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Go Language Go Language Overview Data Structures Maps

Posted by theteamtreehouse
theteamtreehouse
3,472 Points

I'm amazed that this doesn't work?

It seems that the operation here is valid, but modifying p doesn't modify the mapped values, but only a copy of the mapped values. In this case, I think it is passed by value.

I tried this locally with debugger and indeed it is changed but not saved to the maps. 

package main

import "fmt"

func HalfPriceSale(prices map[string]float64) map[string]float64 {
    for _, p := range prices {
        p = p / 2
    }
    return prices
}

func main() {
    priceList := map[string]float64{"apple": 1.99, "grapes": 5.99, "milk": 0.99}
    fmt.Println(priceList)
    fmt.Println(HalfPriceSale(priceList))
}
src/sales/sales.go 
package sales

func HalfPriceSale(prices map[string]float64) map[string]float64 {
    for _, p := range prices {
        p = p / 2
    }
    return prices
}

I'm also surprised that this is not valid Golang code, but it will absolutely work in C++

    priceList := map[string]float64 {
        "apple": 1.99,
        "grapes": 5.99,
        "milk": 0.99
    }

1 Answer

Liam Clarke
Liam Clarke
19,938 Points
Liam Clarke
Liam Clarke
19,938 Points

Hello

This behaviour is detailed on their website

If a function takes a slice argument, changes it makes to the elements of the slice will be visible to the caller, analogous to passing a pointer to the underlying array.

Maps and slices are reference types so your loop is referencing over an underlying data structure, this means that changing it like the above, wont change the referenced value.

Here is 2 examples:

type MapOfValus map[string]float64

func HalfPriceSale(prices MapOfValus) MapOfValus {
    halfPrice := MapOfValus{}
    for key, val := range prices {
        halfPrice[key] = val / 2
    }
    return halfPrice
}

func HalfPriceSale2(prices MapOfValus) MapOfValus {
    for key, val := range prices {
        prices[key] = val / 2
    }
    return prices
}