go - Can't create copy of variable of type map[int]Struct in another variable by just assigning one variable to another

Question

I have a Map :

cart := map[10033207:{10033207 3 425 126} 10012761:{10012761 4 40 0}]

I want to create copy of cart in another variable tempCart so that I can modify tempCart for temporary usage in my function. I want that cart value remains the same.

tempCart := cart
//some operation which modifies temp cart and make it
//map[10033207:{10033207 2 425 126} 10012761:{10012761 1 40 0}]

The problem is that when I modify tempCart, somehow cart is also getting modified and becomes equal to tempCart.

Later when I print value of cart I get: map[10033207:{10033207 2 425 126} 10012761:{10012761 1 40 0}] and not the original value that is map[10033207:{10033207 3 425 126} 10012761:{10012761 4 40 0}].

I can't understand the reason behind it and want to know the solution of how to create a copy of cart.

EDIT: This question has been marked as Duplicate to copy one map to another But I knew how to copy one map to anothor, my prime question was why couldn't I just assign one map to another variable. Why do I have to copy it in a loop.

tempCart := cart is just assigning the reference of one struct to another. It is not "copying". There is no in-built way to copy structs in Go. You'll have to initialize the new struct with the values from the other one manually, or use a library like github.com/jinzhu/copier — Nadh
– Nadh, Commented Nov 10, 2016 at 11:11
Possible duplicate of Why is a map value in one function affected by an entry to the map in another function? Also related: Copying all elements of a map into another. — icza
– icza, Commented Nov 10, 2016 at 11:26
@Jagrati: You use pointers specifically to modify the values of the original — Elias Van Ootegem
– Elias Van Ootegem, Commented Nov 10, 2016 at 14:15
@icza thanks I went through the answers of those question. However, I asked this question because I didn't knew I should have searched my problem like that. Thanks anyway. — Jagrati
– Jagrati, Commented Nov 11, 2016 at 5:24

sadlil · Accepted Answer · 2016-11-10 11:12:11Z

5

To Copy a map use

for k,v := range originalMap {
  newMap[k] = v
}

answered Nov 10, 2016 at 11:12

sadlil

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1 Comment

Nadh Over a year ago

This will not do a deep copy of the structs in the map.

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