Built-in String Methods

Hello students, welcome to Kwickprep. How can a program check if a name has only letters, or clean extra spaces from a form? Python has ready-made tools for this, called string methods. Today we will learn the most useful ones, and three exam programs that use them.

First, some new words. A string is text written inside quotes. A method is a ready-made tool that belongs to a value, like a string. We use it with a dot, then the method name, then round brackets. Strings can never be changed, so a method gives back a new value and the old string stays the same.

The len function counts characters, and the space counts too, so it gives nine. Len is a function, not a method, so it has no dot. Capitalize makes only the first letter capital, and all other letters small. Title makes the first letter of every word capital. Lower makes every letter small. Upper makes every letter capital.

Count tells how many times some text appears, so the letter a appears three times in banana. Find gives the index, which is the position number, of the first match. Positions start at zero, so the first letter n is at two. If the text is not there, find gives minus one. Pause and predict: what does find give for the letter b? Zero, because b is at the very first position. Zero means found, not missing.

Index looks just like find, with one big difference. When the text is found, find gives its position. Index also gives the same position. But when the text is missing, find calmly gives minus one, while index stops the program with a value error.

Startswith checks the beginning of a string, and endswith checks the end. The answer is always True or False. New Delhi starts with New, so True. It ends with the text hi, so True. But it does not start with new in small letters, so False. Capital and small letters are different in Python.

Methods that start with is answer a True or False question about the characters. Is alnum is True when all characters are letters or digits. Is alpha needs only letters. Is digit needs only digits. Is lower needs every letter to be small, and it ignores digits and spaces, so class nine in small letters is True. Is upper needs every letter to be a capital. Is space needs only blank characters, like spaces or tabs.

Exams love these traps, and all four answers are False. Riya Shah has a space, and a space is not a letter. Twelve point five has a dot, and a dot is not a digit. Class nine also has a space in the middle. And an empty string, with nothing inside the quotes, gives False for every is-method.

Users often type extra spaces in a form. The string here has two spaces on each side. We print square brackets around it, so you can see the spaces. Strip removes spaces from both ends. Lstrip, the left strip, removes them only from the left. Rstrip, the right strip, removes them only from the right.

Replace swaps every match of old text with new text, so tea becomes chai. Split breaks a string into a list of words. A list is many values inside square brackets. With nothing inside the brackets, split cuts at spaces. If you give it a comma, it cuts at every comma instead.

Join is the opposite of split. It glues a list of strings together, with the string before the dot in between. So we get red, dash, green, dash, blue. Partition cuts a string at the first match into exactly three parts. We get the part before, the separator itself, and the part after. The round brackets show these three parts are kept together, as a tuple.

Now three programs. First, count the vowels in Incredible India. We use lower first, so capital I is also counted. The loop takes one character at a time, called ch. The word in checks, is ch one of a, e, i, o, u? Each time it is, vowels goes up by one. The answer is seven.

Second, a palindrome is a word that reads the same backwards, like Malayalam. We make it lower case, so capital M matches small m. The slice with a step of minus one reverses the string. Double equals asks, are the two strings equal? They are, so it prints Malayalam is a palindrome.

Third, count the words in a sentence. This line has extra spaces at the start, the end, and in the middle. Split with nothing inside the brackets ignores all extra spaces. So we get a clean list of five words. Len of that list gives the word count, five.

Let us revise what we learned today. Capitalize, title, lower and upper change the case of letters. Count, find, index, startswith and endswith search inside a string. The is-methods check every character and give True or False. Strip, replace, split, join and partition clean and cut strings. And every method returns a new value, because strings never change.