Conditons are used to describe the logic that will trigger the execution of a sequence of code statements. Conditions can be used to check if a specific controller button has been pressed, a specific sprite collided with another one, etc. As code lines, conditions are getters too and this condition picked value is often used for comparisons.
They are defined into the if key of a when command and does its evaluation on the current picked object.
{
"systemVersion":"0.2",
"metadata":{
"title":"My first game"
},
"data":[{
"id":"A",
"sprites":[
{"id":"A","backgroundColor":5,"width":72}
],
"tilemaps":[{"map":["A"]}],
"code":[
{
"when":[{"id":"A","if":[{"itsAttribute":"backgroundColor","is":"==","smallNumber":5}]}],
"then":[{"set":[{"text":[{"string":"I AM RED!"}]}]}]
}
]
}]
}
This cartridge defines an empty red sprite but right after the code execution its text is set to I AM RED!.
The first code line when picks the sprite with ID A and sets a pretty readable condition in its if key: if itsAttribute backgroundColor is equal to the condition picked value of 5 then its text property is set to I AM RED!. That's why our empty sprite immediately displays text.
Notice that the then picked object is not the backgroundColor attribute but the object that owns that attribute, thanks to the itsAttribute key in the condition.
The key itsAttribute changes the evaluated object from the picked one to one of its attributes. That's handy when you want to build a condition on a sprite and manipulate it that in the next code block.
{
"systemVersion":"0.2",
"metadata":{
"title":"My first game"
},
"data":[{
"id":"A",
"sprites":[
{"id":"A","backgroundColor":4,"width":96},
{"id":"B","backgroundColor":5,"width":96,"x":0,"y":8},
{"id":"C","backgroundColor":6,"width":96,"x":0,"y":16}
],
"tilemaps":[{"map":["ABC"]}],
"code":[
{
"when":[{"id":"A","if":[{"itsAttribute":"backgroundColor","is":"==","smallNumber":4}]}],
"then":[{"set":[{"text":[{"string":"I AM CYAN!"}]}]}],
"else":[{"id":"A","set":[{"text":[{"string":"NOT CYAN!"}]}]}]
},
{
"when":[{"id":"B","if":[{"itsAttribute":"backgroundColor","is":"==","smallNumber":6}]}],
"then":[{"set":[{"text":[{"string":"I AM PURPLE!"}]}]}],
"else":[{"id":"B","set":[{"text":[{"string":"NOT PURPLE!"}]}]}]
},
{
"when":[{"id":"C","attribute":"backgroundColor","if":[{"is":"==","smallNumber":6}]}],
"then":[{"set":[{"text":[{"string":"I AM PURPLE!"}]}]}],
"else":[{"id":"C","set":[{"text":[{"string":"NOT PURPLE!"}]}]}]
}
]
}]
}
This cartridge displays 3 colored sprites but only 2 of them have text on it even if the code tries to change all of them.
The first line picks the A sprite and checks if itsAttribute backgroundColor is equal to 4, which is the cyan color in the default palette. Since it's true the then command is executed. It will set the text to I AM CYAN! for the currently picked object, which is still the A sprite picked by the when command. The else command is ignored.
The second line picks the B one but looks for the purple color. Since it's red the then command is ignored and the else command is triggered. As we've seen in commands chapters the else command picked object is the current code block subject. Since we're on the root code block the current picked object is undefined. To manipulate the B sprite we've to pick it again with the id getter and then use the set statement to change its text key to NOT CYAN!.
The third line is a little nasty: it picks the C sprite and looks for the purple color, which is the right color. The then command is executed but no text is set. That's because this time we've used the attribute key in the getter instead of itsAttribute in the condition. While the if condition is still true since the picked object is equal to 6, the then command picked object is that 6 number instead of the C sprite so setting the text key of that 6 number to I'M PURPLE! is not valid and anything will happen.
The most common way to build a condition is by using the is key. Its value decides the evaluation strategy for the current picked value.
