
The `strconv` package converts between strings and numeric types. `Atoi` is a shorthand for converting a decimal string to `int`; the `Parse*` functions give full control over base and bit size.

<ExamplePair>
<ExampleProse>

`strconv.Atoi` converts a decimal string to `int`. It returns both the value and an error. Always check the error - a failed parse returns `0`, which is also a valid integer.

</ExampleProse>
<ExampleCode>

```go
package main

import (
    "fmt"
    "strconv"
)

func main() {
    n, err := strconv.Atoi("42")
    if err != nil {
        panic(err)
    }
    fmt.Println(n) // 42

    _, err = strconv.Atoi("abc")
    fmt.Println(err) // strconv.Atoi: parsing "abc": invalid syntax
}
```

</ExampleCode>
</ExamplePair>

<ExamplePair>
<ExampleProse>

`strconv.ParseInt` parses a string in any base (2–36) and returns an `int64`. The `bitSize` argument (8, 16, 32, 64) specifies the integer type the result must fit into - it does not limit the input string, only the return type's range.

</ExampleProse>
<ExampleCode>

```go
package main

import (
    "fmt"
    "strconv"
)

func main() {
    // base 10, fits in int64
    i64, _ := strconv.ParseInt("255", 10, 64)
    fmt.Println(i64) // 255

    // base 2 (binary)
    bin, _ := strconv.ParseInt("11111111", 2, 64)
    fmt.Println(bin) // 255

    // base 16 (hex), constrained to int8 range
    _, err := strconv.ParseInt("200", 16, 8) // 0x200 = 512, overflows int8
    fmt.Println(err)
}
```

</ExampleCode>
</ExamplePair>

<ExamplePair>
<ExampleProse>

`strconv.ParseFloat` converts a string to `float64` or `float32`. `strconv.FormatInt` and `strconv.Itoa` go the other direction - number to string.

</ExampleProse>
<ExampleCode>

```go
package main

import (
    "fmt"
    "strconv"
)

func main() {
    f, _ := strconv.ParseFloat("3.14159", 64)
    fmt.Printf("%.5f\n", f) // 3.14159

    // int to string
    s := strconv.Itoa(42)
    fmt.Println(s + " is the answer") // 42 is the answer

    // int64 to hex string
    hex := strconv.FormatInt(255, 16)
    fmt.Println(hex) // ff
}
```

</ExampleCode>
</ExamplePair>

<InProduction>
Always check the error from `strconv` functions - a `0` return on failure is the zero value, not a successfully parsed zero. `ParseInt`'s `bitSize` parameter specifies the target type's range, not a cap on the input string; passing `"200"` with `bitSize=8` returns an overflow error because 200 doesn't fit in an `int8`, but the string itself is still valid. Prefer `strconv` over `fmt.Sscanf` for number parsing: `Sscanf` is slower, less precise, and its error messages are harder to act on. In HTTP handlers that parse numeric path or query parameters, always validate with `strconv` at the boundary and return a 400 before the parsed value reaches business logic.
</InProduction>


strconv.Atoi and ParseInt for integers, ParseFloat for floats, FormatInt and Itoa for the reverse, and error handling on invalid input.

Number Parsing


The `net/url` package parses and builds URLs safely. `url.Parse` decomposes a raw URL string into its components; `url.Values` encodes query parameters without manual escaping.

<ExamplePair>
<ExampleProse>

`url.Parse` returns a `*url.URL` with fields for each component. All fields are strings - no type conversion needed. An error is only returned for malformed input that cannot be parsed at all; many "bad" URLs still parse without error.

</ExampleProse>
<ExampleCode>

```go
package main

import (
    "fmt"
    "net/url"
)

func main() {
    u, err := url.Parse("https://api.example.com:8080/v1/users?page=2&limit=50#results")
    if err != nil {
        panic(err)
    }

    fmt.Println(u.Scheme)   // https
    fmt.Println(u.Host)     // api.example.com:8080
    fmt.Println(u.Path)     // /v1/users
    fmt.Println(u.RawQuery) // page=2&limit=50
    fmt.Println(u.Fragment) // results
}
```

</ExampleCode>
</ExamplePair>

<ExamplePair>
<ExampleProse>

`url.URL.Query()` parses the raw query string into a `url.Values` map. Use it to read individual parameters. `url.Values.Encode()` serialises back to a query string, sorting keys alphabetically for deterministic output.

</ExampleProse>
<ExampleCode>

```go
package main

import (
    "fmt"
    "net/url"
)

func main() {
    u, _ := url.Parse("https://search.example.com?q=golang+channels&sort=date&page=1")

    params := u.Query()
    fmt.Println(params.Get("q"))    // golang channels
    fmt.Println(params.Get("sort")) // date

    // Build a new query string safely
    v := url.Values{}
    v.Set("q", "go generics <1.18")
    v.Set("sort", "relevance")
    fmt.Println(v.Encode()) // q=go+generics+%3C1.18&sort=relevance
}
```

</ExampleCode>
</ExamplePair>

<ExamplePair>
<ExampleProse>

`u.Hostname()` strips the port from `Host`, and `u.Port()` extracts just the port. Use these instead of splitting `Host` manually to handle IPv6 addresses correctly.

