How to change the limit of uploaded file size in php?

Edit the following configurations in the php.ini file, and then restart webserver (apache).

upload_max_filesize = 10M
post_max_size = 20M

You may find the php.ini file in /etc/php5/apache2 directory of ubuntu. If you forget the location of ini file, you may find it by running a simple php file as below.

<?php
   phpinfo();
?>

Note: you will probably require write permission to edit the php.ini file.

For more information, you may visit this page. 

Automatic mapping (or conversion) among different biological databases

biomaRt is an R package to map among different biological databases. Here is a simple code-snippet to convert human gene id to gene symbol.

library(biomaRt)
ensembl = useMart("ensembl",dataset="hsapiens_gene_ensembl")
symbols = getBM(attributes=c('entrezgene','hgnc_symbol'), filters='entrezgene', values=c("7157","5601"), mart=ensembl) 

Installation note:

• In Ubuntu, you may need to install 'libxml2-dev' and 'libcurl4-openssl-dev'.

String Reverse in R

String reversing is a common operation especially in Bioinformatics. However, this function is not provided in R, not even in the "stringr" package. Here you can find the code:

str_reverse <- function(x){
  return(sapply(lapply(strsplit(x, NULL), rev), paste, collapse=""))
}

s <- "abc"
s1 <- str_reverse(s)
print(s1)