Why does the Rust compiler give an uninitialized variable error when initializing an array in a loop?
The compiler complains a variable is not initialized, and it is right.
However, the variable appears on the left side of the expression.
I guess I could easily fix this by initializing the array, but I am more interested in understanding why the compiler thinks this is an error condition.
I don't think this would be flagged as an error in other languages.
Here is my code:
fn main() {
const LEN: usize = 5;
let mut arr: [u32; LEN];
for i in 0..LEN {
arr[i] = fib(i as u32);
}
println!("{:?}", arr);
}
fn fib(n: u32) -> u32 {
match n {
0 => 0,
1 => 1,
_ => fib(n - 1) + fib(n - 2),
}
}
Here is the error:
error[E0381]: use of possibly uninitialized variable: `arr`
--> src/main.rs:6:9
|
6 | arr[i] = fib(i as u32);
| ^^^^^^^^^^^^^^^^^^^^^^ use of possibly uninitialized `arr`
error[E0381]: use of possibly uninitialized variable: `arr`
--> src/main.rs:9:22
|
9 | println!("{:?}", arr);
| ^^^ use of possibly uninitialized `arr`
rust
edited Nov 26 '18 at 13:40
Shepmaster
152k14299436
asked Nov 25 '18 at 21:07
MuffoMuffo
1,11211321
add a comment |
The compiler complains a variable is not initialized, and it is right.
However, the variable appears on the left side of the expression.
I guess I could easily fix this by initializing the array, but I am more interested in understanding why the compiler thinks this is an error condition.
I don't think this would be flagged as an error in other languages.
Here is my code:
fn main() {
const LEN: usize = 5;
let mut arr: [u32; LEN];
for i in 0..LEN {
arr[i] = fib(i as u32);
}
println!("{:?}", arr);
}
fn fib(n: u32) -> u32 {
match n {
0 => 0,
1 => 1,
_ => fib(n - 1) + fib(n - 2),
}
}
Here is the error:
error[E0381]: use of possibly uninitialized variable: `arr`
--> src/main.rs:6:9
|
6 | arr[i] = fib(i as u32);
| ^^^^^^^^^^^^^^^^^^^^^^ use of possibly uninitialized `arr`
error[E0381]: use of possibly uninitialized variable: `arr`
--> src/main.rs:9:22
|
9 | println!("{:?}", arr);
| ^^^ use of possibly uninitialized `arr`
rust
edited Nov 26 '18 at 13:40
Shepmaster
152k14299436
asked Nov 25 '18 at 21:07
MuffoMuffo
1,11211321
1
The compiler isn't smart enough, however Rust give a lot of tool to make better code than this Iterator is the way to go or std::mem::uninitialized if you want keep an array is the "trust me" solution.
– Stargateur
Nov 25 '18 at 22:24
Thanks for pointing out std::mem::uninitialized. A quick search brought me to this page that shows some code very similar to mine: doc.rust-lang.org/std/mem/fn.uninitialized.html
– Muffo
Nov 26 '18 at 2:42
1
See also What is the proper way to initialize a fixed length array?; Initialize array holding struct more efficiently; Is there a way to not have to initialize arrays twice?.
– Shepmaster
Nov 26 '18 at 13:42
add a comment |
The compiler complains a variable is not initialized, and it is right.
However, the variable appears on the left side of the expression.
I guess I could easily fix this by initializing the array, but I am more interested in understanding why the compiler thinks this is an error condition.
I don't think this would be flagged as an error in other languages.
Here is my code:
fn main() {
const LEN: usize = 5;
let mut arr: [u32; LEN];
for i in 0..LEN {
arr[i] = fib(i as u32);
}
println!("{:?}", arr);
}
fn fib(n: u32) -> u32 {
match n {
0 => 0,
1 => 1,
_ => fib(n - 1) + fib(n - 2),
}
}
Here is the error:
error[E0381]: use of possibly uninitialized variable: `arr`
--> src/main.rs:6:9
|
6 | arr[i] = fib(i as u32);
| ^^^^^^^^^^^^^^^^^^^^^^ use of possibly uninitialized `arr`
error[E0381]: use of possibly uninitialized variable: `arr`
--> src/main.rs:9:22
|
9 | println!("{:?}", arr);
| ^^^ use of possibly uninitialized `arr`
rust
edited Nov 26 '18 at 13:40
Shepmaster
152k14299436
asked Nov 25 '18 at 21:07
MuffoMuffo
1,11211321
The compiler complains a variable is not initialized, and it is right.
However, the variable appears on the left side of the expression.
I guess I could easily fix this by initializing the array, but I am more interested in understanding why the compiler thinks this is an error condition.
I don't think this would be flagged as an error in other languages.
