1. String Index, Length, Slicing and Traversal¶

1.1. A Collection Data Type¶

So far we have seen built-in types like: int, float, bool, str and we’ve seen lists. int, float, and bool are considered to be simple or primitive data types because their values are not composed of any smaller parts. They cannot be broken down. On the other hand, strings and lists are different from the others because they are made up of smaller pieces. In the case of strings, they are made up of smaller strings each containing one character.

Types that are comprised of smaller pieces are called collection data types. Depending on what we are doing, we may want to treat a collection data type as a single entity (the whole), or we may want to access its parts. This ambiguity is useful.

Strings can be defined as sequential collections of characters. This means that the individual characters that make up the string are assumed to be in a particular order from left to right.

A string that contains no characters, often referred to as the empty string, is still considered to be a string. It is simply a sequence of zero characters and is represented by ‘’ or “” (two single or two double quotes with nothing in between).

1.2. Concatenation Reminder¶

As we have seen before, you cannot perform mathematical operations on strings, even if the strings look like numbers. The one exception to this rule is that the + operator does work with strings, but for strings, the + operator represents concatenation, not addition. As we have learned previously, concatenation means joining the two operands by linking them end-to-end. For example:

1.3. F-Strings Reminder¶

Although concatenation works well for simple use cases, it is often preferable to use an f-string to format strings, which allows you to insert the value of any variable by enclosing the variable in curly brackets. To indicate it is a formatted string, add a f before the string quotes begin. For example, the concatenation example above could also be written as:

1.4. Index Operator: Working with the Characters of a String¶

The indexing operator (Python uses square brackets to enclose the index) selects a single character from a string. The characters are accessed by their position or index value. For example, in the string shown below, the 14 characters are indexed left to right from postion 0 to position 13.

It is also the case that the positions are named from right to left using negative numbers where -1 is the rightmost index and so on. Note that the character at index 9 (or -5) is the blank/space character.

The expression place[4] selects the character at index 4 from place, and creates a new string containing just this one character. The variable some_char refers to the result.

Remember that computer scientists often start counting from zero. The letter at index zero of "Saskatoon Sask" is S. So at position [4] we have the letter a.

If you want the zero-eth letter of a string, you just put 0, or any expression with the value 0, in the brackets. Give it a try.

The expression in brackets is called an index. An index specifies a member of an ordered collection. In this case the collection of characters in the string. The index indicates which character you want. It can be any integer expression so long as it evaluates to a valid index value.

Note that indexing returns a string — Python has no special type for a single character. It is just a string of length 1.

1.5. Length¶

The len function, when applied to a string, returns the number of characters in a string.

To get the last letter of a string, you might be tempted to try something like this:

That won’t work. It causes the runtime error IndexError: string index out of range. The reason is that there is no letter at index position 6 in "Banana". Since we started counting at zero, the six indexes are numbered 0 to 5. To get the last character, we have to subtract 1 from the length. Give it a try in the example above.

Alternatively in Python, we can use negative indices, which count backward from the end of the string. The expression fruit[-1] yields the last letter, fruit[-2] yields the second to last, and so on. Try it! Most other languages do not allow the negative indices, but they are a handy feature of Python!

1.6. The Slice Operator¶

A substring of a string is called a slice. Selecting a slice is similar to selecting a character:

The slice operator [n:m] returns the part of the string from the n’th character to the m’th character, including the first but excluding the last. In other words, start with the character at index n and go up to but do not include the character at index m. This behavior may seem counter-intuitive but if you recall the range function, it did not include its end point either.

If you omit the first index (before the colon), the slice starts at the beginning of the string. If you omit the second index, the slice goes to the end of the string.

There is no Index Out Of Range exception for a slice. A slice is forgiving and shifts any offending index to something legal.

1.7. Traversal and the for Loop: By Item¶

A lot of computations involve processing a collection one item at a time. For strings this means that we would like to process one character at a time. Often we start at the beginning, select each character in turn, do something to it, and continue until the end. This pattern of processing is called a traversal.

We have previously seen that the for statement can iterate over the items of a sequence (a list of names in the case below).

Recall that the loop variable takes on each value in the sequence of names. The body is performed once for each name. The same was true for the sequence of integers created by the range function.

Since a string is simply a sequence of characters, the for loop iterates over each character automatically.

The loop variable a_char is automatically reassigned each character in the string “Go Spot Go”. We will refer to this type of sequence iteration as iteration by item. Note that it is only possible to process the characters one at a time from left to right.

1.8. Practice Problems¶

Try the following practice problems. You can either work directly in the textbook, or use Thonny. Either way, be sure to save your solution into your Computer Science 20 folder when you finish!