# [R] searching for specific row in matrix

Esmail Bonakdarian esmail.js at gmail.com
Wed Jun 11 13:58:07 CEST 2008

```Hi,

I have matrix of bits and a target vector. Is there an
efficient way to search the rows of the matrix for the target?
I am interested in the first row index where target is found.

Example:

> source("lookup.R")
[,1] [,2] [,3] [,4] [,5]
[1,]    1    0    1    1    0
[2,]    1    1    0    1    0
[3,]    0    0    1    0    0
[4,]    1    0    0    1    1
[5,]    1    0    1    1    1
[6,]    1    1    0    0    1
[7,]    1    0    0    1    1
[8,]    0    0    1    1    1
[9,]    0    1    1    0    1
[10,]    0    0    0    1    0

target:  1 1 0 1 1

Should return -1 (or some other indicator) since the
target was not found in any of the rows.

> source("lookup.R")
[,1] [,2] [,3] [,4] [,5]
[1,]    0    0    1    1    0
[2,]    1    0    0    0    0
[3,]    1    0    0    0    0
[4,]    1    1    0    0    0
[5,]    1    1    1    0    0
[6,]    0    0    1    1    0
[7,]    0    1    1    1    0
[8,]    0    0    1    1    0
[9,]    1    1    0    1    1
[10,]    1    0    1    0    0

target:  1 1 0 1 1

Should return 9 since the target was  found in row 9

If the target is found, it is no longer necessary to keep
searching the rest of the matrix (which may be quite large)

The data/size etc may change of course, but target will
always have the same number of "columns" as the matrix.

I tried variations of "which", and a for loop
comparing pop[i,] to target without much success, nor
did google yield any results. I am hoping someone here
can provide a suggestion.

Thanks,

EB

---------------------------------------------

# Here is the code that generates the above data

create_bin_string <- function(len)
{
sample(0:1, len, replace=T)
}

ROWS = 10
COLS =  5
pop = matrix(create_bin_string(ROWS*COLS), ROWS, COLS, byrow=T)

target=c(1, 1, 0, 1, 1)

# my population
print(pop)

# I am looking for the index of this in pop
# if present (else -1?)
cat("\ntarget: ", target, "\n")

##
## this is NOT working
## plus it would continue to search
## after it found the target
##
for(i in ROWS)
if (pop[i,] == target)
cat("\nfound in row: ", i, "\n\n")

```