Why do I get duplicate output when using a for loop over a list?

Question

Hello I have problem looping when printing data in a list.

def inputScore(scor) :
 for i in range(len(scor)) :
 scor[i] = int(input("Enter Score Number : "))
def Display(scr) :
 for i in scr:
 if i >= 86 and i <= 100 :
 x = "A"
 elif i >= 70 and i < 86 :
 x = "B"
 elif i >= 60 and i < 70 :
 x = "C"
 elif i >= 50 and i < 60 :
 x = "D"
 elif i >= 0 and i < 50 :
 x = "E"
 else :
 print("Invalid Score")
 for i in range(0,3) : # If I remove this code "for..", it will error "IndexError: list index out of range"
 print("Score Number",scr[i],"Letter Grades", x)
def main () :
 scor = [int]*3
 inputScore(scor)
 Display(scor)
main()

Example:

# Input :
85
60
40
# Output that I want :
Score Number 85 Letter Grades A
Score Number 60 Letter Grades C
Score Number 40 Letter Grades E
# Output that I got :
Score Number 85 Letter Grades A
Score Number 60 Letter Grades A
Score Number 40 Letter Grades A
Score Number 85 Letter Grades C
Score Number 60 Letter Grades C
Score Number 40 Letter Grades C
Score Number 85 Letter Grades E
Score Number 60 Letter Grades E
Score Number 40 Letter Grades E

There are 3 looping for Letter Grades (A = 3 times, C = 3 times and E = 3 times), I tried to give for i in range(0,3) for stop looping but it doesn't work, Letter Grades always prints 9 times with 3 A, 3 C, and 3 E. How to solve it and make it output like in the example above?

• Daniel Hao: It’s good time to try debug this program yourself to learn the logic - pythontutor.com.

Answer 1

The inner for loop is unnecessary. The outer for loop already iterates through the scores. In each iteration, i is a score, not an index -- to get an index, you use range. Therefore, there's no need to index the scr list at all -- you would print i itself rather than scr[i].

Also, with the current code, the grade for 85 would be B rather than A. Perhaps you need to adjust the bounds for the A and B grades.

Another issue is that for an invalid score, it would still attempt the final print. This would fail if an invalid score occurred as the first score (since x wouldn't be defined). If an invalid score occurred as a subsequent score, the print would show you the grade for the previous score, which you don't want. You can get around this by setting x to be the empty string at the start of each iteration and checking if it has a non-empty value before doing the final print of the grade.

The following code resolves the issues discussed:

def inputScore(scor):
 for i in range(len(scor)):
 scor[i] = int(input("Enter Score Number: "))
def display(scr):
 for i in scr:
 x = ""
 if i >= 85 and i <= 100:
 x = "A"
 elif i >= 70 and i < 85:
 x = "B"
 elif i >= 60 and i < 70:
 x = "C"
 elif i >= 50 and i < 60:
 x = "D"
 elif i >= 0 and i < 50:
 x = "E"
 else:
 print("Invalid Score")
 if x:
 print("Score Number:", i, "Letter Grade:", x)
def main():
 scor = [int] * 3
 inputScore(scor)
 display(scor)
main()