10-conditionals.ipynb

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    "# Conditionals"
   ]
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    "## Use `if` statements to control whether or not a block of code is executed.\n",
    "\n",
    "- An `if` statement (more properly called a _conditional_ statement) controls whether some block of code is executed or not.\n",
    "- Structure is similar to a `for` statement:\n",
    "  - First line opens with `if` and ends with a colon\n",
    "  - Body containing one or more statements is indented (usually by 4 spaces)"
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    "mass = 3.54\n",
    "if mass > 3.0:\n",
    "    print(mass, 'is large')\n",
    "\n",
    "mass = 2.07\n",
    "if mass > 3.0:\n",
    "    print (mass, 'is large')"
   ]
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   "source": [
    "## Conditionals are often used inside loops."
   ]
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    "- Not much point using a conditional when we know the value (as above).\n",
    "- But useful when we have a collection to process."
   ]
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    "masses = [3.54, 2.07, 9.22, 1.86, 1.71]\n",
    "for m in masses:\n",
    "    if m > 3.0:\n",
    "        print(m, 'is large')"
   ]
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   "source": [
    "## Use `else` to execute a block of code when an `if` condition is _not_ true."
   ]
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    "- `else` can be used following an `if`.\n",
    "- Allows us to specify an alternative to execute when the `if` _branch_ isn’t taken."
   ]
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    "masses = [3.54, 2.07, 9.22, 1.86, 1.71]\n",
    "for m in masses:\n",
    "    if m > 3.0:\n",
    "        print(m, 'is large')\n",
    "    else:\n",
    "        print(m, 'is small')"
   ]
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    "## Use `elif` to specify additional tests.\n",
    "\n",
    "- May want to provide several alternative choices, each with its own test.\n",
    "- Use `elif` (short for “else if”) and a condition to specify these.\n",
    "- Always associated with an `if`.\n",
    "- Must come before the `else` (which is the “catch all”)."
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    "masses = [3.54, 2.07, 9.22, 1.86, 1.71]\n",
    "for m in masses:\n",
    "    if m > 9.0:\n",
    "        print(m, 'is HUGE')\n",
    "    elif m > 3.0:\n",
    "        print(m, 'is large')\n",
    "    else:\n",
    "        print(m, 'is small')"
   ]
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   "source": [
    "## Conditions are tested once, in order.\n",
    "\n",
    "- Python steps through the branches of the conditional in order, testing each in turn.\n",
    "- So ordering matters."
   ]
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    "grade = 85\n",
    "if grade >= 70:\n",
    "    print('grade is C')\n",
    "elif grade >= 80:\n",
    "    print('grade is B')\n",
    "elif grade >= 90:\n",
    "    print('grade is A')"
   ]
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    "- Does _not_ automatically go back and re-evaluate if values change."
   ]
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    "velocity = 10.0\n",
    "if velocity > 20.0:\n",
    "    print('moving too fast')\n",
    "else:\n",
    "    print('adjusting velocity')\n",
    "    velocity = 50.0"
   ]
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   "source": [
    "- Often use conditionals in a loop to “evolve” the values of variables."
   ]
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    "velocity = 10.0\n",
    "for i in range(5): # execute the loop 5 times\n",
    "    print(i, ':', velocity)\n",
    "    if velocity > 20.0:\n",
    "        print('moving too fast')\n",
    "        velocity = velocity - 5.0\n",
    "    else:\n",
    "        print('moving too slow')\n",
    "        velocity = velocity + 10.0\n",
    "print('final velocity:', velocity)"
   ]
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    "## Key Points\n",
    "\n",
    "- **Use `if` statements to control whether or not a block of code is executed.**\n",
    "- **Conditionals are often used inside loops.**\n",
    "- **Use `else` to execute a block of code when an `if` condition is _not_ true.**\n",
    "- **Use `elif` to specify additional tests.**\n",
    "- **Conditions are tested once, in order.**\n",
    "- **Create a table showing variables’ values to trace a program’s execution.**"
   ]
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