Update NB A.2 with f-strings; update .gitignore

Author: jsdodge

.gitignore

@@ -22,4 +22,5 @@ download
 import-file-test.ipynb
 **/Untitled.ipynb
 student_feedback
-**/www.itl.nist.gov/
+**/www.itl.nist.gov/
+mindevfit.eps

notebooks/A.2-Minimum-Deviation-Fit.ipynb

@@ -416,7 +416,7 @@
    },
    "outputs": [],
    "source": [
-    "print(\"Refractive index estimate:    {0:.5f} ± {1:.5f}\".format(nOpt, nAlpha))"
+    "print(f\"Refractive index estimate:    {nOpt:.5f} ± {nAlpha:.5f}\")"
    ]
   },
   {
@@ -427,7 +427,7 @@
     }
    },
    "source": [
-    "The expressions in curly brackets `{}` are known as a *format specifiers*, which format the arguments to the `format` method appended to the end of the string. The `.format()` at the end tells the interpreter that formatting information is contained in the string; in each format specifier, the number the precedes the colon `:` is the position index of the argument to be formatted. The expression that follows the colon then tells the interpreter how to format the variable. For example, the specifier `.5f` will represent the argument in floating-point notation to five decimal places. Examples of the most common numerical format strings are below."
+    "The `f` that precedes the expression in quotation marks indicates that this is a formatted string literal, or [f-string](https://docs.python.org/3/reference/lexical_analysis.html#f-strings). F-strings have the format `f\"Text {variable:format}\"`, where the curly brackets `{}` include instructions for printing the value of a variable with a specified format. For example, the specifier `.5f` will represent the argument in floating-point notation to five decimal places. Examples of the most common numerical format strings are below."
    ]
   },
   {
@@ -440,10 +440,14 @@
    },
    "outputs": [],
    "source": [
-    "print(\"Field width=10, precision=5, floating point format: {0:.5f} ± {1:.5f}\".format(nOpt, nAlpha))\n",
-    "print(\"Field width=10, precision=5, scientific notation format: {0:.5e} ± {1:.5e}\".format(nOpt, nAlpha))\n",
-    "print(\"Field width=10, precision=5, the more compact of e or f format: {0:.5g} ± {1:.5g}\".format(nOpt, nAlpha))\n",
-    "print(\"Field width=10, signed integer format: {0:d} ± {1:d}\".format(nOpt.astype(int), nAlpha.astype(int)))"
+    "print(\"Field width=10, precision=5, floating point format: \"\n",
+    "      f\"{nOpt:.5f} ± {nAlpha:.5f}\")\n",
+    "print(\"Field width=10, precision=5, scientific notation format: \"\n",
+    "      f\"{nOpt:.5e} ± {nAlpha:.5e}\")\n",
+    "print(\"Field width=10, precision=5, the more compact of e or f format: \"\n",
+    "      f\"{nOpt:.5g} ± {nAlpha:.5g}\")\n",
+    "print(\"Field width=10, signed integer format: \"\n",
+    "      f\"{nOpt.astype(int):d} ± {nAlpha.astype(int):d}\")"
    ]
   },
   {
@@ -505,7 +509,7 @@
     "res = delta_deg - model(theta_deg,nOpt,alpha_deg_const)\n",
     "normres = res/err_deg\n",
     "chisq = np.sum(normres**2)\n",
-    "print(\"chi-squared = {0:.1f}\".format(chisq))"
+    "print(f\"chi-squared = {chisq:.1f}\")"
    ]
   },
   {
@@ -533,7 +537,7 @@
     "Ndata = np.size(delta_deg)\n",
     "Npar = np.size(nOpt)\n",
     "dof = Ndata - Npar\n",
-    "print(\"dof = {0:d}\".format(dof))"
+    "print(f\"dof = {dof:d}\")"
    ]
   },
   {
@@ -558,8 +562,8 @@
    "outputs": [],
    "source": [
     "cdf = chi2.cdf(chisq,dof)\n",
-    "print(\"Cumulative probability = {0:.3f}\".format(cdf))\n",
-    "print(\"Significance: {0:.3f}\".format(1 - cdf))"
+    "print(f\"Cumulative probability = {cdf:.3f}\")\n",
+    "print(f\"Significance: {1 - cdf:.3f}\")"
    ]
   },
   {
@@ -690,7 +694,7 @@
     }
    },
    "source": [
-    "You can customize the layout with [`GridSpec`](https://matplotlib.org/users/gridspec.html) by using the the `subplot2grid` helper function."
+    "You can customize the layout with [`GridSpec`](https://matplotlib.org/users/gridspec.html) by using the `subplot2grid` helper function."
    ]
   },
   {