# Homework July, 2014

Import two data.frames we’ll use in the problems.

```
ht_wt_df <- read.csv("http://www3.nd.edu/~steve/computing_with_data/Data/heights_weights_genders.csv")
wisc_bc_df <- read.csv("http://www3.nd.edu/~steve/computing_with_data/Data/wisc_bc_data.csv")
```

## Problems

Compute the mean of the heights separately for the males and the females in `ht_wt_df`

.

Form a sub-data.frame `sub_df1`

of `ht_wt_df`

that contains every third row of `ht_wt_df`

; that is, row 1, 3, 6, 9, …. What are the mean and variance of the people in `sub_df1`

? HINT: You will need the `seq`

function.

Use the `str`

function to identify the variables (columns) in the `wsic_bc_df`

that are numeric. (a) Form a new data.frame `num_bc_df`

that contains only the numeric variables. (b) Compute the correlation matrix of `num_bc_df`

. HINT: Use the `cor`

function. Look up the help on this function to learn the correct way to use it. (c) List the variables that have a correlation > 0.90 with `radius_mean`

.