dataframe.Rmd

## Data frames

A data frame is a 2-dimensional structure.
<br> It is more general than a matrix.
<br><br>
All columns in a data frame:
 + can be of different **types** (numeric, character or logical)
 + must have the same **length**

### Create a data frame 

* With the **data.frame** function:

```{r}
# stringsAsFactors: ensures that characters are treated as characters and not as factors
d <- data.frame(c("Maria", "Juan", "Alba"), 
	c(23, 25, 31),
	c(TRUE, TRUE, FALSE),
	stringsAsFactors = FALSE)
```

* Example why "stringsAsFactors = FALSE" is useful

```{r}
# Create a data frame with default parameters
df <- data.frame(label=rep("test",5), column2=1:5)
# Replace one value
df[2,1] <- "yes"
# Throws an error and doesn't replace the value !
```

```{r}
# Create a data frame with 
df2 <- data.frame(label=rep("test",5), column2=1:5, stringsAsFactors = FALSE)
# Replace one value
df2[2,1] <- "yes"
# Works!
```

* Converting a matrix into a data frame:

```{r}
# create a matrix
b <- matrix(c(1, 0, 34, 44, 12, 4), 
        nrow=3,
        ncol=2)
# convert as data frame
b_df <- as.data.frame(b)
```

### Data frame manipulation:
<br>
Very similar to matrix manipulation.

<img src="images/df_fetch.png" width="450"/>