Append value to line in input file based on column number - shell

Question

I have a function append_val_to_line which appends the $append_val to the line in Input.txt and writes it back. I want to insert 45 at column 1 and it should and since its length is two values, it should at it at column 1 and column 0. But my code below is adding it at column 2 and column 3.
I am not sure why it is so. Can someone help me achieve the goal mentioned above?
My working solution is below but it does not add the 45 at column 0 and 1. I want a generic solution as I can insert the value at any column and the value should be added starting at column N to column N-1 ... N-2 based on the length of the append_val.
As you can see in the sample input before and sample input after the call to append_val_to_line function, the value 45 is added at column 1 and column 2 but I wanted to the from column 0 and end on column 1 as the value 45 is of length two. But my code adds it starting at column 2 and 3 instead.
Space is also a valid column number but I will not be adding any values in those spaces in a line.
#! /bin/bash
function append_val_to_line{
sed -i 's/\(.\{'$1'\}\)/\1'$2'/' "input.txt"
}
column_num=1
append_val=45
append_vals_to_line "$column_num" "$append_val"
Input.txt BEFORE call to function append_vals_to_line
1200 5600 775000 34555
Input.txt AFTER call to function append_vals_to_line
145200 5600 775000 34555
Note 45 has been added at column 2 and 3.

Answer 1

Since OP told OP wants to start character's position from 0 and I believe rather than column its character's position number which we are talking about here, so based on that and shown samples following may help then.
awk -v after="0" -v value="45" '{print substr($0,1,after+1) value substr($0,after+2)}' Input_file
Non one liner form of above:
awk -v after="0" -v value="45" '
{
print substr($0,1,after+1) value substr($0,after+2)
}
' Input_file
Explanation: Adding detailed explanation for above one.
awk -v after="0" -v value="45" ' ##Start awk prorgam from heer and setting after variable to 0 a per OP and value to 45.
{
print substr($0,1,after+1) value substr($0,after+2) ##Printing sub-string from 1 to till after+1 value since OP wants to insert value to 2nd character so printing 1st character here. Then I a printing value here, then printing rest of the current Line.
}
' Input_file ##Mentioning Input_file name here.