CODEWITHSUNDEEP
pythonloops
quacodas
quacodas

Reputation: 273

Check statement for a loop only once

Let’s say I have following simple code:

useText = True

for i in range(20):
    if useText:
        print("The square is "+ str(i**2))
    else:
        print(i**2)

I use the variable useText to control which way to print the squares. It doesn’t change while running the loop, so it seems inefficient to me to check it every time the loop runs. Is there any way to check useText only once, before the loop, and then always print out according to that result?

This question occurs to me quite often. In this simple case of course it doesn’t matter but I could imagine this leading to slower performance in more complex cases.

Upvotes: 1

Views: 2559

Answers (5)

Roland Smith
Roland Smith

Reputation: 43495

The only difference that useText accomplishes here is the formatting string. So move that out of the loop.

fs = '{}'
if useText:
    fs = "The square is {}"

for i in range(20):
    print(fs.format(i**2))

(This assumes that useText doesn't change during the loop! In a multithreaded program that might not be true.)

Upvotes: 4

Austin Phillips
Austin Phillips

Reputation: 15746

The general structure of your program is to loop through a sequence and print the result in some manner.

In code, this becomes

for i in range(20):
    print_square(i)

Before the loop runs, set print_square appropriately depending on the useText variable.

if useText:
    print_square = lambda x: print("The square is" + str(x**2))
else:
    print_square = lambda x: print(x**2)

for i in range(20):
    print_square(i)

This has the advantage of not repeating the loop structure or the check for useText and could easily be extended to support other methods of printing the results inside the loop.

Upvotes: 2

KSFT
KSFT

Reputation: 1774

If you write something like this, you're checking the condition, running code, moving to the next iteration, and repeating, checking the condition each time, because you're running the entire body of the for loop, including the if statement, on each iteration:

for i in a_list:
    if condition:
        code()

If you write something like this, with the if statement inside the for loop, you're checking the condition and running the entire for loop only if the condition is true:

if condition:
    for i in a_list:
        code()

I think you want the second one, because that one only checks the condition once, at the start. It does that because the if statement isn't inside the loop. Remember that everything inside the loop is run on each iteration.

Upvotes: 0

Ozgur Vatansever
Ozgur Vatansever

Reputation: 52103

If you are not going to change the value of useText inside the loop, you can move it outside of for:

if useText:
    for i in range(20):
        print("The square is "+ str(i**2))
else:
    for i in range(20):
        print(i**2)

Upvotes: 1

lionelmessi
lionelmessi

Reputation: 1144

We can move if outside of for since you mentioned useText is not changing.

Upvotes: 0

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