lesson 26 Strings & Text · beginner

Pick one character

Every character in a text has a position, starting at 0. text[n] reads one of them, text[-1] reads the last, and len(text) counts them.

You'll be able to

Read a character with a position from the start (code[0]) or from the end (code[-1])
Use len to count characters and explain why the last position is len(text) - 1
Make a decision from one character, remembering that a character is text, not a number

The idea

Codes and IDs in real data often carry meaning in one position. In the Online Retail data, an invoice number that starts with C is a cancellation. A Runtime Market SKU such as HOM-52782 starts with its category. The first digit of a US ZIP code names a region of the country. To read one character you need its position, also called its index.

Positions start at 0, so sku[0] is the first character and sku[1] the second. A negative position counts back from the end, so sku[-1] is the last character. len(sku) counts the characters, which makes the last position len(sku) - 1.

Run each example to see what it prints. Running examples doesn't count toward progress.

example-1.py · Reading positions in a SKU

sku = "HOM-52782"
print(sku[0], sku[1], sku[2])
print(sku[4])
print(sku[-1])
print(len(sku))
print(sku[len(sku) - 1])

If you ask for a position that doesn't exist, Python stops with an error instead of giving back an empty answer. The most common slip is to use the length itself as a position.

example-2.py · One past the end

sku = "HOM-52782"
print(len(sku))
print(sku[len(sku)])

An index is most useful when a decision depends on one character. Combine it with if (see Conditional Logic) and len to check a record before you trust it. A CFPB complaint has a state code and a ZIP code. A state code has 2 characters and a ZIP code has 5. A ZIP code that starts with 9 is on the West Coast.

example-3.py · Checking a complaint's location

state = "MI"
zip_code = "48301"
if len(state) == 2 and len(zip_code) == 5:
    print("Location looks complete")
else:
    print("Location needs checking")
area = zip_code[0]
if area == "9":
    print("West Coast ZIP")
else:
    print("ZIP area", area)

When to use it: whenever one position has a fixed meaning, such as a prefix letter, a check digit at the end, or a separator at a known place. Mistakes to avoid:

Counting positions from 1 instead of 0.
Using len(text) as the last position.
Comparing a character with a number. zip_code[0] == 4 is always False, because "4" is text. The concept check below runs this one.

Review

Positions start at 0. A negative position counts from the end, so text[-1] is the last character.
len(text) counts characters, and the last position is len(text) - 1. Asking for position len(text) raises IndexError.
A character is text: compare it with "4", not 4, or convert it with int.
Next: read a whole run of characters at once with a slice.