# Control Structures

R has the standard control structures you would expect. **expr** can be multiple (compound) statements by enclosing them in braces { }. It is more efficient to use built-in functions rather than control structures whenever possible.

## if-else

```
if (cond) expr
if (cond) expr1 else expr2
```

## for

```
for (var in seq) expr
```

## while

```
while (cond) expr
```

## switch

```
switch(expr, ...)
```

## ifelse

```
ifelse(test,yes,no)
```

## Example

```
# transpose of a matrix
# a poor alternative to built-in t() function
mytrans <- function(x) {
if (!is.matrix(x)) {
warning("argument is not a matrix: returning NA")
return(NA_real_)
}
y <- matrix(1, nrow=ncol(x), ncol=nrow(x))
for (i in 1:nrow(x)) {
for (j in 1:ncol(x)) {
y[j,i] <- x[i,j]
}
}
return(y)
}
# try it
z <- matrix(1:10, nrow=5, ncol=2)
tz <- mytrans(z)
```

## Going Further

To practice working with control structures in R, try the chapter on conditionals and control flow of this interactive R course.