Print the open range between two numbers in descending order

Question

In the code below, I can make it not show variable value a, but the variable b ends up in the print.

The expected response would be the open interval ]A..B[.

Ex: variable a = 10 and variable b = 5.

Expected response: 9,8,7,6

Response obtained: 9,8,7,6,5. Whereas the values reported in input to be displayed, the variable value a I can make it so that it is not displayed on print, but the value of b always ends up leaving.

a = int(input('Valor de a tem que ser maior do que b: '))
b = int(input('Valor de b tem que ser menor do que a: '))

cont = a #Estou fazendo que a var cont receba o valor de a

if a > b and b < a: # O if estou verificando se a é maior do que b e b é menor que a
    while cont > b: # condição para o while é enquanto contador for maior que b vai repetir
        cont = cont - 1 # estou fazendo para que cont receba cont -1
        print(cont) # exibindo cont na tela
        a = a - 1 # estou fazendo que a receba a - 1 para que no cont fique com o valor de a, e depois ele subtraia - 1

Answer 1

If you want to iterate from one number to another (not including these numbers), make a loop simple:

a = 10
b = 5 
while a > b + 1:
    a -= 1
    print(a)

^{See here the code running}

The while continues running while a is greater than b + 1 (the condition must be such that b not be considered). Within the loop, i subtract 1 from a and print its value (in that order, so I guarantee that the 10 will not be part of the sequence). The above code prints the numbers 9, 8, 7 and 6 in this order.

Notice that at first, you don’t need the if. If a is less than or equal to b + 1, the code will not enter the while, then the if is redundant. It would only be necessary if you wanted to take some action if a is less than or equal to b + 1. Ex:

if a > b + 1:
    while a > b + 1:
        a -= 1
        print(a)
else:
    print('O valor de a deve ser maior que b + 1')

Then it makes sense to have the if. Otherwise, it’s just noise. Note that the condition is if a > b + 1, because if it were only if a > b, could fall in the event that a is 10 and b is 9, in which case no number will be printed (therefore it should also fall on the else).

---

There is also no reason to create this variable cont. OK, maybe it was necessary if you did not want to change the value of a, but within the loop you change the value of a and of cont. Then you can use only the variable a.

But if you really want to use another variable (maybe because the value of a cannot be changed, for example), you could initialize it with the first value of the sequence and do the loop thus:

a = 10
b = 5
n = a - 1
while n > b:
    print(n)
    n -= 1

^{See here the code running}

There are other details that have changed: now the condition is n > b (and not b + 1), and within the loop i print the value and then subtract 1. Note that it is not necessary to subtract 1 from a, because it is not used inside the loop and there is no reason to change its value.

I suggest you do the table test in both codes, to better understand how they work (because in one the condition is b + 1 and the next it’s just b, for example).

---

From what I’ve seen in the comments, this is an exercise, so it’s very likely they want you to do a loop like the one above.

But in Python you can also use a range, which represents precisely a sequence of numbers:

a = 10
b = 5
for i in range(a - 1, b, -1):
    print(i)

^{See here the code running}

The first parameter is the initial value (in the case, a - 1, since the a cannot be part of the sequence).

The second parameter is the final value, but I put b, for in a range the initial value is included, but the final value is not. So it is already guaranteed that b will not be part of the sequence.

The third parameter is the "step". In this case, it is -1, since the sequence begins in a - 1 and goes "walking" from -1 to -1, until you get to b. The above code also prints the numbers 9, 8, 7 and 6, in this order.

And in that case you don’t need if, for if a is less than b, the range will have no number (example). The if only makes sense if you want to take any specific action if the numbers are invalid (like the example above, which shows a message saying that a must be bigger than b + 1).

Answer 2

The problem happens because when the variable cont is equal to b + 1 it continues inside the while loop and gets the value of it minus 1 before having its value printed. To solve this problem, I just added +1 while condition. See the code below:

a = int(input())
b = int(input())
cont = a
if a > b:
    while cont > b + 1:
        cont -= 1
        a -= 1
        print(cont)

You can greatly simplify the code using the function range. See the code below:

a = int(input("Digite o valor de A: "))
b = int(input("Digite o valor de B: "))

for num in range(a-1,b,-1): 
    print(num)

The function range returns an iterable object (sequence) with number of to à b in a step of x.