only difference is that do_while-loop have to be executed at least

   1 time,   unlike while-loop can be executed 0 or more times)

In first example 'sum of array' it is preferred to use for-loop, as we can

directly go through array and make summation for every element.

Meanwhile in while-loop we have to check every time for boundary limit of

array and increase index.

And in do_while-loop we are assuming that while-loop is infinite and have

interrupt it with additional if-break statement. In this example it is

bad practise to use do_while-loop as if array is empty we will receive

an error taking a value of 0-index element (as loop is executed as least

one time).

In second example 'counting occuring value in array' it is preferred to

use while-loop, as it takes away 1 extra operation (checking if value is

in array, coding this we avoid Error of find function, in case there is

no val in b) which have to be done in do_while-loop.

And for-loop is bad coding, because it go through whole array (not using

slices) and check every element and takes O(n)-complexity.

In third example 'product of firsts positive inputs' it is preferred to

use do_while-loop, as the input have to be done at least one time. In this

case we can declare variable for input, and check our condition inside the

loop, unlike while-loop. The code also become more elegant.

And for-loop takes very big range of input which have to be stopped with

if-break statement.