Strings and String Operations

A string (str) is a sequence of characters in quotation marks: s = "informatika". Like a list, every character has an index: s[0] is “i”, s[-1] is “a”. The length is given by len(s). Unlike a list, a string item cannot be changed directly: s[0] = "I" causes an error.

Several operations work on strings. + joins two strings ("info" + "rmatika"), * repeats ("ha" * 3 gives “hahaha”). A slice cuts out a part: s[0:4] is “info”, s[4:] is “rmatika”, and s[::-1] is the string reversed. In a slice the last index is not included.

Python has many ready-made string methods. s.upper() and s.lower() change letters to uppercase or lowercase, s.find("mat") returns the position of a substring (or -1 if not found), s.replace("a", "o") replaces, and s.count("a") counts characters. text.split() splits a text into a list of words, and " ".join(words) puts a list back into a string.

A string can also be walked through character by character with a for loop: for ch in s:. For example, to count vowels the condition if ch in "aeiou": fits. A string of digits becomes a number with int("25"), and a number becomes a string with str(25); otherwise an action like "5" + 5 causes an error.

Key takeaways

• A string is a sequence of characters; indexes and slices (s[1:4]) work.
• A string cannot be changed directly; a new string is created.
• upper, lower, find, replace and split are the most used methods.
• int() and str() convert between strings and numbers.