Building Well-Structured Programs
Once you can write code, the next skill is writing code that stays understandable as it grows. Structured programming is the discipline of building programs from three fundamental control structures, and only these three: sequence (statements run in order), selection (choosing a path with if/else), and iteration (repeating with loops). A famous result, the structured program theorem, proves that these three suffice to express any computation — you never need arbitrary jumps.
This matters because unrestricted goto-style jumps produce spaghetti code: control flow that leaps around unpredictably, impossible to follow or verify. Structured programming keeps control flow local and nested, so each block has one entrance and one exit and can be reasoned about on its own.
Good structure also means top-down decomposition: break a large task into smaller sub-tasks, each a clearly named block or function, refining until each piece is simple. Two design virtues guide this: high cohesion (each unit does one well-defined thing) and low coupling (units depend on each other as little as possible). Together they let you change one part without breaking others.
Common pitfall: believing that "it runs, so the structure is fine." A program can produce correct output while being an unmaintainable tangle. Structure is about future change and human understanding — measured by cohesion, coupling, and single-entry/single-exit blocks, not merely by whether today's output is right.