{
"systemVersion":"0.2",
"metadata":{
"title":"My first game"
},
"data":[{
"id":"A",
"sprites":[
{"id":"A","width":24}
],
"tilemaps":[{"map":["A"]}],
"code":[
{
"when":[{"as":"keyboard","attribute":"up","if":[{"is":"hit"}]}],
"then":[{"id":"A","sum":[{"value0":[{"smallNumber":1}]}]}]
},
{
"when":[{"as":"keyboard","attribute":"down","if":[{"is":"hit"}]}],
"then":[{"id":"A","subtract":[{"value0":[{"smallNumber":1}]}]}]
},
{
"when":[{"id":"A","if":[{"itsAttribute":"value0","is":">","smallNumber":4}]}],
"then":[{"set":[{"backgroundColor":[{"smallNumber":5}]}]}],
"else":[{"id":"A","set":[{"backgroundColor":[{"smallNumber":0}]}]}]
},{
"then":[{"id":"A","set":[{"text":[{"attribute":"value0"}]}]}]
}
]
}]
}
This cartridge shows a counter that increases pressing the up button and decreases pressing the down button. When the counter is 5 or more it becomes red.
The first two lines are using is to check the special object keyboard's up or down attributes. The hit value means that the condition will be true if the picked value is equal to 1 - and that's when the picked button is hit. The then code will increase or decrease the A sprite value0 attribute.
The third line picks the A sprites and uses the > is value to check if its value0 attribute is higher than 4. If it's true the A background is set to red else its background is set to transparent.
You can create a condition that triggers on specific values or a range of values using these is key values:
>triggers when the picked value is greater than the condition picked value.>=triggers when the picked value is greater or equal than the condition picked value.<triggers when the picked value is less than the condition picked value.<=triggers when the picked value is less or equal than the condition picked value.!=triggers when the picked value is not equal to the condition picked value.==triggers when the picked value is equal to the condition picked value.%%triggers when the picked value module the picked condition picked value is less than 1.
If you've coded in other languages before you'll probably recognize these conditions and know how to use them.
{
"systemVersion":"0.2",
"metadata":{
"title":"My first game"
},
"data":[{
"id":"A",
"sprites":[
{"id":"A","x":76,"y":0,"width":24,"backgroundColor":2,"speedX":-1},
{"id":"B","x":76,"y":8,"width":24,"backgroundColor":2,"speedX":1},
{"id":"C","x":76,"y":16,"width":24,"backgroundColor":2,"speedX":-1},
{"id":"D","x":76,"y":24,"width":24,"backgroundColor":2,"speedX":1},
{"id":"E","x":76,"y":32,"width":24,"backgroundColor":2,"speedX":-1},
{"id":"F","x":76,"y":40,"width":24,"backgroundColor":2,"speedX":1},
{"id":"G","x":76,"y":48,"width":24,"backgroundColor":2,"speedX":-1},
{"id":"H","x":76,"y":56,"width":24,"backgroundColor":2,"speedX":1}
],
"tilemaps":[{"map":["ABCDEFGH"]}],
"code":[
{
"when":[{"id":"A","if":[{"itsAttribute":"x","is":"<","smallNumber":20}]}],
"then":[{"set":[{"speedX":[{"smallNumber":0}]}]}]
},{
"when":[{"id":"B","if":[{"itsAttribute":"x","is":">","smallNumber":110}]}],
"then":[{"set":[{"speedX":[{"smallNumber":0}]}]}]
},{
"when":[{"id":"C","if":[{"itsAttribute":"x","is":"<=","smallNumber":20}]}],
"then":[{"set":[{"speedX":[{"smallNumber":0}]}]}]
},{
"when":[{"id":"D","if":[{"itsAttribute":"x","is":">=","smallNumber":110}]}],
"then":[{"set":[{"speedX":[{"smallNumber":0}]}]}]
},{
"when":[{"id":"E","if":[{"itsAttribute":"x","is":"!=","smallNumber":76}]}],
"then":[{"set":[{"speedX":[{"smallNumber":0}]}]}]
},{
"when":[{"id":"F","if":[{"itsAttribute":"x","is":"==","smallNumber":90}]}],
"then":[{"set":[{"speedX":[{"smallNumber":0}]}]}]
},{
"when":[{"id":"G","if":[{"itsAttribute":"x","is":"%%","smallNumber":10}]}],
"then":[{"set":[{"backgroundColor":[{"smallNumber":5}]}]}],
"else":[{"id":"G","set":[{"backgroundColor":[{"smallNumber":2}]}]}]
},{
"when":[
{"id":"H","if":[{"itsAttribute":"x","is":">","smallNumber":90}]},
{"id":"H","if":[{"itsAttribute":"x","is":"<","smallNumber":110}]}
],
"then":[{"set":[{"backgroundColor":[{"smallNumber":5}]}]}],
"else":[{"id":"H","set":[{"backgroundColor":[{"smallNumber":2}]}]}]
},{
"then":[{"as":"allSprites","set":[{"text":[{"attribute":"x"}]}]}]
}
]
}]
}
This cartridge shows some sprites displaying their horizontal coordinates that will slowly move away from the center. The first 6 sprites will stop at a specific position, the 6th will move away blinking red and the last one will become red for a short while.