Here is my code:
fn main() {
const LEN: usize = 5;
let mut arr: [u32; LEN];
for i in 0..LEN {
arr[i] = fib(i as u32);
}
println!("{:?}", arr);
}
fn fib(n: u32) -> u32 {
match n {
0 => 0,
1 => 1,
_ => fib(n - 1) + fib(n - 2),
}
}
Here is the error:
error[E0381]: use of possibly uninitialized variable: `arr`
--> src/main.rs:6:9
|
6 | arr[i] = fib(i as u32);
| ^^^^^^^^^^^^^^^^^^^^^^ use of possibly uninitialized `arr`
error[E0381]: use of possibly uninitialized variable: `arr`
--> src/main.rs:9:22
|
9 | println!("{:?}", arr);
| ^^^ use of possibly uninitialized `arr`
rust
rust
edited Nov 26 '18 at 13:40
Shepmaster
152k14299436
asked Nov 25 '18 at 21:07
MuffoMuffo
1,11211321
edited Nov 26 '18 at 13:40
Shepmaster
152k14299436
asked Nov 25 '18 at 21:07
MuffoMuffo
1,11211321
edited Nov 26 '18 at 13:40
Shepmaster
152k14299436
edited Nov 26 '18 at 13:40
Shepmaster
152k14299436
edited Nov 26 '18 at 13:40
Shepmaster
152k14299436
152k14299436
asked Nov 25 '18 at 21:07
MuffoMuffo
1,11211321
asked Nov 25 '18 at 21:07
MuffoMuffo
1,11211321
asked Nov 25 '18 at 21:07
MuffoMuffo
1,11211321
1,11211321
1
The compiler isn't smart enough, however Rust give a lot of tool to make better code than this Iterator is the way to go or std::mem::uninitialized if you want keep an array is the "trust me" solution.
– Stargateur
Nov 25 '18 at 22:24
Thanks for pointing out std::mem::uninitialized. A quick search brought me to this page that shows some code very similar to mine: doc.rust-lang.org/std/mem/fn.uninitialized.html
– Muffo
Nov 26 '18 at 2:42
1
See also What is the proper way to initialize a fixed length array?; Initialize array holding struct more efficiently; Is there a way to not have to initialize arrays twice?.
– Shepmaster
Nov 26 '18 at 13:42
add a comment |
1
The compiler isn't smart enough, however Rust give a lot of tool to make better code than this Iterator is the way to go or std::mem::uninitialized if you want keep an array is the "trust me" solution.
– Stargateur
Nov 25 '18 at 22:24
Thanks for pointing out std::mem::uninitialized. A quick search brought me to this page that shows some code very similar to mine: doc.rust-lang.org/std/mem/fn.uninitialized.html
– Muffo
Nov 26 '18 at 2:42
1
See also What is the proper way to initialize a fixed length array?; Initialize array holding struct more efficiently; Is there a way to not have to initialize arrays twice?.
– Shepmaster
Nov 26 '18 at 13:42
1
1
The compiler isn't smart enough, however Rust give a lot of tool to make better code than this Iterator is the way to go or std::mem::uninitialized if you want keep an array is the "trust me" solution.
– Stargateur
Nov 25 '18 at 22:24
The compiler isn't smart enough, however Rust give a lot of tool to make better code than this Iterator is the way to go or std::mem::uninitialized if you want keep an array is the "trust me" solution.
– Stargateur
Nov 25 '18 at 22:24
Thanks for pointing out std::mem::uninitialized. A quick search brought me to this page that shows some code very similar to mine: doc.rust-lang.org/std/mem/fn.uninitialized.html
– Muffo
Nov 26 '18 at 2:42
Thanks for pointing out std::mem::uninitialized. A quick search brought me to this page that shows some code very similar to mine: doc.rust-lang.org/std/mem/fn.uninitialized.html
– Muffo
Nov 26 '18 at 2:42
1
1
See also What is the proper way to initialize a fixed length array?; Initialize array holding struct more efficiently; Is there a way to not have to initialize arrays twice?.
– Shepmaster
Nov 26 '18 at 13:42
See also What is the proper way to initialize a fixed length array?; Initialize array holding struct more efficiently; Is there a way to not have to initialize arrays twice?.
– Shepmaster
Nov 26 '18 at 13:42
add a comment |
1 Answer
1
active
oldest
votes
When you do a for loop, the code is sequential: the compiler first set the value at index 0, then 1, etc. but it has no clue that you are initializing the array that way. You could, for example, forgot the last index, and your code would be invalid.
To say it simply: you can modify a variable only when it was initialized, and arr is your variable, not arr[0].
When you index something in Rust, this is desugared into the index_mut method. In your situation, you are calling a method of arr that is an uninitialized variable.
As you said, the solution to your issue is to initialize your array first, with zeroes for example:
fn main() {
const LEN : usize = 5;
let mut arr = [0; LEN];
for i in 0..LEN {
arr[i] = fib(i as u32);
}
println!("{:?}", arr);
}
fn fib(n: u32) -> u32 {
match n {
0 => 0,
1 => 1,
_ => fib(n-1) + fib(n-2)
}
}
answered Nov 25 '18 at 21:11
Darth BoiethiosDarth Boiethios
10.6k43775
Thanks for the great explanation and for the link!
– Muffo
Nov 26 '18 at 2:41
add a comment |
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1 Answer
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When you do a for loop, the code is sequential: the compiler first set the value at index 0, then 1, etc. but it has no clue that you are initializing the array that way. You could, for example, forgot the last index, and your code would be invalid.