</ExampleProse>
<ExampleCode>

```go
package main

import (
    "fmt"
    "net/url"
)

func main() {
    u, _ := url.Parse("https://api.example.com:8080/health")

    fmt.Println(u.Hostname()) // api.example.com
    fmt.Println(u.Port())     // 8080

    // Rebuild the URL with a different path
    u.Path = "/v2/health"
    fmt.Println(u.String()) // https://api.example.com:8080/v2/health
}
```

</ExampleCode>
</ExamplePair>

<InProduction>
Always parse user-supplied URLs with `url.Parse` before use - do not string-concatenate URLs or use `fmt.Sprintf` to build query strings. Use `url.Values.Encode()` to safely encode query parameters; it handles special characters correctly and sorts keys deterministically, making URLs easier to cache and compare. When constructing outbound API URLs in production code, build a `url.URL` struct from components rather than formatting a string: it prevents accidental double-encoding and makes the code easier to read and test. For path segments that may contain slashes or special characters, use `url.PathEscape` rather than `url.QueryEscape` - they escape different character sets.
</InProduction>


net/url.Parse to decompose URLs, url.URL fields for scheme, host, path, and query, url.Values for safe query parameter encoding.

URL Parsing


Go's `math/rand/v2` package (Go 1.22+) provides pseudo-random number generation for simulations and non-security use cases. For anything security-sensitive - tokens, session IDs, nonces - use `crypto/rand`.

<ExamplePair>
<ExampleProse>

`math/rand/v2` is auto-seeded in Go 1.22+. Call global functions directly - no setup required. `rand.IntN(n)` returns a non-negative random integer in [0, n). `rand.Float64()` returns a float in [0.0, 1.0).

</ExampleProse>
<ExampleCode>

```go
package main

import (
    "fmt"
    "math/rand/v2"
)

func main() {
    // Random int in [0, 100)
    fmt.Println(rand.IntN(100))

    // Random float in [0.0, 1.0)
    fmt.Println(rand.Float64())

    // Simulate a d6 roll
    roll := rand.IntN(6) + 1
    fmt.Println("d6:", roll)

    // Shuffle a slice
    s := []int{1, 2, 3, 4, 5}
    rand.Shuffle(len(s), func(i, j int) {
        s[i], s[j] = s[j], s[i]
    })
    fmt.Println(s)
}
```

</ExampleCode>
</ExamplePair>

<ExamplePair>
<ExampleProse>

Create a local `rand.Rand` with an explicit source when you need reproducible output (e.g., in tests) or an isolated random stream that does not share state with global functions.

</ExampleProse>
<ExampleCode>

```go
package main

import (
    "fmt"
    "math/rand/v2"
)

func main() {
    // Seeded source - deterministic output
    src := rand.NewPCG(42, 0)
    r := rand.New(src)

    fmt.Println(r.IntN(100))
    fmt.Println(r.IntN(100))

    // Same seed → same sequence
    src2 := rand.NewPCG(42, 0)
    r2 := rand.New(src2)
    fmt.Println(r2.IntN(100)) // same as first value above
}
```

</ExampleCode>
</ExamplePair>

<ExamplePair>
<ExampleProse>

Use `crypto/rand` for cryptographically secure randomness. `rand.Read` fills a byte slice with random bytes from the OS. Encode with `base64.RawURLEncoding` to produce a URL-safe token string.

</ExampleProse>
<ExampleCode>

```go
package main

import (
    "crypto/rand"
    "encoding/base64"
    "fmt"
)

func secureToken(n int) string {
    b := make([]byte, n)
    if _, err := rand.Read(b); err != nil {
        panic(err)
    }
    return base64.RawURLEncoding.EncodeToString(b)
}

func main() {
    token := secureToken(32) // 256 bits of entropy
    fmt.Println(token)       // e.g. "dGhpcyBpcyBhIHRlc3Q..."

    // Random UUID v4 (simplified)
    var uuid [16]byte
    rand.Read(uuid[:])
    uuid[6] = (uuid[6] & 0x0f) | 0x40 // version 4
    uuid[8] = (uuid[8] & 0x3f) | 0x80 // variant bits
    fmt.Printf("%x-%x-%x-%x-%x\n",
        uuid[0:4], uuid[4:6], uuid[6:8], uuid[8:10], uuid[10:])
}
```

</ExampleCode>
</ExamplePair>

<InProduction>
`math/rand/v2` is auto-seeded and safe for simulation, load testing, random sampling, and shuffling. Never use it for security-sensitive randomness - tokens, passwords, nonces, CSRF values, API keys. Use `crypto/rand` for all of those. The practical pattern: generate N bytes from `crypto/rand.Read`, base64url-encode them, and you have an opaque token with N*8 bits of entropy. 32 bytes (256 bits) is the conventional floor for tokens. For generating UUIDs in production, use the `github.com/google/uuid` package rather than rolling your own - it handles version 4 correctly and is extensively tested. In tests that need deterministic sequences (property tests, golden file generators), use `rand.NewPCG` with a fixed seed so failures are reproducible without relying on a global random state.
</InProduction>


math/rand/v2 for general-purpose randomness, global functions vs local Source, and crypto/rand for cryptographically secure randomness.