Updated question 37

100_Numpy_exercises_with_hints_with_solutions.md

@@ -193,10 +193,10 @@ print(Z)
 `hint: np.dtype`
 
 ```python
-color = np.dtype([("r", np.ubyte, 1),
-                  ("g", np.ubyte, 1),
-                  ("b", np.ubyte, 1),
-                  ("a", np.ubyte, 1)])
+color = np.dtype([("r", np.ubyte),
+                  ("g", np.ubyte),
+                  ("b", np.ubyte),
+                  ("a", np.ubyte)])
 ```
 #### 24. Multiply a 5x3 matrix by a 3x2 matrix (real matrix product) (★☆☆)
 `hint:`
@@ -333,7 +333,6 @@ print(Z)
 ```python
 A = np.ones(3)*1
 B = np.ones(3)*2
-C = np.ones(3)*3
 np.add(A,B,out=B)
 np.divide(A,2,out=A)
 np.negative(A,out=A)
@@ -355,8 +354,7 @@ print(np.trunc(Z))
 `hint: np.arange`
 
 ```python
-Z = np.zeros((5,5))
-Z += np.arange(5)
+Z = np.tile(np.arange(0, 5), (5,1))
 print(Z)
 ```
 #### 38. Consider a generator function that generates 10 integers and use it to build an array (★☆☆)
@@ -472,7 +470,7 @@ for dtype in [np.float32, np.float64]:
 
 ```python
 np.set_printoptions(threshold=float("inf"))
-Z = np.zeros((16,16))
+Z = np.zeros((40,40))
 print(Z)
 ```
 #### 50. How to find the closest value (to a given scalar) in a vector? (★★☆)
@@ -681,10 +679,24 @@ print(F)
 # Author: Fisher Wang
 
 w, h = 256, 256
-I = np.random.randint(0, 4, (w, h, 3)).astype(np.ubyte)
+I = np.random.randint(0, 4, (h, w, 3)).astype(np.ubyte)
 colors = np.unique(I.reshape(-1, 3), axis=0)
 n = len(colors)
 print(n)
+
+# Faster version
+# Author: Mark Setchell
+# https://stackoverflow.com/a/59671950/2836621
+
+w, h = 256, 256
+I = np.random.randint(0,4,(h,w,3), dtype=np.uint8)
+
+# View each pixel as a single 24-bit integer, rather than three 8-bit bytes
+I24 = np.dot(I.astype(np.uint32),[1,256,65536])
+
+# Count unique colours
+n = len(np.unique(I24))
+print(n)
 ```
 #### 67. Considering a four dimensions array, how to get sum over the last two axis at once? (★★★)
 `hint: sum(axis=(-2,-1))`
@@ -778,7 +790,7 @@ G = F.view( dtype=[('p0',F.dtype),('p1',F.dtype)] )
 G = np.unique(G)
 print(G)
 ```
-#### 74. Given an array C that is a bincount, how to produce an array A such that np.bincount(A) == C? (★★★)
+#### 74. Given a sorted array C that corresponds to a bincount, how to produce an array A such that np.bincount(A) == C? (★★★)
 `hint: np.repeat`
 
 ```python

100_Numpy_exercises_with_solutions.md

@@ -193,10 +193,10 @@ print(Z)
 
 
 ```python
-color = np.dtype([("r", np.ubyte, 1),
-                  ("g", np.ubyte, 1),
-                  ("b", np.ubyte, 1),
-                  ("a", np.ubyte, 1)])
+color = np.dtype([("r", np.ubyte),
+                  ("g", np.ubyte),
+                  ("b", np.ubyte),
+                  ("a", np.ubyte)])
 ```
 #### 24. Multiply a 5x3 matrix by a 3x2 matrix (real matrix product) (★☆☆)
 
