Question

Write a Python program that takes the value of n from the user and 
calculates the value of S using the following equation:
S = 1! - 2! + 3! - 4! + 5! - 6! + ……..  n!
Here, the value of n should be a positive (n>0) integer. If the user gives 
a negative input or zero, then print “Invalid Input!”. Also, print the 
sentence “End of program.” in the last line no matter what the input is.
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Sample Input 1:
4
Sample Output 1:
Value of S is: -19
End of program.
---------------------------------------------------------------------
Sample Input 2:
11
Sample Output 2:
The value of S is: 36614981
End of program.
---------------------------------------------------------------------
Sample Input 3:
-4
Sample Output 3:
Invalid Input!
End of program.
---------------------------------------------------------------------
Explanation/Hints:
You may import the ‘math’ module and use the built-in math.factorial() 
function in this problem. Alternatively, if you use a nested loop, the 
equation of S can be rewritten as follows:
S = 1 - (1*2) + (1*2*3) - (1*2*3*4) + …….. (1*2*3….*n)

For Sample Input 1 & 2, if you simply implement the given equation in your 
code, you will get the desired values. For Sample Input 3, as the input is 
negative, the corresponding output is displayed. The sentence “End of 
program.” is displayed every time.

Answer 1

def factorial_val(num):   
if num > 1:   
return factorial_val(num-1) * num
else:   
return num
  

limit = int(input("enter number of terms : "))
if(limit>0):
result = []   
count = 1
while(count <= limit):
result.append(factorial_val(count))   
count += 1
  
s=0
for i in range(0,limit):
if i%2==0:
s+=result[i]
else:
s-=result[i]
print(result)
print("sum of series : " , s)

else:
print("Enter a positive number invalid input")