characterize our approach as “depth-first.” It is also “concrete-first” and “concept-based.” First, we quickly (well, relatively quickly, Chapters 1–11) assemble a set of skills needed for writing small practical programs. In doing so, we present a lot of tools and techniques in minimal detail.
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More generally, a textbook or a course must lead students through a series of subsets. We consider it our responsibility to select topics and to provide emphasis. We can’t just present everything, so we must choose; what we leave out is at least as important as what we leave in — at each stage of the journey
The most basic building block of programs is an expression. An expression computes a value from a number of operands. The simplest expression is simply a literal value, such as 10, 'a', 3.14, or "Norah". Names of variables are also expressions. A variable represents the object of which it is the name.
Note that a<b<c means (a<b)<c and that a<b evaluates to a Boolean value: true or false. So, a<b<c will be equivalent to either true<c or false<c. In particular, a<b<c does not mean “Is b between a and c?” as many have naively (and not unreasonably) assumed. Thus, a<b<c is basically a useless expression.