@@ -333,7 +333,6 @@ print(Z)
 ```python
 A = np.ones(3)*1
 B = np.ones(3)*2
-C = np.ones(3)*3
 np.add(A,B,out=B)
 np.divide(A,2,out=A)
 np.negative(A,out=A)
@@ -355,8 +354,7 @@ print(np.trunc(Z))
 
 
 ```python
-Z = np.zeros((5,5))
-Z += np.arange(5)
+Z = np.tile(np.arange(0, 5), (5,1))
 print(Z)
 ```
 #### 38. Consider a generator function that generates 10 integers and use it to build an array (★☆☆)
@@ -472,7 +470,7 @@ for dtype in [np.float32, np.float64]:
 
 ```python
 np.set_printoptions(threshold=float("inf"))
-Z = np.zeros((16,16))
+Z = np.zeros((40,40))
 print(Z)
 ```
 #### 50. How to find the closest value (to a given scalar) in a vector? (★★☆)
@@ -681,10 +679,24 @@ print(F)
 # Author: Fisher Wang
 
 w, h = 256, 256
-I = np.random.randint(0, 4, (w, h, 3)).astype(np.ubyte)
+I = np.random.randint(0, 4, (h, w, 3)).astype(np.ubyte)
 colors = np.unique(I.reshape(-1, 3), axis=0)
 n = len(colors)
 print(n)
+
+# Faster version
+# Author: Mark Setchell
+# https://stackoverflow.com/a/59671950/2836621
+
+w, h = 256, 256
+I = np.random.randint(0,4,(h,w,3), dtype=np.uint8)
+
+# View each pixel as a single 24-bit integer, rather than three 8-bit bytes
+I24 = np.dot(I.astype(np.uint32),[1,256,65536])
+
+# Count unique colours
+n = len(np.unique(I24))
+print(n)
 ```
 #### 67. Considering a four dimensions array, how to get sum over the last two axis at once? (★★★)
 
@@ -1179,4 +1191,4 @@ idx = np.random.randint(0, X.size, (N, X.size))
 means = X[idx].mean(axis=1)
 confint = np.percentile(means, [2.5, 97.5])
 print(confint)
-```
+```

100_Numpy_random.ipynb

@@ -2,6 +2,7 @@
  "cells": [
   {
    "cell_type": "markdown",
+   "id": "68fdc73a",
    "metadata": {},
    "source": [
     "# 100 numpy exercises\n",
@@ -17,6 +18,7 @@
   },
   {
    "cell_type": "markdown",
+   "id": "fb354250",
    "metadata": {},
    "source": [
     "File automatically generated. See the documentation to update questions/answers/hints programmatically."
@@ -24,6 +26,7 @@
   },
   {
    "cell_type": "markdown",
+   "id": "a6d059d8",
    "metadata": {},
    "source": [
     "Run the `initialize.py` module, then call a random question with `pick()` an hint towards its solution with\n",
@@ -33,6 +36,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
+   "id": "390f283b",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -42,6 +46,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
+   "id": "ac498ee7",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -49,25 +54,7 @@
    ]
   }
  ],
- "metadata": {
-  "kernelspec": {
-   "display_name": "Python 3",
-   "language": "python",
-   "name": "python3"
-  },
-  "language_info": {
-   "codemirror_mode": {
-    "name": "ipython",
-    "version": 3
-   },
-   "file_extension": ".py",
-   "mimetype": "text/x-python",
-   "name": "python",
-   "nbconvert_exporter": "python",
-   "pygments_lexer": "ipython3",
-   "version": "3.7.7"
-  }
- },
+ "metadata": {},
  "nbformat": 4,
- "nbformat_minor": 4
+ "nbformat_minor": 5
 }

source/exercises100.ktx

@@ -451,8 +451,7 @@ Create a 5x5 matrix with row values ranging from 0 to 4 (★★☆)
 hint: np.arange
 
 < a37
-Z = np.zeros((5,5))
-Z += np.arange(5)
+Z = np.tile(np.arange(0, 5), (5,1))
 print(Z)
 
 < q38