当输入无效时，如何停止 for 循环迭代？ （不允许使用 while 循环）- Python

Question

for i in range (0, 3): 
    
    print() # When iterates creates a space from last section
    
    raw_mark = int(input("What was student: " + str(student_id[i]) + "'s raw mark (0 - 100)?: "))
    
    days_late = int(input("How many days late was that student (0 - 5)?: "))
    
    penalty = (days_late * 5)
    
    final_mark = (raw_mark - penalty)

    # Selection for validation 
    
    if 0 <= raw_mark <= 100 and 0 <= days_late <= 5 and final_mark >= 40:
        
        print("Student ID:", str(student_id[i]))
        
        print() # Spacing for user readability
        
        print("Raw mark was:", str(raw_mark),"but due to the assignment being handed in",
              str(days_late),"days late, there is a penalty of:", str(penalty),"marks.")
        
        print()
        
        print("This means that the result is now:", final_mark,"(this was not a capped mark)")

         
        
    elif 0 <= raw_mark <= 100 and 0 <= days_late <= 5 and final_mark < 40: # Final mark was below 40 so mark must be capped
        
        print("Student ID:", str(student_id[i]))
        
        print()
        
        print("Raw mark was:", str(raw_mark),"but due to the assignment being handed in",
              str(days_late),"days late, there is a penalty of:", str(penalty),"marks.")
        
        print()
        
        print("Therefore, as your final mark has dipped below 40, we have capped your grade at 40 marks.")

        
    else:
        print("Error, please try again with applicable values")

At the else i would like the loop to loop back through but not having iterated i to the next value, so that it can be infinite until all 3 valid inputs are entered... cannot use a while loop nor can i put the if - elif- else outside the loop. Nor can i use a function :(

Answer 1

Try something like this. You can keep track of the number of valid inputs, and only stop your loop (the while ) once you hit your target number.

valid_inputs = 0

while valid_inputs <= 3:
   ...

   if ...:
      ...
   elif ...:
      ...
   else:
      # Immediately reset to the top of the while loop
      # Does not increment valid_inputs
      continue

   valid_inputs += 1

Answer 2

You can put the input statement inside while loop containing try block.

for j in range(3): # or any other range
    try:
        raw_mark = int(input("What was student: " + str(student_id[i]) + "'s raw mark (0 - 100)?: "))

        days_late = int(input("How many days late was that student (0 - 5)?: "))
        break
    except:
        pass

Answer 3

If you really cannot use a while loop...

def get_valid_input():
   user_input = input(...)
   valid = True
   if (...):  # Do your validation here
      valid = False

   if valid:
      return user_input
   else:
      return get_valid_input()

for i in range (0, 3): 
   input = get_valid_input()
   # Do what you need to do, basically your program from the question
   ...
   ...

There are some additional gotchas here, as in you need to worry about the possibility of hitting the maximum recursion limit if you keep getting bad input values, but get_valid_input should ideally only return when you are sure you have something that is valid.

Answer 4

You can have your for loop behave like a while loop and have it run forever and implement your i as a counter. Then the loop can be terminated only if it ever hits 3 (or 2 so that you indices dont change), while otherwise it is set back to 0:

cnt_i = -1
for _ in iter(int, 1):
    cnt_i += 1
    if ...

    else:
        cnt_i = 0
    if cnt_i == 2:
        break

But seriously, whatever the reason for not using a while loop, you should use it anyhow..