String-handling operations

Overview

Recall questions on length, position, substring and concatenation, on where positions are counted from, and on converting between characters and their character codes.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What does the length operation do? What is the length of 'Computing' and of the empty string?

   Model answer

   It returns the number of characters in a string. 'Computing' has length 9 and the empty string has length 0. Spaces and punctuation are characters and count towards the length, so 'A B' has length 3.

2. 2

   What does the position operation do, and what does it return when the item is not there?

   Model answer

   It searches a string for a character or a substring and returns the index at which the first occurrence starts. If the item does not occur, it returns an agreed value that cannot be a valid index, typically -1 or 0 depending on the language. Only the first occurrence is reported, so finding later ones means searching again from beyond that point.

3. 3

   Where does the position of a character get counted from, and why must you check this?

   Model answer

   Some languages number the characters of a string from 0 and others from 1, so in 'Computing' the letter 'm' is at position 2 counting from zero but at position 3 counting from one. Getting the base wrong produces answers that are consistently out by one, which is one of the commonest string-handling bugs.

4. 4

   What does the substring operation do? Extract the first three characters of 'Computing'.

   Model answer

   It returns a section of a string, given a starting position and either a length or an end position. Taking three characters from the start of 'Computing' gives 'Com'. The original string is unchanged; the operation returns a new string.

5. 5

   What does concatenation do, and what does 'Comp' joined to 'uting' give?

   Model answer

   It joins two strings end to end to produce a single longer string, so 'Comp' and 'uting' concatenate to 'Computing'. Nothing is inserted between them, so joining a first name to a surname needs a space concatenated in as well. In most languages the operator is +, though some use & or a join function.

6. 6

   What does the character to character code operation do? What is the code for 'A'?

   Model answer

   It returns the numeric code that represents a given character in the coding scheme being used, so in ASCII the character 'A' gives 65. It is often written ord(c) or ASC(c).

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