Solution and benchmarking to http://stackoverflow.com/questions/14684539/sample-with-a-max/14684701#14684701

SO_sample_max

library(inline)

set.seed(123)
x <- sample(1:100,200, replace = TRUE)

cpp_src <- '
Rcpp::IntegerVector xa = clone(x); // Vector to be sampled
Rcpp::IntegerVector na(n); // Number of elements in solution
Rcpp::IntegerVector sa(s); // Sum of solution

int nsampled;
int currentSum;
int dropRandomIndex;
int numZeroes;
Rcpp::IntegerVector remainingQuantity(1);
int maxAttempts = 100;

// Create container for our results
Rcpp::IntegerVector res(maxAttempts);
std::fill( res.begin(), res.end(), NA_INTEGER );

// Calculate min/max so that we can draw random integers from within range
Rcpp::IntegerVector::iterator mn = std::min_element(xa.begin(), xa.end()) ;
Rcpp::IntegerVector::iterator mx = std::max_element(xa.begin(), xa.end()) ;
std::cout << "mx = " << *mx << std::endl;

// Now draw repeatedly
nsampled = 0;
for( int i = 0; i < maxAttempts; i++ ) {
  std::cout << "\\n" << i;
  int r = *mn + (rand() % (int)(*mx - *mn + 1));
  res[i] = xa[r+1];
  // Calculate n and s for current loop iteration
  numZeroes = 0;
  for( int j = 0; j < maxAttempts; j++) 
    if(res[j]==0) numZeroes++;
  std::cout << " nz= " << numZeroes ;
  nsampled = maxAttempts - sum( is_na(res) ) - numZeroes - 1;
  currentSum = std::accumulate(res.begin(),res.begin()+i,0); // Cant just use Rcpp sugar sum() here because it freaks at the NAs
  std::cout << " nsamp= " << nsampled << " sum= " << currentSum;
  if(nsampled == na[0]-1) {  
    std::cout << " One element away. ";
    remainingQuantity[0] = sa[0] - currentSum;
    std::cout << "remainingQuantity = " << remainingQuantity[0];
    if( (remainingQuantity[0] > 0) && (remainingQuantity[0]) < *mx ) {
      std::cout << "Within range.  Prepare the secret (cheating) weapon!\\n";
      std::cout << sa[0] << " ";
      std::cout << currentSum << " ";
      std::cout << remainingQuantity[0] << std::endl;
      if( i != maxAttempts ) {
        std::cout << "Safe to add one last element on the end.  Doing so.\\n";
        res[i] = remainingQuantity[0];
      }
      currentSum = sa[0];
      nsampled++;
      if(nsampled == na[0] && currentSum == sa[0]) std::cout << "It should end after this...nsamp= " << nsampled << " and currentSum= " << currentSum << std::endl;
      break;
    } else {
      std::cout << "Out of striking distance.  Dropping random element\\n";
      dropRandomIndex = 0 + (rand() % (int)(i - 0 + 1));
      res[dropRandomIndex] = 0;
    }
  }
  if(nsampled == na[0] && currentSum == sa[0]) {
      std::cout << "Success!\\n";
      for(int l = 0; l <= i+1; l++) 
        std::cout << res[l] << " " ;
      break;
  }
  if(nsampled == na[0] && currentSum != sa[0]) {
    std::cout << "Reached number of elements but sum is ";
    if(currentSum > sa[0]) {
      std::cout << "Too high. Blitz everything and start over!\\n";
      for(int k = 0; k < res.size(); k++) {
        res[k] = NA_INTEGER;
      }
    } else {
      std::cout << "Too low.  \\n";
      
    }
  }
  if( nsampled < na[0] && currentSum >= sa[0] ) {
    std::cout << "Too few elements but at or above the sum cutoff.  Dropping a random element and trying again.\\n";
    dropRandomIndex = 0 + (rand() % (int)(i - 0 + 1));
    res[dropRandomIndex] = 0;
  }
}
return res;
'

sumto <- cxxfunction( signature(x="integer", n="integer", s="integer"), body=cpp_src, plugin="Rcpp", verbose=TRUE )

testresult <- sumto(x=x, n=20L, s=1000L)
testresult <- testresult[!is.na(testresult)]
testresult <- testresult[testresult!=0]
testresult
cumsum(testresult)
length(testresult)

##### Test Other functions ######

# ABF's R function

n <- 20L
target <- 1000L
vec <- seq(100)
set.seed(123)

# R repeat loop
sumto_repeat <- function(vec,n,target) {
  res <- integer()
  repeat {
    cat("begin:",sum(res),length(res),"\n")
    res <- c( res, sample(vec,1) )
    if( sum(res)<target & length(res)==(n-1) ) {
      res[length(res)+1] <- target - sum(res)
    }
    # cat("mid:",sum(res),length(res),"\n")
    if(sum(res)>target) res <- res[-length(res)]
    if( length(res)>n | length(res)<n & sum(res)==target ) {
      res <- res[-sample(seq(length(res)),1)]
    }
    # cat("end:",sum(res),length(res),"\n")
    # cat(dput(res),"\n")
    if( sum(res)==target & length(res)==n ) break
  }
  res
}

# Ananda's R function

SampleToSum <- function(Target = 100, VecLen = 10, 
                        InRange = 1:100, Tolerance = 2, 
                        showSum = TRUE) {
  Res <- vector()
  while ( TRUE ) {
    Res <- round(diff(c(0, sort(runif(VecLen - 1)), 1)) * Target)
    if ( all(Res > 0)  & 
           all(Res >= min(InRange)) &
           all(Res <= max(InRange)) &
           abs((sum(Res) - Target)) <= Tolerance ) { break }
  }
  if (isTRUE(showSum)) cat("Total = ", sum(Res), "\n")
  Res
}

# Greg Snow's elegant solution

# But doesn't seem to always work if the vector differs from the sample size:
#> sum(as.vector(table( c( seq(n), sample(vec, target, replace=TRUE) ) )))
#[1] 1020

gs <- function(vec,n,target) {
  as.vector(table( c( seq(n), sample(vec, target, replace=TRUE) ) ))
}

# Benchmark

m <- microbenchmark( 
  SampleToSum(Tolerance=0),
  sumto(vec, n, target),
  sumto_repeat(vec,n,target),
  gs(vec,n,target),
  times=10
)
m

library(ggplot2)
library(taRifx)
autoplot(m)

You spent too much time on this question ;)

I don't totally understand Greg Snow's solution. It just doesn't seem to work for me. The length is all wrong, the range in the results are very narrow---I think it answers a different question. Also, to get the correct output value, do remember that vec should be vec - n (which is why the sum is 20 off in your test).