R help - Apr 2012 - Performing repeated T tests in R

I need to perform 10000 T tests

#I have two populations with different means
Popc1<-rnorm(100000,10,2)
Popc2<-rnorm(100000,8,2)

#I created two sets of samples - each set has 10000 samples, and I made a
matrix of 20 rows and 10000 columns to fit the data

sampc1<-matrix(,20,10000)
for(j in 1:10000){sampc1[1:20,j]<-sample(Popc1,20)}
sampc2<-matrix(,20,10000)
for(j in 1:10000){sampc2[1:20,j]<-sample(Popc2,20)}

#I wrote a blank matrix with 1 row and 10000 columns. I am planning to fill
this matrix with the t values that i calculate for each pair of samples
t<-matrix(,1,10000)

#What i need to do is to calculate the t value for each pair of samples: So
if i were to do it manually, for the first sample in each set, I would do
something like:

t.test(sampc1[,1],sampc2[,1])
#And i would continue doing this all the way until the 10000th pair of
samples
so t.test(sampc1[,2],sampc2[,2])
    t.test(sampc1[,3],sampc2[,3]) and so on

#But i need to do this for all 10000 pairs of samples:
#I tried to write a loop

for(j in 1:10000) {t[,1:10000]<-t.test(sampc1[, j],sampc2[, j])$statistic}

#but this fills each column in the matrix with the same exact t value.  

#Can someone tell me how to write the code to perform each pair of t tests?
I think i am missing something in my loop

Thanks 

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Hello,
>
> #I wrote a blank matrix with 1 row and 10000 columns. 
>
Why not a vector? (It would be a column vector, which might not fit your
needs.)
And though not really a problem, the matrix is called 't', the name of
R's
transpose function.
I can become confusing, maybe you could call it 'tstat' or something
like
that.
>
> #But i need to do this for all 10000 pairs of samples:
> #I tried to write a loop
> 
> for(j in 1:10000) {t[,1:10000]<-t.test(sampc1[, j],sampc2[,
j])$statistic}
> 
> #but this fills each column in the matrix with the same exact t value.  
> 
And that value is the test statistic of the last test.
This is because the index of the results matrix/row vector should be

for(j in 1:10000) {t[, j]<-t.test(sampc1[, j],sampc2[, j])$statistic}

Or, since it has only one row,  t[1, j]

Or, with the suggestions above,

tstat <- double(10000)
for(j in 1:10000) {tstat[j]<-t.test(sampc1[, j],sampc2[, j])$statistic}


Hope this helps

Rui Barradas


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