Lựa chọn và Lặp lại

Beyond straight-line code

• So far, code has run top to bottom, one statement after another.
• Real algorithms need to choose and to repeat — not just run in a straight line.
• Selection 选择: pick which code to run based on a condition (if).
• Repetition (iteration 迭代): run some code many times (while, for).

Selection: making decisions

• Selection lets a program take different paths depending on the data.
• "If the score is above $90$, print an A; otherwise print a B."
• The program branches — only one path runs each time.
• Unit 2 builds this with Boolean conditions and if statements.

The three control structures: sequence, selection, and iteration

Repetition: doing it again

• Repetition runs a block of code over and over until a condition says stop.
• "Keep asking for a password until the user types the right one."
• A loop saves you from copying the same code many times.
• Loops are how programs process lists, totals, and searches.

Combining the three

• The three building blocks of every algorithm: sequence, selection, repetition.
• Sequence = do steps in order; selection = choose; repetition = repeat.
• Almost every program is these three, nested and combined.
• Mastering them lets you express any computation.