CSC 110 -Intro. to Computing

Lecture 13:

PALGO

Announcements

Midterm is in one weekTime to start reviewingWe will do more review in class Tuesday

Loops

Computers are fast, but VERY stupidGood for simple computations: computing what

happens when atomic bombs detonatesBad at complex ideas: designing better atomic

bombs; not needing to explode bombs Use computers to take advantage of this

Problems requiring lots of little calculationsChoosing best option from MANY possibilities

Loops

Do not want to write same instructions over and over and over again90% of time spent running 10% of code

Instead, loops tell computer to execute certain instructions repeatedly

Loops in Palgo

Palgo contains three different loops:repeat

Execute commands a fixed number of times

for Execute commands a fixed number of times and

record how many times execution has occurred

while Execute commands while some condition applies

Repeat loops

Repeat Loops

99x

Variables in repeat loops

More than Movie References

What would this code do?

x = 5y = 3total = 0repeat y times total = total + xendprint total

Tracing a Loop

1 x = 52 y = 33 total = 04 repeat y times5 total = total + x6 end7 print total

Line

Cond-ition

x ytotal

output

1 5

2 3

3 0

4 1 of 3

5 5

4 2 of 3

5 10

4 3 of 3

5 15

7 15

Line

Cond-ition

x ytotal

outputLine

Cond-ition

x ytotal

output

1 5

Line

Cond-ition

x ytotal

output

2 3

Line

Cond-ition

x ytotal

output

3 0

Line

Cond-ition

x ytotal

output

4 1 of 3

Line

Cond-ition

x ytotal

output

5 5

Line

Cond-ition

x ytotal

output

4 2 of 3

Line

Cond-ition

x ytotal

output

5 10

Line

Cond-ition

x ytotal

output

4 3 of 3

Line

Cond-ition

x ytotal

outputCond-ition

5 15

For loops

For loops are similar to repeat loops, but they use a variable to count the loopsCan loop between any two values

Tracing a for loop

What would this code do?

num = input_fact(“What number?”)fact = 1for i = 1 to num fact = fact * iendprint fact

Tracing n! 1 num = input_num(“What number?”)

2 fact = 13 for i = 1 to num4 fact = fact * i5 end6 print fact

LineCond-ition

num

fact i OutputCond-ition

1 4

Cond-ition

2 1

Cond-ition

3 1 < 4 1

Cond-ition

4 1

Cond-ition

3 2 < 4 2

Cond-ition

4 2

Cond-ition

3 3 < 4 3

Cond-ition

4 6

Cond-ition

3 4 < 4 4

Cond-ition

3

4 24

LineCond-ition

num

fact i Output

2 1

3 1 < 4 1

4 1

3 2 < 4 2

4 2

3 3 < 4 3

4 6

3 4 < 4 4

4 24

3 5 < 4 5

LineCond-ition

num

fact i Output

3 1 < 4 1

4 1

3 2 < 4 2

4 2

3 3 < 4 3

4 6

3 4 < 4 4

4 24

3 5 < 4 5

6 24

While loops

While loops iterate while a condition is metCan be as many, or as few, times as neededValues can change within the loop

Can convert for, repeat, & while loops Not always easy to change one to anotherUsually choose the loop that makes the most

sense

More code examples

What will this print out? n = 10while n > 0 if n != 1 then print n + “ bottles of beer on

the wall” else print n + “ bottle of beer”

endend

Oops!

More code examples

n = 10while n > 0 if n != 1 then print n + “ bottles of beer on the wall”

else print n + “ bottle of beer”

end n = n - 1end

Success!

Coding Loops

Any code is allowed within a loopCan even write loops or if-then-else-end

statements within loops But what if wanted to remember something

from every pass of a loopNeed new variable for each piece of dataExcel: 16,384 rows x 256 columns

Over 4,000,000 different variables!

Lists

Lists are variables which can hold multiple pieces of data at onceLists are similar to columns in ExcelEach list/column includes numbered locationsWhile each location can hold only 1 value, the

list can include many locations

Lists

To be used, a list must be assigned to a variableThese variable follow the normal Palgo rules of

variables, however Assign to students a new list by:

students = list() Initially this list is empty (does not have any “cells”) It grows as we assign new data into it

Access data in lists

Add to a list by assigning its locations:students[0] = “Ashley”students[1] = “Sarah”students[2] = “David”students[3] = “Lisa”

Use these locations like a normal variable So, print students[2] would make

Palgo print out Lisa

Using Lists

Have to create locations in a list in orderscores[0] = 100scores[4] = 99

Important: Lists are numbered from 0First assignment must be to location “0”

Using Lists

Get number of locations in a list using length functionlen = length(scores)assigns to num

the number of elements in the list scores

Since lists are numbered from 0, last defined location in scores would be scores[length(scores)-1]

Looping over lists

Lists are really made for loops Could compute sum and average by:data = new list()data[0] = input_num(“1st score?”)data[1] = input_num(“2nd score?”)data[2] = input_num(“3rd score?”)sum = 0for i = 0 to length(data)-1 sum = sum + data[i]endavg = sum / length(data)print “Total ” + sum + “Average “ + avg

Palgo Loop Selection

Each of Palgo’s loops serves different purpose repeat

Executing the commands a specific number of times for

Remembering the number of times that we are executing the instructions in a loop

Very useful for lists! while

Executing a loop for as long as needed to meet a specific condition

Your Turn: Tracing Practice

Show program trace if input was (in order):94, 34, 67, 90, 97, -1 i = 0data = new list()num = input_number(“Next score?”)stupidName = numwhile (num != -1) data[i] = num if stupidName > num then stupidName = num end i = i + 1 num = input_number(“Next score?”)end

Your Turn

Write a program which has the user input the number of hits for each player on a baseball team (only 9 players on a team) Store the values in a list

Once the user has entered all the hits, use a loop to find the highest number of hits any player hit and print this value out

For Next Lecture

Continue getting ready for the midtermGiven in class on Mar. 9th

Continue playing with PalgoRun the demo programsTry seeing what happens if you enter some of

the code from the handoutTry writing a program of your own