For loop in Python working, but not doing it's job

Question

I'm new to Python, i wanted to start learning it and here i try to rewrite a little code that i wrote in C# before. The program counts factorial, so if i type in 6 it would be 720. This is the python code:

   import array

def countFactorial(tal):
    summ = 1

    for i in range(len(tal)):
            summ *= i
            i += 1
    print("{} is".format(tal))
    print("{}".format(summ))
    return summ

def main():

    lst = []
    for i in range(5):
        values = input("Type in a value: ")
        lst.append(values)

    for s in lst:
        countFactorial(s)
       # print(s)


main()

If the user would type in for example 6, this is what the program for now will type out: "6 is 0" (5 times).

This is a part of the for loop that i tried to rewrite from C# that doesn't work here. Obviously "tal" is passed on as a parameter here in the C# aswell.

for (int i = 1; i <= tal; i++)
        {
            summ *= i;

        }
        Console.WriteLine(tal + " is " + summ);

What i think i need is a while loop that needs to rund the summ code trough like for example 6 times. I've tried this but cannot work it out.

In countFactorial you modify i twice, once in for i in range... and once in i += 1, the second modification is lost. — finite graygreen
– finite graygreen, Commented Jan 21, 2017 at 14:47
@finitegraygreen Well what i tried to create there was as you can se in the c# code(if you know c#). Like a "while loop" where i want "i" to increment untill it equals "tal" — kastravec
– kastravec, Commented Jan 21, 2017 at 14:50
You are also using the length of tal as the number (a string), so you are using the number of digits as the loop value. — Martijn Pieters
– Martijn Pieters, Commented Jan 21, 2017 at 14:52
So if i understood right, im using the user input to declare how many times it loops? But it will always loop 5 times anyways? @MartijnPieters — kastravec
– kastravec, Commented Jan 21, 2017 at 14:53
@kastravec: you are always asking for 5 numbers. Each of those entries remains a str object, because you never convert the value to an int object. String objects are sequences and have length, the number of characters in the string (so len('5') == 1). — Martijn Pieters
– Martijn Pieters, Commented Jan 21, 2017 at 14:59

Martijn Pieters · Accepted Answer · 2017-01-21 14:55:06Z

1

Ranges without an explicit starting value start at 0, so the first time you multiply 1 by 0. From there on out summ stays 0.

Your C# code on the other hand, starts at 1 and include the end value.

Either start your Python range() objects at 1 too and stop at tal + 1:

summ = 1
for i in range(1, tal + 1):
    summ *= i

or add 1 to i when multiplying:

summ = 1
for i in range(tal):
    summ *= (i + 1)

The i += 1 you added was redundant; i is set to the next value in the range on the next iteration, replacing whatever i was bound to before.

You do need to pass in integers; input() in Python 3 returns a string, so you need to convert it to an integer first:

for i in range(5):
    value = int(input("Type in a value: "))
    lst.append(value)

edited Jan 21, 2017 at 14:55

answered Jan 21, 2017 at 14:49

Martijn Pieters

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2 Comments

kastravec Over a year ago

Ok now i understand the error, but how would i "make" ranges start at 1, or the "i" start at 1. do i just "i = 1" under the for loop?

kastravec Over a year ago

Thanks man it worked! You also explained everything well so now i understand it! Thank you yet again!

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