David Stotts Computer Science Department UNC Chapel Hill

0. data (types, simple information) 1. data storage (variables, assignment) 2. data retrieval (expressions, evaluation) 3. repetition (loops) 4. decision making (conditionals) 5. procedure abstraction (functions) 6. data abstraction (arrays) 7. objects: all-the-above, wrapped up

 Often we need to “bump up” a variable value… add a little to what’s stored there  The pattern is to have the same variable name on both LHS and RHS of assignment  Here variable is “incremented” by 1 (all forms mean the same thing) x = x + 1 x += 1 x++  This form appears in loops often

 We can increment by more than 1’s x = x+2 x += 2 age = age+9 age += 9  We can increment by some variable amount, it doesn’t have to be just constants distance = distance + currMiles speed = speed + (time*accel) speed += time*accel

We can use more than just “+” (although the word “increment” is usually used for bumping up with addition) speed = speed * 2 speed *= 2 factorial = factorial * num factorial *= num

We can bump down a variable… decrement max = max – 1 max -= 1 one max-- highend = highend – 10 constant highend -= 10 upper = upper – num variable upper -= num upper = upper / 2

Repetition (loops)  Often we have a set of operations we would like to repeat over and over  We do this with loops (iteration)  World’s most famous loop: “ lather, rinse, repeat “

var first_num; // variable declaration var second_num; // variable declaration var total; // variable declaration first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); What if we want to repeat this a few times? Perhaps the user has more than one number pair to add

Cut and paste the code? var first_num; // variable declaration var second_num; // variable declaration var total; // variable declaration first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); It works, but … is it good?

var first_num; // variable declaration var second_num; // variable declaration var total; // variable declaration first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); 10 repetitions !! What if we need 100? 500,000 ? What if we want to repeat until the user is tired of it, or out of data? How many times will that be? Cut and paste on code lines clearly does not scale well

In a loop the syntax specifies two important components  the collection of program statements we want to repeat ( the loop body )  how many times to repeat the collection ( the number of iterations )

How to create a 20 tulip bouquet repeat // number of iterations 20 times all these steps { // body cut a tulip trim to proper length put it in the vase arrange to a pleasing look }

Human task example Spruce the train For // definite condition ( 11 times, one per car ) repeat { // body actions wash car windows grease wheel axles touch up car paint }

Human task example Spruce the train For start with car 1 stop after car 11 repeat { // body actions wash car windows grease wheel axles touch up car paint move to next car } “for” loops are a lot like this Work with a sequence

Definite loop: specific number of times, like counting items in a collection Concept for (N times) { do all these body statements} where N is some specific number ( this is NOT quite JavaScript syntax )

real JavaScript syntax var first_num; // variable declaration var second_num; // variable declaration var total; // variable declaration for (var i=1; i<=3; i++) { // says “do 3 times” // loop body is between the “curly braces” first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); } start stop Move to next time

Need to do this 500,000 times? var first_num; // variable declaration var second_num; // variable declaration var total; // variable declaration // user should get a cup of coffee and settle in … // here comes a LOT of data input typing for (var i=1; i<=500000; i++) { first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); } stop

Human task example Cut the Iron Bar while // indefinite condition (hot iron bar is one piece) repeat { // body actions heat bar red hot raise the hammer smack the chisel } How many hammer hits will it take? Who knows…

Indefinite loop: repeat the loop body an unpredictable number of times Concept while ( some condition holds ) { do all these body statements } ( this is NOT quite JavaScript syntax )

real JavaScript syntax var first_num; // variable declaration var second_num; // variable declaration var total; // variable declaration var more_input = “yes” ; // variable declaration while( more_input == “yes” ) { // loop body is between the “curly braces” first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); more_input = prompt(“do again? "); } Loop another time? Extra code does “Move to next” condition

real JavaScript syntax var first_num; // variable declaration var second_num; // variable declaration var total; // variable declaration var more_input = “yes” ; // variable declaration while( more_input == “yes” ) { first_num = Number(prompt(“First number? ")); second_num = Number(prompt(“Second number? “)); total = first_num + second_num; alert(“Their sum is “ + total); more_input = prompt(“do again? "); } User says “I’m done… let’s stop looping”

real JavaScript syntax var target; var counter = 2; var fact = 1; target = Number(prompt(“what number ?”)); while ( counter <= target ) { fact = fact * counter; counter = counter + 1; } alert(target + “ factorial is ” + fact); Data says “It’s done… stop the loop” not a user input, an internal variable change

In general … for loop: definite, specific # iterations while loop: indefinite, unknown # iterations ◦ loop until user says “I’m done” ◦ loop until data says “It’s done”

 Counter and Accumulator are common patterns of variable use  Remember the increment concept (or decrement) from earlier?  A counter is an increment inside a loop ◦ we usually increment by 1 (count by 1’s) but not always x = x + 1

var max; var kount = 0; // variable declaration // AND initialization combined max = Number(prompt(“what upper limit?”)); for (var i=1; i<=max; i++) { kount = kount + 1; } alert(“we counted to “ + kount); A counter MUST be initialized outside the loop, before the loop is entered and executed

 Accumulator : generalized counter  a counter in a loop is incremented by a variable amount  An accumulator is used to build a result incrementally -- one piece at a time ◦ we “grow” the result value some every time we execute the body of the loop result = result + i*offset factorial = factorial * num factorial *= num

var max; var fact = 1; // variable declaration // AND initialization combined max = Number(prompt(“factorial of what?”)); for (var i=1; i<=max; i++) { fact = fact * i; } alert(max + “ factorial is “ + fact); A multiply accumulator is initialized to 1 (why?)

var n; var total = 0; // variable declaration // AND initialization combined for (var i=1; i<=5; i++) { n = Number(prompt(“gimme a num”)); total = total + num; } alert(“5 user inputs sum to “ + total); An add accumulator is initialized to 0 (why?)