Fixing Function Calculating Wrong Answers in R Programming Language

Understanding the Issue with Function Calculating Wrong Answers

Introduction

In this article, we’ll delve into a common issue faced by many users of R programming language - specifically, the problem of incorrect function results when processing vector inputs versus standalone user inputs. We’ll explore the root cause of this issue and provide several solutions to resolve it.

The Function Overview

The provided function analyzeGPS_DirectionChange calculates directional changes between consecutive bearings. These bearings are relative to the North-South line, making them either positive (0 - 180) or negative (-0 - 180). We’ve accounted for signs (positive or negative) and magnitudes (> 90 or < 90) using basic trigonometry to create “rules” that determine directional changes.

The Function

analyzeGPS_DirectionChange <- function(bearingVec1, bearingVec2) {
  # Determine the signs of bearings
  a1 <- ifelse(bearingVec1 >= 0.00, "positive", "negative")
  a2 <- ifelse(bearingVec2 >= 0.00, "positive", "negative")
  Bearing_Signs <- paste(a1, a2)

  # Determine the values of bearings
  b1 <- ifelse(abs(bearingVec1) < 90.00, "less", "greater")
  b2 <- ifelse(abs(bearingVec2) < 90.00, "less", "greater")
  Bearing_Values <- paste(b1, b2)

  # Print intermediate results
  print(Bearing_Signs)
  print(Bearing_Values)

  # Calculate directional change based on signs and values
  if (Bearing_Signs == "positive positive" & Bearing_Values == "less less") {
    Direction_Change <- (90 - bearingVec1) + (90 + bearingVec2)
  } else if (Bearing_Signs == "positive positive" & Bearing_Values == "less greater") {
    Direction_Change <- (180 - bearingVec2) + bearingVec1
  } else if (Bearing_Signs == "positive positive" & Bearing_Values == "greater less") {
    Direction_Change <- 180 - ((bearingVec1 - 90) + (90 - bearingVec2))
  } else if (Bearing_Signs == "positive positive" & Bearing_Values == "greater greater") {
    Direction_Change <- (bearingVec1 - 90) + (180 - bearingVec2) + 90
  }

  # Handle other cases
  if (Bearing_Signs == "positive negative" & Bearing_Values == "less less") {
    Direction_Change <- 180 - (abs(bearingVec2) + bearingVec1)
  } else if (Bearing_Signs == "positive negative" & Bearing_Values == "less greater") {
    Direction_Change <- 180 - (90 + (90 - bearingVec1) + (180 - abs(bearingVec2)))
  } else if (Bearing_Signs == "positive negative" & Bearing_Values == "greater less") {
    Direction_Change <- 90 - ((bearingVec1 - 90) + abs(bearingVec2))
  } else if (Bearing_Signs == "positive negative" & Bearing_Values == "greater greater") {
    Direction_Change <- 180 - ((180 - bearingVec1) + (180 - abs(bearingVec2)))
  }

  # Handle other cases
  if (Bearing_Signs == "negative positive" & Bearing_Values == "less less") {
    Direction_Change <- 180 - (abs(bearingVec1) + bearingVec2)
  } else if (Bearing_Signs == "negative positive" & Bearing_Values == "less greater") {
    Direction_Change <- 180 - (90 + (90 - abs(bearingVec1)) + (180 - bearingVec2))
  } else if (Bearing_Signs == "negative positive" & Bearing_Values == "greater less") {
    Direction_Change <- 90 - ((abs(bearingVec1) - 90) + bearingVec2)
  } else if (Bearing_Signs == "negative positive" & Bearing_Values == "greater greater") {
    Direction_Change <- 180 - ((180 - abs(bearingVec1)) + (180 - bearingVec2))
  }

  # Handle other cases
  if (Bearing_Signs == "negative negative" & Bearing_Values == "less less") {
    Direction_Change <- (90 - abs(bearingVec2)) + (90 + abs(bearingVec1))
  } else if (Bearing_Signs == "negative negative" & Bearing_Values == "less greater") {
    Direction_Change <- (180 - abs(bearingVec2)) + abs(bearingVec1)
  } else if (Bearing_Signs == "negative negative" & Bearing_Values == "greater less") {
    Direction_Change <- 180 - (abs(bearingVec1) - 90) + abs(bearingVec2)
  } else if (Bearing_Signs == "negative negative" & Bearing_Values == "greater greater") {
    Direction_Change <- (abs(bearingVec1) - 90) + 90 + (180 - abs(bearingVec2))
  }

  return(Direction_Change)
}

The Issue

The problem arises from the use of if statements in combination with logical AND (&). As explained in the R help documentation, & and && indicate logical AND, while | and || indicate logical OR. However, the shorter form performs element-wise comparisons in a similar way to arithmetic operators, whereas the longer form evaluates left to right examining only the first element of each vector.

In this case, the issue is that the function falls back to the first element when using the shorter form (&). To fix this, we need to use the longer form of logical AND (&&) to ensure that both conditions are evaluated correctly.

Solutions

Using Logical AND with Element-wise Comparisons

One solution is to replace all if clauses with ifelse constructs, which are vectorized and can handle multiple elements. However, this approach may become very complex given the nested ifs.

analyzeGPS_DirectionChange <- function(bearingVec1, bearingVec2) {
  # Determine the signs of bearings
  a1 <- ifelse(bearingVec1 >= 0.00 & bearingVec2 >= 0.00, "positive", 
               ifelse(bearingVec1 < 0.00 | bearingVec2 < 0.00, "negative", "unknown"))
  a2 <- ifelse(bearingVec1 >= 0.00 & bearingVec2 >= 0.00, "positive",
               ifelse(bearingVec1 < 0.00 | bearingVec2 < 0.00, "negative", "unknown"))

  # Determine the values of bearings
  b1 <- ifelse(abs(bearingVec1) < 90.00 & abs(bearingVec2) < 90.00, "less",
               ifelse(abs(bearingVec1) >= 90.00 | abs(bearingVec2) >= 90.00, "greater", "unknown"))
  b2 <- ifelse(abs(bearingVec1) < 90.00 & abs(bearingVec2) < 90.00, "less",
               ifelse(abs(bearingVec1) >= 90.00 | abs(bearingVec2) >= 90.00, "greater", "unknown"))

  # ... rest of the function remains the same ...
}

Using `map2_dbl` from `purrr`

Another solution is to use purrr::map2_dbl, which applies a function element-wise to two vectors.

library(purrr)

analyzeGPS_DirectionChange <- function(bearingVec1, bearingVec2) {
  map2_dbl(bearingVec1, bearingVec2, ~ analyzeGPS_DirectionChange(.x, .y))
}

Using `Vectorize`

A third solution is to use Vectorize, which vectorizes the entire function.

analyzeGPS_DirectionChange <- Vectorize(function(x, y) {
  # ... rest of the function remains the same ...
})

Conclusion

In conclusion, the issue with incorrect function results when processing vector inputs versus standalone user inputs arises from the use of if statements in combination with logical AND (&). By using Vectorize, purrr::map2_dbl, or rewriting the if clauses to use element-wise comparisons with logical OR (|) and ||, we can resolve this issue and ensure accurate results.

Last modified on 2024-08-03