To say it simply: you can modify a variable only when it was initialized, and arr is your variable, not arr[0].
When you index something in Rust, this is desugared into the index_mut method. In your situation, you are calling a method of arr that is an uninitialized variable.
As you said, the solution to your issue is to initialize your array first, with zeroes for example:
fn main() {
const LEN : usize = 5;
let mut arr = [0; LEN];
for i in 0..LEN {
arr[i] = fib(i as u32);
}
println!("{:?}", arr);
}
fn fib(n: u32) -> u32 {
match n {
0 => 0,
1 => 1,
_ => fib(n-1) + fib(n-2)
}
}
answered Nov 25 '18 at 21:11
Darth BoiethiosDarth Boiethios
10.6k43775
Thanks for the great explanation and for the link!
– Muffo
Nov 26 '18 at 2:41
add a comment |
When you do a for loop, the code is sequential: the compiler first set the value at index 0, then 1, etc. but it has no clue that you are initializing the array that way. You could, for example, forgot the last index, and your code would be invalid.
To say it simply: you can modify a variable only when it was initialized, and arr is your variable, not arr[0].
When you index something in Rust, this is desugared into the index_mut method. In your situation, you are calling a method of arr that is an uninitialized variable.
As you said, the solution to your issue is to initialize your array first, with zeroes for example:
fn main() {
const LEN : usize = 5;
let mut arr = [0; LEN];
for i in 0..LEN {
arr[i] = fib(i as u32);
}
println!("{:?}", arr);
}
fn fib(n: u32) -> u32 {
match n {
0 => 0,
1 => 1,
_ => fib(n-1) + fib(n-2)
}
}
answered Nov 25 '18 at 21:11
Darth BoiethiosDarth Boiethios
10.6k43775
Thanks for the great explanation and for the link!
– Muffo
Nov 26 '18 at 2:41
add a comment |
When you do a for loop, the code is sequential: the compiler first set the value at index 0, then 1, etc. but it has no clue that you are initializing the array that way. You could, for example, forgot the last index, and your code would be invalid.
To say it simply: you can modify a variable only when it was initialized, and arr is your variable, not arr[0].
When you index something in Rust, this is desugared into the index_mut method. In your situation, you are calling a method of arr that is an uninitialized variable.
As you said, the solution to your issue is to initialize your array first, with zeroes for example:
fn main() {
const LEN : usize = 5;
let mut arr = [0; LEN];
for i in 0..LEN {
arr[i] = fib(i as u32);
}
println!("{:?}", arr);
}
fn fib(n: u32) -> u32 {
match n {
0 => 0,
1 => 1,
_ => fib(n-1) + fib(n-2)
}
}
answered Nov 25 '18 at 21:11
Darth BoiethiosDarth Boiethios
10.6k43775
When you do a for loop, the code is sequential: the compiler first set the value at index 0, then 1, etc. but it has no clue that you are initializing the array that way. You could, for example, forgot the last index, and your code would be invalid.
To say it simply: you can modify a variable only when it was initialized, and arr is your variable, not arr[0].
When you index something in Rust, this is desugared into the index_mut method. In your situation, you are calling a method of arr that is an uninitialized variable.
As you said, the solution to your issue is to initialize your array first, with zeroes for example:
fn main() {
const LEN : usize = 5;
let mut arr = [0; LEN];
for i in 0..LEN {
arr[i] = fib(i as u32);
}
println!("{:?}", arr);
}
fn fib(n: u32) -> u32 {
match n {
0 => 0,
1 => 1,
_ => fib(n-1) + fib(n-2)
}
}
answered Nov 25 '18 at 21:11
Darth BoiethiosDarth Boiethios
10.6k43775
edited Nov 25 '18 at 21:43
answered Nov 25 '18 at 21:11
Darth BoiethiosDarth Boiethios
10.6k43775
answered Nov 25 '18 at 21:11
Darth BoiethiosDarth Boiethios
10.6k43775
answered Nov 25 '18 at 21:11
Darth BoiethiosDarth Boiethios
10.6k43775
10.6k43775
Thanks for the great explanation and for the link!
– Muffo
Nov 26 '18 at 2:41
add a comment |
Thanks for the great explanation and for the link!
– Muffo
Nov 26 '18 at 2:41
Thanks for the great explanation and for the link!
– Muffo
Nov 26 '18 at 2:41
Thanks for the great explanation and for the link!
– Muffo
Nov 26 '18 at 2:41
add a comment |
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1
The compiler isn't smart enough, however Rust give a lot of tool to make better code than this Iterator is the way to go or std::mem::uninitialized if you want keep an array is the "trust me" solution.
– Stargateur
Nov 25 '18 at 22:24
Thanks for pointing out std::mem::uninitialized. A quick search brought me to this page that shows some code very similar to mine: doc.rust-lang.org/std/mem/fn.uninitialized.html
– Muffo
Nov 26 '18 at 2:42
1
See also What is the proper way to initialize a fixed length array?; Initialize array holding struct more efficiently; Is there a way to not have to initialize arrays twice?.
– Shepmaster
Nov 26 '18 at 13:42