Grasshopper Python 103

Conditional Statements

Working with single values improves a lot when you can perform what is called conditional logic. This introduces a basic concept called program flow: parts of our code that get executed depending on some conditions. Conditional statements require operators, expressions that compare two (mostly numerical) properties and check for a logical state.

Basic Conditional Statement

if x > y:  #Check if x is strictly larger than y
    A = x   #If so, Assign x to A
else:
    A = 0.0 #Assign 0.0 to A

Every conditional statement has a logical test and an action to perform if the test is true. The “else:” is optional and will be executed in case the test is false.

Important: Conditional statements and other flow control expression require we define blocks of code by carefully using indents in our lines. You can use whatever spaces you want but it is better to indent blocks using the TAB key. A minimum condition is that all lines belonging to the same block must be indented the same number of positions.

This is correct:

if x > y:
    A = x
    B = 0.0  
else:
       A = 0.0 
       B = 1.0

This is NOT correct:

if x > y:
    A = x
  B = 0.0  
else:
       A = 0.0 
     B = 1.0

Operators

More info can be found in the corresponding Python Operators

Comparison Operators:

Comparison operators are used to check two elements against each other. Most of the times the elements will be numbers (integers, doubles, floats,…) but also other types, specially when determining if two elements are the same.

A < B – Will check if the A is strictly smaller than B.

A <= B – Will check if A is smaller or equal than B.

A == B – Will check if A is equal to B.

A > B – Will check if the A is strictly larger than B.

A >= B – Will check if A is larger or equal than B.

A != B – Will check if A is not equal to B.

A <> B – Will check if A is not equal to B.

Math Operators:

A = B – Will make A the same as B.

A + B – Will add A to B provided they are of the same type, this method sums two numbers and concatenates two strings.

A – B – Will subtract B from A provided they are of compatible types

A / B – Will divide A over B and return a float (decimal) result

A // B – Will divide A over B and return the floor integer result

A * B – Will multiply A times B

A % B – Will return the remainder of integer dividing A over B

Assignment Operators:

A ^= B – Will raise A to the power of B and replace A with the result.

A *= B – Will multiply A times B and replace A with the result.

A /= B – Will divide A over B and replace A with the result.

A %= B – Will integer divide A over B and replace A with the result.

A += B – Will add B to A and replace A with the result.

A -= B – Will subtract B from A and replace A with the result.

Logical Operators:

Used to perform logical proofs between several comparison tests. Consider A and B as booleans.

A and B – AND Operator. True if both tests are true, checks both of them.

A or B – OR Operator. True if either A or B are true.

not A – NOT Operator. True if A is false, False if A is true.

Examples:

#Example 0. Copy the contents of the script to your component

if x > 1:
    A = x

#Example 1. Copy the contents of the script to your component

if x > z:
    A = "large"
elif x <= z and x > y:
    A = "medium"
else:
    A = "small";

#Example 2. Copy the contents of the script to your component

if x == y:
    A = "match!"
elif x < y:
    A = "smaller"
else: 
    A = "bigger"

#Example 3. Copy the contents of the script to your component

if x != 0.0:
    if x < y:
        x += z # the same as x = x + z
    else:
        x -= z # the same as x = x - z
A = x

Challenge

1. Write a component that filters a lists of input integers and returns only those which are even numbers.

2. Write a component that filters a list of input integers and returns only those which are a multiple of a Y integer.