How to extend cap in golang

Golang is a programming language for developing high concurrency and excellent performance. It provides built-in support at the language level, such as goroutine and channel. However, when processing large-scale data, Golang's cap (capacity) may not be able to meet the demand, causing performance problems in the program. This article will introduce how to extend Golang's cap, optimize program performance, and make your program run faster.

1. What is cap

First of all, we need to know what cap is. In Golang, the two data structures of slice and map have an attribute called cap, which represents the capacity of the underlying array of the slice or map. Capacity is the number of elements pre-allocated to the slice or map. When a program adds elements to a slice or map, if the capacity of the slice or map is exceeded, the underlying array will be expanded. Expansion will cause memory reallocation and data copying, thus affecting program performance.

2. Expand the cap of slice

2.1. Pre-allocate cap

We can pre-allocate a larger cap when creating the slice instead of Dynamically expand the capacity while the program is running. This can reduce the number of expansions and improve program performance.

arr := make([]int, 0, 1000)
// 从空slice开始，容量为1000

2.2. Manual expansion

We can also manually expand the cap of the slice. When we know the number of elements that need to be stored in the slice, we can first allocate a smaller cap to the slice and expand it as needed.

arr := make([]int, 0, 10)
// 先分配一个容量为10的底层数组
for i := 0; i < 1000; i++ {
    if len(arr) == cap(arr) { // 当容量不足时
        newCap := cap(arr) * 2 // 扩容为原来的2倍
        newArr := make([]int, len(arr), newCap)
        copy(newArr, arr)
        arr = newArr // 更换为新数组
    }
    arr = append(arr, i)
}

3. Extended map cap

3.1. Estimating the number of elements

We can estimate the number of map elements when creating the map. This can make the capacity of the underlying array larger, thereby reducing the number of expansions.

m := make(map[int]int, 1000)
// 预估map元素数量为1000

3.2. Manually adjust the capacity

We can also manually adjust the map capacity. When there are a large number of elements in the map, we can increase the capacity of the map according to certain rules. In Golang's standard library, the map expansion rule is: when the number of elements is greater than 2/3 of the size of the underlying array, the map will expand to 2 times the original size.

m := make(map[string]int) // 创建空map
for i := 0; i < 10000; i++ {
    key := fmt.Sprintf("key%d", i)
    m[key] = i
    if len(m) >= int(0.67 * float64(cap(m))) { // 当元素数量超过2/3时
        newCap := cap(m) * 2 // 扩容为原来的2倍
        newMap := make(map[string]int, newCap)
        for k, v := range m {
            newMap[k] = v // 复制元素到新map
        }
        m = newMap //更换为新map
    }
}

4. Summary

Golang is an efficient programming language, but when processing large-scale data, cap may become a performance bottleneck. We can use the above method to expand the cap of slice and map to optimize program performance. It should be noted that expansion will cause memory reallocation and data copying, so the number of elements and capacity should be estimated before adding elements.

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