String length

Computer Science · Strings

Is this password long enough?

This Python program keeps asking for a password until it has at least 8 characters. Three attempts are typed in. Step through and watch len(password) drive the loop.

Before you step: the third attempt, "red fox!", has only six letters. Will the loop let it through?

 1  password = input("Choose a password: ")
 2  while len(password) < 8:
 3      print("Too short")
 4      password = input("Choose a password: ")
 5  print("Accepted:", len(password), "characters")

Predict, then check

Count first, then pick. Commit to one answer before you look.

What does len("Year 11!") give back?

What you need to know

• len(s) gives back a whole number (an integer): how many characters are in the string s.
• Every character counts: letters, digits, spaces and punctuation each add 1.
• The empty string "" has no characters, so its length is 0.
• Because the length is a number, a program can compare it in an if statement or use it to control a loop.

The big picture

The length operation takes a string and gives back a whole number: how many characters it contains. Every character counts: letters, digits, spaces and punctuation. A string with nothing in it has length 0. Because the answer is a number, a program can test it in an if statement or a loop, for example to keep asking for a password until it is long enough.

⚠ Watch out

Counting only the letters (or skipping the spaces and punctuation). len("red fox!") is 8, not 6: the space and the ! are characters too, and a wrong count sends the program down the wrong branch.

Memory hook

Picture the string written one character per box: letters, digits, spaces and symbols each get their own box. len is the number of boxes. No boxes at all? Length 0.

Check yourself

Without running any code: what is len("") and what is len(" ") (two spaces)? Why are they different, even though neither shows any letters?