1. Algorithms: Branching and Iteration (in Scratch)
© 2014 Project Lead The Way, Inc.
Unpublished work © 2013 Project Lead The Way, Inc.

2. Learn from Scratch! http://scratch.mit.edu/projects/11516969/
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Course Name
Unit # – Lesson #.# – Lesson Name
Learn from Scratch!
Edit your picture in paint editor

3. Algorithms are step-by-step procedures
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Algorithms are step-by-step procedures

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

5. If-else structure Executes one chunk or the other
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Unit # – Lesson #.# – Lesson Name
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

6. If-else structure Continues with next block Presentation Name
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Unit # – Lesson #.# – Lesson Name
If-else structure
Continues with next block

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

8. Nested if-else-structures
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Unit # – Lesson #.# – Lesson Name
Nested if-else-structures
One inside the other
-240
240 x y
snap
Hello
clap
squeak
Fella
sing
Within the outer if-else structure, one group of blocks or the other is executed.
Each of the two options here contains an if-else structure plus one more block. In each case, the “if” or “else” section will execute and then the group continues with a block to say something.
Then the script could continue below the outer if-else structure.

9. Iteration Iteration: an algorithm that repeats,
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Iteration
Iteration: an algorithm that repeats,
one cycle after another

10. Your turn! Presentation Name Course Name
Unit # – Lesson #.# – Lesson Name
Your turn!

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

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

13. How to get exactly-one-of-several
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Unit # – Lesson #.# – Lesson Name
How to get exactly-one-of-several
How about 1-of-4?
1-of-4 ?
1-of-3 if if
else-if
else-if
else-if
else
else
These structures are very common. If you want to check the user input for one of four menu items, for instance, you can use this structure. If you wanted heaters, fans, or air conditioners to come on at different temperatures, you could use this structure. The “else” block at the end is a catch-all; if none of the conditions are met, the “squeak” block executes. 10 20 30

14. How to get exactly-one-of-several
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
How to get exactly-one-of-several
1-of-5 if
else-if
else-if
else-if
These structures are very common. If you want to check the user input for one of four menu items, for instance, you can wuse this structure. If you wanted heaters, fans, or air conditioners to come on at different temperatures, you could use this structure. The “else” block at the end is a catch-all; if none of the conditions are met, the “squeak” block executes.
else 10 20 30 40