# How to add/subtract values in a dataframe or array column?

**URL:** <https://discourse.julialang.org/t/how-to-add-subtract-values-in-a-dataframe-or-array-column/45455>\
**Category:** New to Julia\
**Created:** [August 24, 2020, 2:04pm UTC](https://discourse.julialang.org/t/how-to-add-subtract-values-in-a-dataframe-or-array-column/45455 "2020-08-24T14:04:35Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![HelgavonLichtenstein](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/helgavonlichtenstein/32/5005_2.png) [@HelgavonLichtenstein](https://discourse.julialang.org/u/HelgavonLichtenstein)\
**Post date:** [August 24, 2020, 2:04pm UTC](https://discourse.julialang.org/t/how-to-add-subtract-values-in-a-dataframe-or-array-column/45455/1 "2020-08-24T14:04:35Z")

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I’m going over the [array tutorial](https://en.wikibooks.org/wiki/Introducing_Julia/Arrays_and_tuples#Setting_the_contents_of_arrays) trying to see how to add 1 to the content of an array but am having difficulties. I do not want to replace the contents of the column by 1, but add 1 to the value already in the column. What is the correct way to do this for an array and dataframe?

```julia
QWER = rand(5, 5)

QWER[:,2:4] .+1 #this adds 1 to the selected columns
QWER # but this change does not occur in the array

QWER[:,2:4] .= .+1 # this changes the column values to 1
QWER # this change occurs in the array

```

```julia
using DataFrames
df_QWER = DataFrame(QWER)
df_QWER[:,2:4] = .+1 # this changes the column values to 1
df_QWER # this change occurs in the array

df_QWER = DataFrame(QWER)
df_QWER[:,2:4] .+1 # this adds 1 to the selected columns
df_QWER # but this change does not occur in the array

```

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**Author:** ![Rudi79](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rudi79/32/3884_2.png) [@Rudi79](https://discourse.julialang.org/u/Rudi79)\
**Post date:** [August 24, 2020, 3:04pm UTC](https://discourse.julialang.org/t/how-to-add-subtract-values-in-a-dataframe-or-array-column/45455/2 "2020-08-24T15:04:58Z")

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> [@HelgavonLichtenstein](#):
>
> ```julia
> QWER = rand(5, 5)
> 
> QWER[:,2:4] .+1
> 
> ```

Try

```julia
QWER[:,2:4] .+= 1

```

This is a shorthand notation for

```julia
QWER[:,2:4] = QWER[:,2:4] .+ 1

```

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<div class="post-metadata">

**Author:** ![HelgavonLichtenstein](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/helgavonlichtenstein/32/5005_2.png) [@HelgavonLichtenstein](https://discourse.julialang.org/u/HelgavonLichtenstein)\
**Post date:** [August 24, 2020, 3:16pm UTC](https://discourse.julialang.org/t/how-to-add-subtract-values-in-a-dataframe-or-array-column/45455/3 "2020-08-24T15:16:44Z")

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Thank you!

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**Author:** ![grahamstark](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/grahamstark/32/5200_2.png) [@grahamstark](https://discourse.julialang.org/u/grahamstark)\
**Post date:** [August 24, 2020, 3:37pm UTC](https://discourse.julialang.org/t/how-to-add-subtract-values-in-a-dataframe-or-array-column/45455/4 "2020-08-24T15:37:54Z")

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In a DataFrame, (say, `d`), and the columns have names, you can just do:

```julia
d.my_col .+= 1

```

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**Author:** ![Rudi79](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rudi79/32/3884_2.png) [@Rudi79](https://discourse.julialang.org/u/Rudi79)\
**Post date:** [August 24, 2020, 3:40pm UTC](https://discourse.julialang.org/t/how-to-add-subtract-values-in-a-dataframe-or-array-column/45455/5 "2020-08-24T15:40:52Z")

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> [@grahamstark](#):
>
> `d.my_col += 1`

you are missing a `.` like

```julia
d.my_col .+= 1

```

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**Author:** ![grahamstark](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/grahamstark/32/5200_2.png) [@grahamstark](https://discourse.julialang.org/u/grahamstark)\
**Post date:** [August 24, 2020, 3:42pm UTC](https://discourse.julialang.org/t/how-to-add-subtract-values-in-a-dataframe-or-array-column/45455/6 "2020-08-24T15:42:54Z")

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Yes, sorry, fixed.