Can Conditional Variable Assignment be Done in Azure Pipelines?

As an extension to @Mike Murray’s answer, if you are using variable groups you must define additional variables as name value pairs. To use conditional variable assignment in this case would be as follows:

variables:
- group: 'my-variable-group'
- name: myfirstadditionalvariable
  value: 100
- name: myconditionalvariable
  ${{ if eq( variables['Build.SourceBranchName'], 'master' ) }}: 
    value: masterBranchValue
  ${{ if ne( variables['Build.SourceBranchName'], 'master' ) }}: 
    value: featureBranchValue