What a String Is

A string is an ordered sequence of characters: letters, digits, spaces, and punctuation, kept in order and treated as text. In Python you write a string inside quotation marks, like "Hello" or "grade: A". The order matters, so "ab" and "ba" are different strings. Because a string is text, the characters inside it are not treated as numbers even when they look like digits: "7" is the character seven, not the value 7.

The two string operations you must know for AP CSP are concatenation (joining strings together) and finding a string's length (how many characters it contains). Everything in this lesson is built from those two ideas plus one conversion rule for mixing numbers into text.

Concatenation: Joining Strings with +

Concatenation uses the + operator to glue two strings into one longer string, in order, left to right. The result contains every character of the first string followed by every character of the second. Nothing is added between them, so if you want a space, you must include it yourself.

name = "Ada"
greeting = "Hello, " + name
print(greeting)

name <- "Ada"
greeting <- "Hello, " + name
DISPLAY(greeting)

Here "Hello, " + name produces "Hello, Ada". The space and comma are part of the first string, so they appear in the result. This is how programs build a message out of fixed text and a variable that changes.

x = "2" + "3"
print(x)

Mixing Numbers and Text: str()

You often need to put a number inside a message, like a score or a count. But Python will not concatenate a string and a number directly. Writing "score: " + 7 raises a TypeError, because one side is text and the other is a number and Python refuses to guess what you meant. The fix is to convert the number to a string first with str().

score = 95
label = "Score: " + str(score)
print(label)

score <- 95
label <- "Score: " + score
DISPLAY(label)

The call str(score) turns the number 95 into the text "95", which can then be concatenated. On the pseudocode side, AP pseudocode is looser about combining text and numbers, but in real Python the conversion is required. Whenever you build a message that includes a number, wrap the number in str().

n = 7
print("score: " + n)

Length: Counting Every Character

The length of a string is how many characters it contains. In Python you find it with len(); AP describes this as finding the length of the string. The count includes every character: letters, digits, punctuation, and spaces all count. Only the quotation marks are left out, because they mark where the string begins and ends and are not part of the text.

word = "banana"
print(len(word))

word <- "banana"
DISPLAY(LENGTH(word))

The string "banana" has 6 characters, so len(word) is 6. Watch spaces carefully: "go team" has 7 characters because the space between the words counts. A blank space is a real character, exactly like a letter.

s = "a b"
print(len(s))

Building Output from Pieces

Real programs assemble their output from several parts: fixed text, variables, and numbers converted with str(). You build the whole message by concatenating those pieces in the right order, remembering to add spaces and punctuation yourself.

a = "AP"
b = "CSP"
print(a + b)
print(a + " " + b)

a <- "AP"
b <- "CSP"
DISPLAY(a + b)
DISPLAY(a + " " + b)

The first print joins "AP" and "CSP" with nothing between them, so it shows APCSP. The second inserts " " between them, so it shows AP CSP. The only difference is one space that you chose to include. Building a clean output string is just deciding, piece by piece, exactly which characters belong and in what order.

Key Vocabulary