Creating an (Intervals) Frequency Table Histogram

Question

I have the following frequency table of intervals:

Height (inches)	Class Mark	Frequency
0–10	5	5
10–20	15	18
20–30	25	12
30–40	35	6
40–50	45	9

For which I tried to create a histogram using the following R code:

# Example data frame for a continuous frequency table
df <- data.frame(
  ClassInterval = c("[0, 10)", "[10, 20)", "[20, 30)", "[30, 40)", "[40, 50)"),
  Frequency = c(5, 8, 12, 6, 9)
)

# Extract numeric values from class intervals
class_intervals <- gsub("\\[|\\)|\\(|\\]", "", df$ClassInterval)
class_midpoints <- sapply(strsplit(class_intervals, ","), function(x) mean(as.numeric(x)))

# Manually specify breaks based on unique values in ClassInterval
breaks <- as.numeric(gsub("\\[|\\)|\\(|\\]", "", unique(df$ClassInterval)))

# Create a histogram
hist(class_midpoints, breaks = breaks, weights = df$Frequency, col = "skyblue", xlab = "Value", main = "Continuous Frequency Histogram")

However, the command that creates breaks seems to generate only NAs, and consequently the hist gives me this error:

Error in hist.default(class_midpoints, breaks = breaks, weights = df$Frequency,  : 
  negative length vectors are not allowed
In addition: Warning messages:
1: In min(x) : no non-missing arguments to min; returning Inf
2: In max(x) : no non-missing arguments to max; returning -Inf
3: In min(h[h > 0]) : no non-missing arguments to min; returning Inf

Can someone please tell me what I'm doing wrong? And how to fix the problem? Thank you very much in advance.

Answer 1

Given your toy data, we can create a bar plot where we display the weighted frequencies against the point ranges ("Class Intervals"):

df <- data.frame(
  ClassInterval = c("[0, 10)", "[10, 20)", "[20, 30)", "[30, 40)", "[40, 50)"),
  Frequency = c(5, 8, 12, 6, 9)
)

with(df, barplot(height = Frequency / sum(Frequency), names.arg = ClassInterval, 
                 ylab = "Frequency", xlab = "Class Interval"))

^{Created on 2023-11-22 with reprex v2.0.2}