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ID: past-exam-of-the-computer-science-course-of-the-university-of-oxford/2015/object-oriented-programming/1/b/solution

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Past exam of the computer science course of the University of Oxford / 2015 / Object oriented programming / 1 / b / Solution by

Codex  0 2026-10-31

First determine the width of each column by finding the longest rendered String data type value in it. Independently determine the number of rows as the largest column size. An empty column has width zero. Then traverse rows first and columns second: render a present entry with spaces on the appropriate side, or render an absent entry as an entire column of spaces. Put one separating space between adjacent columns and a newline after each row. In particular, an exhausted column must not shift later entries into its position.

Here is executable Java for the algorithm. Column, defined in part (e), is an ordinary list of Data. The object interface in part (c) supplies the three queries used below. Each query sees stable content during this call; concurrent changes to formatting are outside this contract.

static String replicate(char c, int n) {
    StringBuilder s = new StringBuilder();
    for (int i = 0; i < n; ++i) s.append(c);
    return s.toString();
}

static String printData(List<Column> columns) {
    int[] width = new int[columns.size()];
    int rows = 0;
    for (int c = 0; c < columns.size(); ++c) {
        Column column = columns.get(c);
        rows = Math.max(rows, column.size());
        for (Data d : column)
            width[c] = Math.max(width[c], d.length());
    }
    StringBuilder out = new StringBuilder();
    for (int r = 0; r < rows; ++r) {
        for (int c = 0; c < columns.size(); ++c) {
            if (c != 0) out.append(' ');
            Column column = columns.get(c);
            if (r >= column.size()) {
                out.append(replicate(' ', width[c]));
            } else {
                Data d = column.get(r);
                String text = d.toString();
                String pad = replicate(' ', width[c] - text.length());
                out.append(d.leftJustified() ? text + pad : pad + text);
            }
        }
        out.append('\n');
    }
    return out.toString();
}

The algorithm uses the stipulated fixed-width characters and assumes entries contain no embedded newlines. If the column widths are

w_{0}, \dots, w_{k - 1}

and

k > 0

, every emitted row has the same width,

c = 0 \sum k - 1 w_{c} + (k - 1) characters before its newline .

(1)

An empty collection, or a collection consisting entirely of empty columns, gives the empty String data type value. Spaces at the end of a short last row are harmless and preserve the rectangular layout.

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