Algorithms Branching and Iteration in Scratch Computer Science

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Algorithms are step-by-step procedures

If-else structure: conditional • Always evaluates condition at top of the if-else structure • Boolean expression: true or false

If-else structure • Executes one chunk or the other • If-else always executes one chunk: not zero, not two • If (no else) might execute one or zero chunks

If-else structure • Continues with next block

Stacked if-else-structures • These are separate structures, one after another y 240 clap squeak snap squeak -240 0 snap sing -240 clap sing x

Nested if-else-structures • One inside the other y 240 squeak Fella clap Hello -240 0 snap Hello -240 sing Fella x

Iteration • Iteration: an algorithm that repeats, one cycle after another

Your turn!

Don’t miss the “else” opportunity • These stacks run slightly differently. Why? Better Usually wasteful, risky Pair of opposite conditions

How to get exactly-one-of-several • If-else selects 1 -of-2 • If-elseif-else selects 1 -of-3 10 20 30

How to get exactly-one-of-several • How about 1 -of-4? 1 -of-4 ? 1 -of-3 if if else-if else 10 else 20 30

How to get exactly-one-of-several 1 -of-5 if else-if else 10 20 30 40