# Why is it possible to change the values of a row of a given matrix with a new matrix?

**URL:** <https://discourse.julialang.org/t/why-is-it-possible-to-change-the-values-of-a-row-of-a-given-matrix-with-a-new-matrix/84322>\
**Category:** Internals & Design\
**Created:** [July 16, 2022, 5:27pm UTC](https://discourse.julialang.org/t/why-is-it-possible-to-change-the-values-of-a-row-of-a-given-matrix-with-a-new-matrix/84322 "2022-07-16T17:27:05Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Shayan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/shayan/32/33025_2.png) [@Shayan](https://discourse.julialang.org/u/Shayan)\
**Post date:** [July 16, 2022, 5:27pm UTC](https://discourse.julialang.org/t/why-is-it-possible-to-change-the-values-of-a-row-of-a-given-matrix-with-a-new-matrix/84322/1 "2022-07-16T17:27:05Z")

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I tried to pick an exact and explicit title for this question but failed. So I guess you must read my entire question to understand the issue.  
In Julia, we can define a random 2\*5 Matrix using `rand(2,5)`:

```julia
julia> a = rand(2,5)
2×5 Matrix{Float64}:
 0.471737 0.726199 0.575673 0.760658 0.300233
 0.443542 0.92782 0.501209 0.902491 0.171455

```

And what amazed me (Also warned me a lot) is that you can change the values of `a[1, 1:4]` by assigning (=) a 2\*2 Matrix:

```julia
julia> a[1 , 1:4] = [1 2;3 4]
2×2 Matrix{Int64}:
 1 2
 3 4

julia> a
2×5 Matrix{Float64}:
 1.0 3.0 2.0 4.0 0.300233
 0.443542 0.92782 0.501209 0.902491 0.171455

```

I mean, This is not good at all and can cause many silent problems in many algorithms! How and most importantly, why even is this possible to assign a matrix into a piece of a vector! I mean, `a[1, 1:4]` is a vector:

```julia
julia> a[1, 1:4]
4-element Vector{Float64}:
 1.0
 3.0
 2.0
 4.0

```

Which belongs to the `a` matrix, and here I changed that vector with the values of a matrix!! I expected that Julia would throw an error saying: _“that dimensions don’t match, so how the hell are you expecting me (Julia) to do that?”_.  
But not also Julia didn’t throw anything, but it also amazed me in that way! Why is that possible?

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**Author:** ![mkitti](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mkitti/32/12459_2.png) [@mkitti](https://discourse.julialang.org/u/mkitti)\
**Post date:** [July 16, 2022, 7:05pm UTC](https://discourse.julialang.org/t/why-is-it-possible-to-change-the-values-of-a-row-of-a-given-matrix-with-a-new-matrix/84322/2 "2022-07-16T19:05:33Z")

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You may be interested in using `.=` instead.

```julia
julia> a = rand(2,5)
2×5 Matrix{Float64}:
 0.652209 0.773756 0.0126887 0.320912 0.758584
 0.483548 0.278898 0.902765 0.428947 0.0341019

julia> a[1, 1:4] .= [1 2; 3 4]
ERROR: DimensionMismatch("array could not be broadcast to match destination")
Stacktrace:

```

Otherwise, `setindex_shape_check` is designed to do the following:

> <https://github.com/JuliaLang/julia/blob/bc9fdcef490f32c48e92fe93c373b2aa7076e55b/base/indices.jl#L197-L202>

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<div class="post-metadata">

**Author:** ![jar1](https://avatars.discourse-cdn.com/v4/letter/j/c0e974/32.png) [@jar1](https://discourse.julialang.org/u/jar1)\
**Post date:** [July 16, 2022, 7:25pm UTC](https://discourse.julialang.org/t/why-is-it-possible-to-change-the-values-of-a-row-of-a-given-matrix-with-a-new-matrix/84322/3 "2022-07-16T19:25:34Z")

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I find implicit raveling is often surprising and error prone. I would rather create a linear view explicitly when I want it, but I don’t think that’s possible in 1.X. A nonraveling view like `noravel(b)` that errors when raveled could help avoid these issues.