Week 4: Conditionals & Testing

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# Week 4: .fancy[Conditionals & Testing]

### EMSE 4574: Intro to Programming for Analytics
### John Paul Helveston
### September 22, 2020
]

---

# Week 4: .fancy[Conditionals & Testing]

## 1. Conditionals
## 2. Testing
## 3. Tips

---

# Week 4: .fancy[Conditionals & Testing]

## 1. .orange[Conditionals]
## 2. Testing
## 3. Tips

---
# "Flow Control"

### Code that alters the otherwise linear flow of operations in a program.

--
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### This week:

- `if` statements
- `else` statements
]
--
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### Next week:

- `for` loops
- `while` loops
- `break` statements
- `next` statements
]

---
# The `if` statement

--
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### Basic format

```r
if ( CONDITION ) {
    # Do stuff here
}
```
]]
--
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### Flow chart:
<img src="images/condition_if.png" width="400">
]

---
# Quick code tracing

```r
f <- function(x) {
    cat("A")
    if (x == 0) {
        cat("B")
        cat("C")
    }
    cat("D")
}
```
]]
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What will this print?

```r
f(1)
f(0)
```
]]

---
class: inverse

# Quick practice

### Write the function `absValue(n)` that returns the absolute value of a number (and no cheating - you can't use the built-in `abs()` function!)

Tests:
- `absValue(7) == 7`
- `absValue(-7) == 7`
- `absValue(0) == 0`

---
# Adding an `else` to an `if`

```r
if ( CONDITION ) {
  # Do stuff here
} else {
  # Do other stuff here
}
```
]]
--
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### Flow chart:
<img src="images/condition_if_else.png" width="500">
]

---
# Quick code tracing

```r
f <- function(x) {
    cat("A")
    if (x == 0) {
        cat("B")
        cat("C")
    } else {
        cat("D")
        if (x == 1) {
            cat("E")
        } else {
            cat("F")
        }
    }
    cat("G")
}
```
]]
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What will this print?

```r
f(0)
f(1)
f(2)
```
]]

---
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# `else if` chains
Example - "bracketing" problems:

```r
getLetterGrade <- function(score) {
    if (score >= 90) {
        grade = "A"
    } else if (score >= 80) {
        grade = "B"
    } else if (score >= 70) {
        grade = "C"
    } else if (score >= 60) {
        grade = "D"
    } else {
        grade = "F"
    }
    return(grade)
}
```
]
--
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Check function output:

```r
getLetterGrade(99)
```

```
## [1] "A"
```

```r
getLetterGrade(88)
```

```
## [1] "B"
```

```r
getLetterGrade(70)
```

```
## [1] "C"
```

```r
getLetterGrade(61)
```

```
## [1] "D"
```

```r
getLetterGrade(22)
```

```
## [1] "F"
```
]

---
class: inverse

# Think-Pair-Share

Write the function `getType(x)` that returns the type of the data (either `integer`, `double`, `character`, or `logical`). Basically, it does the same thing as the `typeof()` function (but you can't use `typeof()` in your solution).

Tests:
- `getType(3) == "double"`
- `getType(3L) == "integer"`
- `getType("foo") == "character"`
- `getType(TRUE) == "logical"`

---

# Week 4: .fancy[Conditionals & Testing]

## 1. Conditionals
## 2. .orange[Testing]
## 3. Tips

---
# Why write test functions?
<br>

--
## 1. They help you understand the problem

--
## 2. They verify that a function is working as expected

---
# Test function "syntax"

--
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### Function:

```r
functionName <- function(arguments) {
    # Do stuff here
    return(something)
}
```
]]
--
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### Test function:

```r
test_functionName <- function() {
    cat("Testing functionName()...")
    # Put test cases here
    cat("Passed!\n")
}
```
]]

---
# Writing test cases with `stopifnot()`

`stopifnot()` stops the function if whatever is inside the `()` is not `TRUE`.

--
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### Function:

```r
isEven <- function(n) {
    return((n %% 2) == 0)
}
```
- `isEven(1)` should be `FALSE`
- `isEven(2)` should be `TRUE`
- `isEven(-7)` should be `FALSE`
]]
--
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### Test function:

```r
test_isEven <- function() {
    cat("Testing isEven()...")
    stopifnot(isEven(1) == FALSE)
    stopifnot(isEven(2) == TRUE)
    stopifnot(isEven(-7) == FALSE)
    cat("Passed!\n")
}
```
]]

---
# Writing test cases with `stopifnot()`

`stopifnot()` stops the function if whatever is inside the `()` is not `TRUE`.

```r
isEven <- function(n) {
    return((n %% 2) == 0)
}
```
- `isEven(1)` should be `FALSE`
- `isEven(2)` should be `TRUE`
- `isEven(-7)` should be `FALSE`
]]
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### Test function:

```r
test_isEven <- function() {
    cat("Testing isEven()...")
    stopifnot(isEven(1) == FALSE)
    stopifnot(isEven(2) == TRUE)
    stopifnot(isEven(-7) == FALSE)
    cat("Passed!\n")
}
```

```r
test_isEven()
```

```
## Testing isEven()...Passed!
```
]]

---
# Write the test function _first_!

```r
test_isEven <- function() {
    cat("Testing isEven()...")
    stopifnot(isEven(1) == FALSE)
    stopifnot(isEven(2) == TRUE)
    stopifnot(isEven(-7) == FALSE)
    cat("Passed!\n")
}
```
]]
--
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### Step 2: Write the function

```r
isEven <- function(n) {
    return((n %% 2) == 0)
}
```
### Step 3: Test the function

```r
test_isEven()
```

```
## Testing isEven()...Passed!
```
]]