How can I change values in a column without knowing how many columns I have in the dataframe in r?

I have a matrix

df <- matrix(c(9185, 3661, 9539, 4266, 9650, 3897, 9160, 4451), nrow = 2, ncol = 4)

and I want calculate: (element - mean(by row))/ sd(by row) . Ex: (9185 - 9383,5)/248,03 and substitute the result in a matrix. Like that:

matrix(c(-0.80030564, -1.144503446, 0.62694, 0.553656, 1.074466, -0.482080851, -0.9011, 1.072928123), nrow = 2, ncol = 4)

The problem is that in the real case I need to iterate and I don't know how many columns I will have, but they will be all the columns present in the df dataframe.

Solution

You can use apply() to do row-wise calculations on a matrix.

df <- matrix(c(9185, 3661, 9539, 4266, 9650, 3897, 9160, 4451), nrow = 2, ncol = 4)
t(apply(df, 1, function(x)(x-mean(x))/sd(x)))
#>            [,1]      [,2]       [,3]       [,4]
#> [1,] -0.8003056 0.6269397  1.0744658 -0.9010998
#> [2,] -1.1445034 0.5536562 -0.4820809  1.0729281

Also, scale() is a function that does the calculation you want

t(apply(df, 1, scale))
#>            [,1]      [,2]       [,3]       [,4]
#> [1,] -0.8003056 0.6269397  1.0744658 -0.9010998
#> [2,] -1.1445034 0.5536562 -0.4820809  1.0729281

^{Created on 2023-10-02 with reprex v2.0.2}