The code shows multiple examples of the is key. In the first 6 cases, the sprites are stopped at a specific condition. The 7th sprite will blink red since the %% module operator triggers a color change every time the sprite position is multiple of 10. The 8th sprite has a when command with 2 conditions: since both of them have to be true in order to trigger the color change it will happen in a range between 90 and 110 excluded.
To make your game react to user input you've to create conditions on the keyboard special object attributes. There are 3 is values shorthands to check a button state: up returns true if the picked button is not pressed, hit when has been just pressed and down when is hold down.
{
"systemVersion":"0.2",
"metadata":{
"title":"My first game"
},
"data":[{
"id":"A",
"sprites":[{"id":"A","x":76,"y":68}],
"tilemaps":[{"map":["A"]}],
"code":[
{
"when":[{"as":"keyboard","attribute":"down","if":[{"is":"down"}]}],
"then":[{"id":"A","sum":[{"y":[{"smallNumber":1}]}]}]
},{
"when":[{"as":"keyboard","attribute":"right","if":[{"is":"down"}]}],
"then":[{"id":"A","sum":[{"x":[{"smallNumber":1}]}]}]
},{
"when":[{"as":"keyboard","attribute":"up","if":[{"is":"down"}]}],
"then":[{"id":"A","subtract":[{"y":[{"smallNumber":1}]}]}]
},{
"when":[{"as":"keyboard","attribute":"left","if":[{"is":"down"}]}],
"then":[{"id":"A","subtract":[{"x":[{"smallNumber":1}]}]}]
},{
"when":[{"as":"keyboard","attribute":"buttonA","if":[{"is":"down"}]}],
"then":[{"id":"A","set":[{"backgroundColor":[{"smallNumber":2}]}]}]
},{
"when":[{"as":"keyboard","attribute":"buttonA","if":[{"is":"up"}]}],
"then":[{"id":"A","set":[{"backgroundColor":[{"smallNumber":5}]}]}]
},{
"when":[{"as":"keyboard","attribute":"buttonA","if":[{"is":"hit"}]}],
"then":[{"id":"A","set":[{"backgroundColor":[{"smallNumber":3}]}]}]
}
]
}]
}
In this cartridge you move a red square around the screen using the up, down, left, and right buttons. Holding down the A button it becomes blue but flashes cyan when the button is just hit.
The first four lines get the keyboard object attributes up, down, left, and right and apply the related movement when the buttons are down.
Then the 4th line changes the sprite color to blue when the buttonA button is down. The next line changes the it to blue when the same button is up. Since in Rewtro a button can't be up and down at the same time only one of the two then code will run.
The last line instead checks if the A button has been hit and changes the sprite color to cyan. A button that's down is also hit during the single game frame the button has been pressed. Since this color change happens after the down condition it overwrites its color change and that's why the sprite blinks for a single game frame to cyan right when the player hits the A button.
Using the is key you can also check another very common condition in games: sprite collisions. The collidingWith key value checks if all of the picked sprites are colliding with at least one of the condition picked sprites. It's also possible to check if the picked sprite collision applying first a translation setting the deltaX and deltaY keys to a getter to the position gap.
