For a single peak, you can use the `which.max()`

function, which returns the index of the maximum value in a vector:

```
# Fake data
set.seed(3)
y = cumsum(rnorm(100))
x = 151:250
```

```
which.max(y)
```

`[1] 87`

```
x[which.max(y)]
```

`[1] 237`

```
plot(x,y, type="l")
abline(v=x[which.max(y)], col="red")
```

To find multiple peaks, R has many options, for example, `findpeaks`

from the `pracma`

package. `findpeaks(y)`

returns a matrix, the second column of which gives the indices of the peaks.

```
library(pracma)
plot(x,y, type="l")
abline(v=x[findpeaks(y)[,2]], col="red")
```

For more information on tools for spectral analysis in R, see, for example, the CRAN Task Views for Chemometrics and Time Series.