Comparisons and conditional expressions
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What Are Computers Good At?
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Basic math
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Simple decision making
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Doing things over and over again very, very fast
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And storing data
Comparisons
Java also allows me to compare different variables: either against literal values, or against other variables.
This is basis for simple decision making.
Simple Comparisons
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A Common Mistake
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a = 10means set a to 10 -
a == 10means test if a is equal to 10
Don’t Get Confused
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Comparisons Against Variables
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More Complex Comparisons
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Questions About Simple Comparisons?
Compound Comparisons
We can combine multiple comparisons together using logical and (&&) and or
(||) operators.
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Evaluation Order
Conditionals are always evaluated from left to right, and evaluation stops as soon as the result is known.
Comparisons Enable Decisions
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if-else
if-else statements are the building block for programmatic decision making:
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if something is true, do one thing;
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otherwise, if something else is true, do some other thing;
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otherwise, if something else is true, do some other thing;
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etc.
if-elseif-elseif-else
if statements can have multiple clauses:
if (firstThing) {
// do one thing
} else if (secondThing) {
// do another thing
} else if (thirdThing) {
// another thing
} else {
// run if firstThing, secondThing, and thirdThing are all false
}else statements are run if no if statement matches.
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Remember: Only One Statement Runs
In any if statement, only one statement is executed.*
Somewhat Useful Example
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Nested Conditionals
if statements can be nested inside other if statements
if (testMe) {
if (testMeAgain) {
// I am well tested
} else {
// I'm only moderated well-tested
}
} else {
// You need to write better tests
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Variable Scope
if statements provide our first example of a block of code.
if (...) {
// I'm a block of code
}-
Blocks are enclosed by braces and can have multiple statements
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Variables declared inside a block are not visible outside it…
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But variables declared outside (and prior to) a block are visible inside it
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Indentation Hints
Assuming you are indenting your code properly, a general rule of thumb about scope:
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You can access variables "to the left"
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You cannot access variables "to the right"
(Where directions are defined relative to the start of the line of code you are writing.)
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Next Time: Algorithms and Loops
Announcements
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The first set of Turing’s Craft exercises are due tomorrow by midnight.
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CS 199 EMP and CS 196 (the honors section) open for registration today at noon.
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MP0 is out and due Friday! At this point you should be able to finish
Quizzer. Office hours until 7PM today. -
Please fill out the intro survey! 1% extra credit for anyone who does by today at noon.