Immutability and String Updates

Many everyday string tasks come down to two questions: which piece of this string do I need to look at, and how do I change one piece without rebuilding everything else? Substring operations are the tools for answering both questions. A substring is a contiguous piece of a larger string, and substring operations either read a piece out or write a new piece in.

Before looking at any individual operation, hold one picture in mind. The string is the whole. A substring operation either takes a piece of the whole out to examine it, or puts a new piece in its place. Each character in the string occupies a position, and a substring is the run of characters that sits between a starting position and an ending point. The diagram below numbers each character of 'status: pending' from 1 for this illustration, and highlights the run that the word 'pending' occupies.

Read the diagram as an index map. A start position tells you where the piece begins, and a length tells you how many characters to take from there. For 'pending', the piece starts at position 9 and runs for 7 characters, covering positions 9 through 15. Every character outside that range, including the colon and the space, stays where it is when you work with the rest of the string.

The source states that, directly or indirectly, substr() is the operation used to read substrings. Reading means you start with a string, specify the part you want, and receive that part as a new value. The original string is not the thing handed back to you; the extracted piece is.

The phrase directly or indirectly is worth noticing. It signals that you may call substr() yourself, or you may reach it through another operation or library function that does the same job underneath. Either way the concept is the same: extraction of a piece from a larger string. A read leaves the original string as it was. If you extract 'pending' from 'status: pending', the status string still reads 'status: pending' afterward.

The source pairs substr() for reading with replace() for writing. Writing a substring means you take a string, locate a section of it, and substitute different content into that section. The result is a string whose contents have changed in that region, while the rest of the string is carried over. In the status example, the section 'pending' is located and 'done' is substituted in, and the 'status: ' part comes along unchanged.

Reading and writing are different kinds of work. A read leaves the original string as it was and gives you a piece of it. A write produces a changed version of the string. Keeping that distinction clear prevents a common confusion, which is expecting a read to change the string, or expecting a write to return only the piece you inserted.

Now trace the full sequence on a short string. Work through it one step at a time and track the string after each step. The read is the extraction step; the write is the replacement step.

The source states that a string can grow and shrink as needed. This matters for substring writes in particular. The replacement text does not have to be the same length as the text it replaces. If the new piece is longer, the string gets longer. If it is shorter, the string gets shorter. In the status example, 'pending' has 7 characters and 'done' has 4, so the string shrinks by 3 characters.