平成22年度秋期 ITパスポート試験問66

This question's explanation and your answers so far.

問66

頻出度★★☆

ものとものとのつながりを抽象化してとらえるとき，X から Y へのつながり（順序関係という）を（X，Y）と記し，(X)──→(Y) と図示するものとする。3 組の順序関係（A，B），（A，C），（B，D）を図で示したものはどれか。

出典　平成22年度秋期 ITパスポート試験問66

解説

問 66

Incorrectア
The arrows are A→B, C→A, and D→B. C→A represents the ordering relation (C, A), which is the reverse of the required (A, C). D→B likewise represents (D, B), the reverse of (B, D). Two of the three arrows point in the wrong direction.
Incorrectイ
The arrows are A→B, A→C, and D→B. The first two are correct, but the third points from D to B, so it ends up representing (D, B). What is required is (B, D), that is, an arrow from B to D, so its direction is reversed.
Incorrectウ
The arrows are B→A, A→C, and B→D. A→C and B→D are correct, but the arrow joining A and B points from B to A, so it represents (B, A). What is required is (A, B), so this one arrow points in the reverse direction.
エ
正解 An ordering relation (X, Y) means “a directed connection that starts at X and ends at Y,” and in a figure it is drawn as a single arrow pointing from X to Y. So what must be shown is (A, B) → from A to B, (A, C) → from A to C, and (B, D) → from B to D, three arrows in all. Figure エ consists of three arrows: one pointing right from A to B, one pointing down from A to C, and one pointing down-right from B to D. Both the number of arrows and the start and end point of each arrow match all three pairs exactly, with nothing missing and nothing extra. Therefore エ is correct.

ここがポイント

In an ordering relation (X, Y), “X is the start point and Y is the end point”; the direction of the arrow is itself the substance of the information. (X, Y) and (Y, X) are different relations, so mistaking the direction makes the answer wrong.
A figure in which nodes are joined by directed edges (arrows that have a direction) is called a directed graph. To check the choices, it is enough to confirm two things in order: whether the number of arrows matches the number of pairs, and whether the start and end point of each arrow are as specified.

用語

解答問66エ