CODEWITHSUNDEEP
pythonvariablesmathsyntaxsubtraction
Vijayx7
Vijayx7

Reputation: 11

Why is this simple subtraction code not working

This is a minimal representation of my original code

x = 0
count = 0
a=30-count

while x < 10:
    if True:
         count = count+1
         x = x+1

print("count is ",count)
print("a is ",a)

This is the output:

count is  10
a is  30

The a in the output was supposed to be 20 but the line 'a=30-count' is not working

To make it work I have to move the line 'a=30-count' just above the 'print("a is ",a)' but I want the line 'a=30-count' in the beginning of the code

I have tried setting a as a global variable but that didn't worked

Thank you in advance also sorry if the format of my question wasn't right its my first time posting a question on this site

Upvotes: 0

Views: 985

Answers (6)

Stef
Stef

Reputation: 15505

When you write a = 30 - count, it doesn't "bind" variable a to variable count. It simply calculates a value for a, using the current value for count.

If you want to "save" the expression 30 - count to be reevaluated later, you can put it in a function:

def a(count):
  return 30 - count

x = 0
count = 0

while x < 10:
    count = count+1
    x = x+1

print("count is ",count)
print("a is ",a(count))

# count is  10
# a is  20

Upvotes: 1

Abk
Abk

Reputation: 154

x = 0
count = 0
a=30-count

while x < 10:
    if True:
         count = count+1
         x = x+1

print("count is ",count)
print("a is ",a)

is equavalent to

x = 0
count = 0
a=30-count

while x < 10:
  count = count + 1
  x = x + 1
print("count is ",count)
print("a is ",a)

Discussing the problem , python evaluates the code form top to bottom. So as the time you set a = 30-count the value of count is 0 so 'a' gets the value 30. As you said the most significant way to solve this problem is by putting line before the print statement or create a function

def updateA():
  global a
  a = 30 - count

and call the function updateA() before the print statement. I hope this will work

Upvotes: 1

Kanishk Mewal
Kanishk Mewal

Reputation: 419

This is not working because you declared the value of a before you changed the value of count, So a has the value : 30 - 0 which is nothing but 30.

After running through the WHILE loop, the value of count changes to 10, but the value of a is not updated because it has already been declared as (30-0) earlier.

The Variable a will not keep updating every time the value of count changes, that's why you have to update it's value again.

here's the code :

count = 0
a = 30

for x in range(10):
    count += 1

a -= count

print("count is ",count)
print("a is ",a)

The Output should look like this :

count is 10
a is 20

Upvotes: 1

user15801675
user15801675

Reputation:

Python will interpret count as 0 and a = 30-0 hence the answer. And then it will proceed to the while loop.

You while have to move the line a=30-count after the while loop.

x = 0
count = 0

while x < 10:
    count +=1
    x = x+1
a=30-count

print("count is",count)
print("a is",a)

Upvotes: 1

Alex S
Alex S

Reputation: 41

a=30-count

is only called once when you initially call it. To rectify this you should call it at the end of the while loop.

x = 0
count = 0

while x < 10:
    if True:
         count = count+1
         x = x+1
a=30-count

this gives you

count is  10
a is  20

Also

if True:

is redundant as your while loop is essentially saying while x < 10 is true continue.

Upvotes: 1

kayuapi_my
kayuapi_my

Reputation: 508

Python is a procedural imperative programming language and the code is run from top to bottom. When you put a=30-count at the beginning of the code, python will interpret it as a = 30 - 0 by replacing count = 0 to that line of code. So, it prints 30.

Try moving print("a is ",a) to the line right below a=30-count and you will see that a is 30.

You already know the correct way to make it works, which is to move a=30-count just above print("a is ", a) because then python is replaying count = 10 there.

Upvotes: 1

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