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"<a href=\"https://colab.research.google.com/github/coder9051/Tips-and-tricks/blob/main/Recursive_function.ipynb\" target=\"_parent\"><img src=\"https://colab.research.google.com/assets/colab-badge.svg\" alt=\"Open In Colab\"/></a>"
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]
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},
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{
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"cell_type": "markdown",
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"source": [
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" **NOTE ON RECURSION**\n",
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"Recursive function:\n",
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"\n",
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"A recursive function calls itself. This mean that a new function is called from within the original function.\n",
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"\n",
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"\n",
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"For example look at the code below and assume a linked list counting from 0 to n by 1.\n"
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],
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"metadata": {
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"id": "b6Q_RILdTh_v"
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}
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},
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{
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"cell_type": "code",
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"source": [
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"def print_list(head):\n",
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" if head is not None:\n",
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" print(head.data)\n",
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" print_list(head.next)"
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],
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"metadata": {
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"id": "k9CYsraPUWmn"
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},
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"execution_count": null,
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"outputs": []
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},
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{
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"cell_type": "markdown",
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"source": [
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"The first function looks at the head of the list and prints it. It then creates a new function to print the next element. However,\n",
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"first function hasn't ended yet. It can't end until all the functions within it end. This is key to understanding.\n",
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"\n",
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"If we look at the call stack (the list of running functions) it looks like this:"
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],
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"metadata": {
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"id": "ogRmJFUOUg0Q"
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}
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},
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{
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"cell_type": "code",
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"source": [
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"print_list() # the original outer most function, waiting for internal functions to end.\n",
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" print_list() # the new function called within the original func."
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],
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"metadata": {
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"id": "eFoPtNS2Umkl"
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},
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"execution_count": null,
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"outputs": []
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},
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{
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"cell_type": "markdown",
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"source": [
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"This just keeps on going. The new function calls another function. Now our stack looks like this:"
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],
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"metadata": {
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"id": "uqeJ2HR4UsYG"
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}
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},
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{
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"cell_type": "code",
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"source": [
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"print_list() # the original function\n",
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" print_list() # level 1 function.\n",
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"\tprint_list() # level 2 function."
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],
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"metadata": {
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"id": "UVJkNiz7Uw1h"
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},
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"execution_count": null,
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"outputs": []
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},
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{
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"cell_type": "markdown",
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"source": [
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"The call stack keeps getting bigger and bigger because none of the outer functions can terminate because they are waiting on inner\n",
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"functions."
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],
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"metadata": {
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"id": "Cbn1bcI-U6Tw"
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}
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},
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{
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"cell_type": "code",
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"source": [
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"print_list() # the original function\n",
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" print_list() # level 1 function.\n",
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"\tprint_list() # level 2 function.\n",
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" ...\n",
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" ...\n",
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" ...\n",
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" print_list() # level 1000 - Recursion depth hit. Program ends and error displayed."
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],
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"metadata": {
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"id": "9Na1zr-sU-uE"
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},
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"execution_count": null,
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"outputs": []
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},
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{
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"cell_type": "markdown",
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"source": [
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"The call stack keeps getting bigger and bigger because none of the outer functions can terminate because they are waiting on inner\n",
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"functions.\n",
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"\n",
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"Recursion depth is a count of how many layers are active inside your recursive function. The default recursion depth limit for \n",
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"Python 3 is 1000. The limit exists to prevent a stack overflow caused by too much (possibly infinite) recursion.\n",
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"\n",
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"Recursion is a cool tool and you should use it (or at least understand it). However, some languages have optimized tail call \n",
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"recursion (what is the kind of recursion being shown above) and avoid this problem by ending the outer function when the inner one is called. Python has not.\n",
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"\n",
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"If you are writing a function that may hit the recursion limit it may be better to approach it iteratively (at least in Python)."
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