add unique id for exercise

Author: rlanzafame

book/flow/Exercises/01.ipynb

@@ -57,6 +57,7 @@
    },
    "source": [
     "## Exercise\n",
+    "% was exercise 2.2.1\n",
     "\n",
     "One of the most crucial applications of <code>if</code> statements is filtering the data from errors and checking whether an error is within a certain limit.\n",
     "\n",
@@ -167,6 +168,7 @@
    },
    "source": [
     "## (Searching) Exercise\n",
+    "% was exercise 2.2.4\n",
     "\n",
     "Conditional expression is a way how one can compress an <code>if</code> statement to a more compact (and logical) statement. Your task is to rewrite the below <code>if</code> statement by using the <i>'conditional expression'</i> technique."
    ]
@@ -290,6 +292,7 @@
    },
    "source": [
     "## Exercise\n",
+    "% was exercise 2.4.3\n",
     "\n",
     "Here you need to write a function that is able to sort any list consisting only of real numbers, in the descending order. For example, the list $[19, 5, 144, 6]$ becomes $[144, 19, 6, 5]$. However there are three possible options to complete correctly the code where the dots `...` are placed.\n",
     "\n",