add index + if else statement

session_1/session_1.Rmd

@@ -742,6 +742,8 @@ of the modulo is equal to `0`.
 13 %% 2
 ```
 
+3. Using your `even_test` function, write a new function `even_print` which will print "This number is even" or "This number is odd". You will need the `if else` statement and the function `print`. Find help on how to use them.
+
 </div>
  
 <details><summary>Solution 1 </summary>
@@ -772,7 +774,7 @@ even_test(3)
 
 **Note :** A function can be written in several forms.
 
-```{r VandAstep11small, include=TRUE}
+```{r VandAstep11smal2, include=TRUE}
 even_test2 <- function(x){
   (x %% 2) == 0
 }
@@ -783,6 +785,38 @@ even_test(3)
 </details>
 
 
+
+<details><summary>Solution 3 </summary>
+<p>
+```{r VandAstep13, include=TRUE}
+even_print <- function(x){
+  if(even_test(x) == TRUE) {
+    print("This number is even")
+  } else {
+    print("This number is odd")
+  }
+}
+even_print(4)
+even_print(3)
+```
+
+**Note :** There is no need to test whether a boolean variable (TRUE/FALSE) is TRUE or FALSE.
+
+```{r VandAstep11small14, include=TRUE}
+even_print <- function(x){
+  if(even_test(x)) {
+    print("This number is even")
+  } else {
+    print("This number is odd")
+  }
+}
+even_print(4)
+even_print(3)
+```
+</p>
+</details>
+
+
 ## Cleaning up
 
 We can now clean your environment
@@ -872,7 +906,6 @@ To determine the type of the elements of a vector:
 typeof(x)
 ```
 
-
 ```{r Vecstep6, include=TRUE}
 typeof(x + 0.5)
 x + 0.5
@@ -898,6 +931,51 @@ y
 x == y
 ```
 
+## Accessing values
+
+There are multiple ways to access or replace values in vectors or other data structures. The most common approach is to use "indexing". 
+In the below, note that brackets `[ ]` are used for indexing, whereas you have already seen that parentheses `( )` are used to call a function and `{ }` to define function. It is very important not to mix these up.
+
+
+Here are some examples that show how elements of vectors can be obtained by indexing.
+
+
+You can use the position(s) of the value(s) in the vector
+
+```{r index1, include=TRUE}
+x <- c(1,5,7,8)
+x[4]
+x[c(1,3,4)]
+```
+You can use booleans to define which values should be kept.
+
+```{r index2, include=TRUE}
+x <- c(1,5,7,8,15)
+x[c(TRUE,FALSE,TRUE,FALSE,TRUE)]
+x[c(FALSE,TRUE)] # Bolean vector is reuse if it is not of the same size of the vector to index
+
+y <- c(TRUE,FALSE,FALSE,FALSE,TRUE)
+x[y]
+```
+
+You can use names in the case of a named vector.
+
+```{r index3, include=TRUE}
+x <-c(a = 1, b = 2, c = 3, d = 4, e = 5)
+x[c("a","c")]
+```
+
+You can also use an index to change values
+
+```{r index4, include=TRUE}
+x <- c(1,5,7,8,15)
+x[1] <- 3
+x
+
+x[x>5] <- 5
+x
+```
+
 <div class="pencadre">
   **Summary so far**