Windows

Variable Conversion

Loops in Errortrapping

Windows

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Lesson Goal

• Create a program that displays a multiple of 4
• The user will enter the multiple
• If input isn’t valid an error window will appear
• We’ll ask until the input is valid

Don’t worry about making this now, we’ll build towards this for the end of the lesson.

Variable Types Review

• Every variable has a type
• This limits the values it can have
• Ints only hold whole numbers
• You can’t put text into an int or a real
• Chars only hold 1 character at a time
• Booleans can only be true or false

Type Incompatibility

• Trying to store values out of a variable’s type in a variable will crash
• Trying to add number types to text types will crash
• Ints and reals can be added safely, but you can’t put a real in an int
• Chars and strings can be added safely

Variable Types Summary

Variable Conversion

Example

Safe Conversion

• Some strings can’t be converted to reals and ints
• Eg. strint(“ABCD”) and strreal(“100.0..0”)
• strrealok() and strintok() return whether the string can be converted

Error-Trapping Input

• Protect your code from bad inputs
• Take all input as strings
• Check if the strings can be converted
• This “traps” any errors, your code will never crash on the user

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Error-Trapping Example

Pauses and Delays

Two methods are pausing a program

1. getchar

var ch : char

ch := getchar

• delay(milliseconds)

delay(1000)

Loops

• Will run until exit command is called
• You can call exit inside a conditional structure
• You can also call exit when condition
• If exit is never reached, it loops on forever
• Use when you don’t know how many repetitions are needed

Loop Example

Common Uses for Loops

• Animations that loop over (eg. GIFs)
• Asking for input until an expected answer is given
• Error-trapping
• Loop until input can be safely converted
• Loop until input is in expected range

Practice

Ask the user for an integer, and output its value doubled. If the input isn’t an int, ask for input again until it’s valid.

Solution

Windows

• Output and input happens in Turing’s default window
• Instantly made when you use put, draw, or get
• You can create windows from within your program
• Input and output only happens to 1 window at a time
• This is called the current window

Creating Windows

• This creates a new window
• Specifies position on screen with x and y
• Specifies width and height with w and h
• You can put max;max for fullscreen
• The position and dimensions are in pixels
• You can omit position or graphics

Practice

Create a window with:

1. Position (200, 250) and dimensions 100x250
2. Position (50, 50) and dimensions 800x500
3. Position (0, 0) and full-screen dimensions

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Solution

• There are more ways you can customize windows
• “title:name”, “text:rows;columns”, “nocursor”
• For more options:
• http://compsci.ca/holtsoft/doc/view_set.html

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Window Commands

Program States

• Your program is made of multiple “states”
• Each window can be its own state
• Each state has a purpose (intro, input, output, etc.)
• Events cause the program to change states
• When planning, know what states you’ll have
• Know what events cause states to change

Switching States

There are 2 ways to handle windows:

1. Create all windows at the beginning
1. Hide and show windows with each event
2. You can still use offscreen windows

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• Each event, create the window you need
• Close the window after the event
• Easier to switch states

States Example

STATE: SLEEPING

STATE: EATING

EVENT: GIVEN FOOD

EVENT: DONE EATING

Windows Example

(Window 1)

I’M SLEEPY!

(Window 2)

I’M HUNGRY!

1. User enters input in window 1
2. User enters input in window 2

1

2

Code Example

Create windows

Event 1 hides win1, shows and selects win2

Event 2 hides win1, shows and selects win2

Events are triggered by user input in a loop

Main Challenge

• Create a program that displays a multiple of 4
• The user will enter the multiple
• If input isn’t valid an error window will appear
• They can press a key to enter another input
• We’ll ask until the input is valid

Main Challenge Plan

(Input)

ENTER

AN INTEGER

(Output)

THE INTEGER

MULTIPLIED BY 4 IS...

(Error-Trap)

YOUR INPUT

WAS INVALID

• User enters valid input
• User enters invalid input
• User presses key

2

3

1

Solution