15.1. From files to containers

A file stores characters; a program gives those characters meaning. In this chapter we read and write text files, parse records, and choose a container that fits the data. The examples use the C++20 standard library.

Our running example records distances between cities. A record contains two city names and a nonnegative distance. City names can contain spaces, so the file surrounds each name with double quotes:

"San Diego" "Los Angeles" 120
"Los Angeles" "San Francisco" 380

These are illustrative values, not a source for travel planning. We want to look up a known distance, list the cities without duplicates, and distinguish an unknown route from a route whose distance is zero.

The standard library provides containers for different jobs:

Choosing a container

Container	Organization	A useful example
std::array<T, N>	A fixed number of elements in sequence	The four suit names or seven daily readings
std::vector<T>	A sequence that can grow	Records read from a file of unknown length
std::set<T>	Unique keys in sorted order	City names without duplicates
std::map<Key, Value>	Unique keys with associated values, sorted by key	A city's neighbors and their distances

We can combine these containers. A rectangular table can be an array of arrays when its dimensions are fixed, or a vector of vectors when dimensions are chosen at run time. A map of maps can represent only the routes we know. There is no need to implement another container just to store this data.

Why aren't we filling in every value in our table, who are we leaving blank space above the diagonal of 0's?

Because we only need the half of the dataset contained by the triangle.

Incorrect! All of the data above the 0 diagonal is a mirror image of the triangle! So, the triangle contains the whole dataset.

Because triangles are the most effective shape to use when presenting data to others.

Incorrect! Triangles do look cool, but they aren't necessarily the most effective shape to use when presenting data.

Because matrices are triangles.

Incorrect! This triangle is PART OF an apmatrix.

Because the triangle contains the entire dataset.

Correct! The triangle contains all data points with no repeat data. If we included all datapoints, the would just be repeats of the points we already have.

Check answer

Based on how it is used to create the above table, what do you think a matrix is?

a geometric shape

Incorrect! A matrix is not a geometric shape, although they ARE rectangles.

a two-dimensional vector

Correct!

a material in which something develops

Incorrect! This is a definition for matrix, but not in the programming sense.

a mold used to shape things

Incorrect! This is a definition for matrix, but not in the programming sense.

Check answer