{
"systemVersion":"0.2",
"metadata":{
"title":"My first game"
},
"data":[{
"id":"A",
"sprites":[
{"id":"A","x":0,"y":0,"speedX":1,"zIndex":1},
{"id":"B","x":0,"y":16,"speedX":1},
{"id":"C","x":76,"y":0,"backgroundColor":8}
],
"tilemaps":[{"map":["ABC"]}],
"code":[
{
"when":[{"id":"A","if":[{"is":"collidingWith","id":"C"}]}],
"then":[{"set":[{"backgroundColor":[{"smallNumber":5}]}]}],
"else":[{"id":"A","set":[{"backgroundColor":[{"smallNumber":3}]}]}]
},
{
"when":[{
"id":"B",
"if":[{
"is":"collidingWith",
"deltaX":[{"smallNumber":8}],
"deltaY":[{"smallInteger":-16}],
"id":"C"
}]
}],
"then":[{"set":[{"backgroundColor":[{"smallNumber":5}]}]}],
"else":[{"id":"B","set":[{"backgroundColor":[{"smallNumber":3}]}]}]
}
]
}]
}
This cartridge shows two green sprites on the left that slowly moves to the white one at center of the screen. According to the code the two green sprites will turn red when colliding with the white one. If the first one is pointing toward the white sprite and will eventually collide, the second one is off-track... and still it will collide even before the first one. How's possible?
The first condition in code check if the picked A sprite is collidingWith the condition picked sprite C and changes the A sprite color. The second one instead is a little more elaborate.
It checks if the picked sprite B is collidingWith the condition picked sprite C as the line before but the collision is checked as if the B sprite was moved 8 pixels to the right (the deltaX value) and 16 pixels to the top (the deltaY value). During the collision evaluation, the B sprite will be a little tilted to the top, making it on-track with the B sprite and making it collide. Moreover, the B sprite is also tilted to the right making the collision happen a little before.
You can check if a sprite is under, over, on the right, or on the left of another one using the is values under, over, onRightOf, or onLeftOf. Two sprites can be at just one of these relative positions: if it's under is not over but it's also not onRightOf or onLeftOf.
{
"systemVersion":"0.2",
"metadata":{
"title":"My first game"
},
"data":[{
"id":"A",
"sprites":[
{"id":"A","x":76,"y":24,"backgroundColor":3},
{"id":"B","x":76,"y":68,"backgroundColor":6,"textAlignment":"center"}
],
"tilemaps":[{"map":["AB"]}],
"code":[
{
"when":[{"as":"keyboard","attribute":"down","if":[{"is":"down"}]}],
"then":[{"id":"A","sum":[{"y":[{"smallNumber":3}]}]}]
},{
"when":[{"as":"keyboard","attribute":"right","if":[{"is":"down"}]}],
"then":[{"id":"A","sum":[{"x":[{"smallNumber":3}]}]}]
},{
"when":[{"as":"keyboard","attribute":"up","if":[{"is":"down"}]}],
"then":[{"id":"A","subtract":[{"y":[{"smallNumber":3}]}]}]
},{
"when":[{"as":"keyboard","attribute":"left","if":[{"is":"down"}]}],
"then":[{"id":"A","subtract":[{"x":[{"smallNumber":3}]}]}]
},{
"when":[{"id":"B","if":[{"is":"under","id":"A"}]}],
"then":[{"set":[{"text":[{"string":"~I AM~UNDER"}]}]}]
},{
"when":[{"id":"B","if":[{"is":"over","id":"A"}]}],
"then":[{"set":[{"text":[{"string":"~I AM~OVER"}]}]}]
},{
"when":[{"id":"B","if":[{"is":"onRightOf","id":"A"}]}],
"then":[{"set":[{"text":[{"string":"~I AM~ON RIGHT"}]}]}]
},{
"when":[{"id":"B","if":[{"is":"onLeftOf","id":"A"}]}],
"then":[{"set":[{"text":[{"string":"~I AM~ON LEFT"}]}]}]
}
]
}]
}
This cartridge show a green sprite that you can move around and a purple one in the middle of the screen. Under the purple sprite you'll read its relative position to the green one. Move the square around to see how this text changes.
Moving the green square around you'll discover that there are 4 quadrants around the purple square and the under condition will trigger when the green sprite is on the lower one, the over on the top one, the onRightOf on the one to the right, and onLeftOf on the one to the left.
The existing value is used to check if the picked element is defined and to check if it contains at least one element on lists. To learn more about how existing works have a look at the iterators chapter.