How to convert this while loop to for loop

Question

Hello, I am fairly new to python and wondering how to convert this while loop to a for loop. I am not sure how to keep looping the inputs so it keeps asking Enter a line until GO and then say Next until STOP then display the count of lines.

def keyboard():
    """Repeatedly reads lines from standard input until a line is read that 
    begins with the 2 characters "GO". Then a prompt Next: , until a line is 
    read that begins with the 4 characters "STOP". The program then prints a 
    count of the number of lines between the GO line and the STOP line (but 
    not including them in the count) and exits."""
    line = input("Enter a line: ")
    flag = False
    count = 0
    while(line[:4] != "STOP"):
        if (line[:2] == "GO"):
            flag = True
        if flag:
            line = input("Next: ")
            count += 1
        else:
            line = input("Enter a line: ")
    
    print(f"Counted {count-1} lines")
    
keyboard()

With these inputs:

ignore me
and me
GO GO GO!
I'm an important line
So am I
Me too!
STOP now please
I shouldn't even be read let alone printed
Nor me

Should display/result in:

Enter a line: ignore me
Enter a line: and me
Enter a line: GO GO GO!
Next: I'm an important line
Next: So am I
Next: Me too!
Next: STOP now please
Counted 3 lines

if you (or better your code) don't know the total number of lines that will come a while loop is the best way. Why do you want to change it to a for loop? — Finn
– Finn, Commented Nov 5, 2020 at 9:15
I'm just trying to understand the code better and be able to switch from while to for and for to while — thatguy123
– thatguy123, Commented Nov 5, 2020 at 9:17
@Finn nailed it. So in this case it makes no sense to switch to a for loop. — Matthias
– Matthias, Commented Nov 5, 2020 at 9:19

cozek · Accepted Answer · 2020-11-05 09:22:09Z

You just need an infinity for loop which is what while True essentially is. This answer has it: Infinite for loops possible in Python?

#int will never get to 1
for _ in iter(int, 1):
    pass

So just replace your while loop with the above and add a break condition.

def keyboard():
    """Repeatedly reads lines from standard input until a line is read that 
    begins with the 2 characters "GO". Then a prompt Next: , until a line is 
    read that begins with the 4 characters "STOP". The program then prints a 
    count of the number of lines between the GO line and the STOP line (but 
    not including them in the count) and exits."""
    line = input("Enter a line: ")
    flag = False
    count = 0
    for _ in iter(int, 1):
        if line[:4] == "STOP":
            break
        if (line[:2] == "GO"):
            flag = True
        if flag:
            line = input("Next: ")
            count += 1
        else:
            line = input("Enter a line: ")
    
    print(f"Counted {count-1} lines")
    
keyboard()

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