3.4 Strings

What you need to know

• A string is an ordered sequence of characters (letters, digits, spaces, symbols), written in quotes: "Hello".
• Concatenation joins strings end to end. "AP" + "CSP" → "APCSP". Note there's no automatic space.
• A substring is any contiguous run of characters within a string. "Comp" is a substring of "Computer"; "Cptr" is not.
• Strings can be compared, searched, and have their length measured. The exact operation names vary by language; the CED expects the concepts, not a specific syntax.
• A string containing digits, like "42", is text, not a number — concatenating "4" + "2" gives "42", not 6.
• Strings are immutable in many languages: operations produce a new string rather than changing the original.

Worked example

first ← "Grace"
last ← "Hopper"
full ← first + " " + last
DISPLAY(full)

Concatenation with an explicit space in the middle → Grace Hopper. Without the + " " +, the output would be GraceHopper. Questions love that missing space.

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

• The CED's essential knowledge on strings is minimal: they're ordered sequences of characters, and you can concatenate and take substrings. Everything else — length, indexing, searching — the exam defines inside the question if it's needed.
• Concatenation with + works on strings in AP pseudocode. Combining a string and a number is not defined on the reference sheet; if a question does it, it will explain the behavior.
• A substring must be contiguous. "cat" is a substring of "concatenate" (positions 4–6). "cnt" is not, even though all three letters appear in order.
• The empty string "" is a valid string with zero characters. Concatenating it changes nothing. It's often the starting value when building a string in a loop.
• When a question defines a string procedure — say, SUBSTRING(str, start, length) or SUBSTRING(str, start, end) — the parameters matter enormously. One version takes a length, one takes an end index, and the end may or may not be inclusive. Read the definition every time.
• Building a string character by character in a loop is the standard pattern for reversing or filtering text: start with "", add one character per iteration.

Mistakes that cost points

• Adding a space that isn't there. "a" + "b" is "ab". If a space is needed it must be concatenated explicitly.
• Treating a numeric string as a number. "4" + "2" is "42" if both are strings. Arithmetic isn't happening.
• Assuming a substring definition. Don't apply Java's or Python's rules. Use whatever the question says, including whether indices start at 1 and whether the end is inclusive.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.

a ← "12"
b ← "3"
DISPLAY(a + b)