Why Do Golang's For Loops Produce Different Outputs When Using Closures?

Understanding Disparate Loop Behavior in Golang

In this code, we create two for loops (loop1 and loop2) that iterate over a slice of strings (cmds). However, the resulting output from these loops differs.

Loop1: Unexpected Output

In loop1, after iterating over cmds, a map (actions) is created where each command is mapped to a function that prints the command. When these functions are executed, they all print "update," which is the last element in cmds.

Loop2: Desired Output

In loop2, a similar approach is taken, but instead of storing direct command references in the actions map, a copy of the command is made using its index. This ensures that each function has its own copy of the command, preventing the unexpected "update" output from loop1.

Explanation

The peculiar behavior in loop1 occurs due to how closures (functions stored in the actions map) reference loop variables.

• In loop1, cmd is a loop variable that is modified in each iteration. When functions are created within the loop, they reference this loop variable.
• However, after the loop completes, the loop variable cmd is no longer accessible, and all functions refer to the last value it held ("update").

By making a copy of the loop variable for each function (as done in loop2), each function has its own isolated variable that is not affected by the subsequent loop iterations.

Conclusion

This example highlights the importance of variable scoping in Golang and the distinction between loop variables and detached variables created through copying. When working with closures that reference loop variables, it's crucial to ensure that copies are made to prevent unexpected behavior due to variable reuse.

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