How to find x and y of a point of a mathematical function in Rstudio ..?

[GiacomoLam]

hello everyone!
I have a function in R

A2 <- function(f) abs(1/(cos(2*pi*f*H*(1-1i*D)/Vs)))

I have plotted this function and found the maximum, but I only know how to find the maximum in y, I need of corresponding value in x, but I do not know how to do it.
I tried it in different ways, but i'm not a programmer, is an exercise for the university.
I built a vector in which the maxima were contained

N = seq(0, 100, by=0.001)
VettoreMassimo <- vector(mode = "numeric")
for(j in N)
{
  VettoreMassimo[j]=abs(1/(cos(2*pi*j*H*(1-1i*D)/Vs)))
}
m = max(VettoreMassimo)

and if I try to use the command which.max(VettoreMassimo) i find the index of the vector.
Can anybody help me?
Sarei molto grato
(I apologize for my bad english)

[jrlewi]

Hi @GiacomoLam try subsetting your vector N: N[which.max(VettoreMassimo)]. You may also want to check out the function optim. To see the documentation you can use help(optim).

[GiacomoLam]

thank you all!!
But I solved this way:

ESERCITAZIONE 1 - IGS 2018/2019

#Funzione di amplificazione A2
H = 10 # [m] Spessore strato
Vs = 200 # [m/s] Velocità onde S
D = 0.05 # [%] Smorzamento

Funzione di amplificazione A2

A2 <- function(f) abs(1/(cos(2pifH(1-1i*D)/Vs)))

Grafico

curve(A2, 0, 100, n = 100000, xlab="f [Hz]", ylab= "A2(f)", col="blue", main="Funzione di amplificazione A2", sub="Smorzamento D = 5 %", lwd="1")

Calcolo massimo

passoFrequenza = 0.001
FrequenzaMax = 100
N = FrequenzaMax/passoFrequenza
VettoreMassimo <- vector(mode = "numeric")
VetFreqA2 <- vector(mode = "numeric")
for(k in 1:N)
{
j=(k0.001)
VettoreMassimo[k]=abs(1/(cos(2pijH*(1-1i*D)/Vs)))
VetFreqA2[k]=j
}
m = max(VettoreMassimo)
#Calcolo frequenza fondamentale
ind = which.max(VettoreMassimo)
f0 = VetFreqA2[ind]
#Stampa risultati
print("Valore massimo di A2")
print(m)
print("Frequenza fondamentale f0")
print(f0)

I know it's not the most elegant way to do it, but it works!

[Yarnabrina]

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However, please note that if this is your actual code, you once again omitted the * in these two lines:

A2 <- function(f) abs(1/(cos(2 pi f H (1-1i*D)/Vs)))

and

VettoreMassimo[k]=abs(1/(cos(2 pi j H*(1-1i*D)/Vs)))

Also, let me give you a better way for this problem. You actually don't need the for loop here:

# Funzione di amplificazione A2
H <- 10 # [m] Spessore strato
Vs <- 200 # [m/s] Velocità onde S
D <- 0.05 # [%] Smorzamento

# Funzione di amplificazione A2
A2 <- function(f) abs(x = (1/(cos(2 * pi *f * H * (1 - 1i * D) / Vs))))

# Calcolo massimo
N <- seq(from = 0, to = 100, by = 0.001)
VettoreMassimo <- A2(f = N)
m <- max(VettoreMassimo)
f0 <- N[which.max(x = VettoreMassimo)]

# Stampa risultati
cat("Valore massimo di A2:", m)
#> Valore massimo di A2: 12.73528
cat("Frequenza fondamentale f0:", f0)
#> Frequenza fondamentale f0: 4.987

^{Created on 2019-02-23 by the reprex package (v0.2.1)}

[GiacomoLam]

Thank you all for your answers and help!
Thanks I'll try your way, it seems more efficient.
In the code I assure you that the * sign is there, but I do not know why when I copy the code it is not reported too ..