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Fixes to questions 2, 9, 18, 31, 35, 44, 52, 56, 66 & 78 #106

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Fixes to questions 2, 9, 18, 31, 35, 44, 52, 56, 66 & 78 #106

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bffecc9
Fix hint 2
Mar 11, 2020
4dd6265
Fix answer 9
Mar 11, 2020
ebd2b73
Optimize answer 18
Mar 11, 2020
d3a5117
Fix alternative answer 31
Mar 11, 2020
861e5d6
Simplify setup of answer 35
Mar 11, 2020
e5a13fd
Simplify answer 44 using np.hypot
Mar 11, 2020
c36f064
Simplify answer 52 using np.hypot
Mar 12, 2020
7d40613
Simplify answer 56 using np.hypot
Mar 12, 2020
d43e208
Fix answer 66
Mar 13, 2020
02b7c17
Simplify answer 78
Mar 14, 2020
52fa50e
Run generators
Mar 14, 2020
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2 changes: 1 addition & 1 deletion 100_Numpy_exercises.ipynb
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Expand Up @@ -564,7 +564,7 @@
"cell_type": "markdown",
"metadata": {},
"source": [
"#### 36. Extract the integer part of a random array using 5 different methods (★★☆)"
"#### 36. Extract the integer part of a random array of positive numbers using 4 different methods (★★☆)"
]
},
{
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2 changes: 1 addition & 1 deletion 100_Numpy_exercises.md
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Expand Up @@ -113,7 +113,7 @@ np.sqrt(-1) == np.emath.sqrt(-1)

#### 35. How to compute ((A+B)*(-A/2)) in place (without copy)? (★★☆)

#### 36. Extract the integer part of a random array using 5 different methods (★★☆)
#### 36. Extract the integer part of a random array of positive numbers using 4 different methods (★★☆)

#### 37. Create a 5x5 matrix with row values ranging from 0 to 4 (★★☆)

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8 changes: 4 additions & 4 deletions 100_Numpy_exercises_with_hints.md
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Expand Up @@ -15,7 +15,7 @@ File automatically generated. See the documentation to update questions/answers/
#### 1. Import the numpy package under the name `np` (★☆☆)
`hint: import … as`
#### 2. Print the numpy version and the configuration (★☆☆)
`hint: np.__version__, np.show_config)`
`hint: np.__version__, np.show_config`
#### 3. Create a null vector of size 10 (★☆☆)
`hint: np.zeros`
#### 4. How to find the memory size of any array (★☆☆)
Expand Down Expand Up @@ -113,8 +113,8 @@ np.sqrt(-1) == np.emath.sqrt(-1)
`hint: np.arange(dtype=datetime64['D'])`
#### 35. How to compute ((A+B)*(-A/2)) in place (without copy)? (★★☆)
`hint: np.add(out=), np.negative(out=), np.multiply(out=), np.divide(out=)`
#### 36. Extract the integer part of a random array using 5 different methods (★★☆)
`hint: %, np.floor, np.ceil, astype, np.trunc`
#### 36. Extract the integer part of a random array of positive numbers using 4 different methods (★★☆)
`hint: %, np.floor, astype, np.trunc`
#### 37. Create a 5x5 matrix with row values ranging from 0 to 4 (★★☆)
`hint: np.arange`
#### 38. Consider a generator function that generates 10 integers and use it to build an array (★☆☆)
Expand All @@ -130,7 +130,7 @@ np.sqrt(-1) == np.emath.sqrt(-1)
#### 43. Make an array immutable (read-only) (★★☆)
`hint: flags.writeable`
#### 44. Consider a random 10x2 matrix representing cartesian coordinates, convert them to polar coordinates (★★☆)
`hint: np.sqrt, np.arctan2`
`hint: np.hypot, np.arctan2`
#### 45. Create random vector of size 10 and replace the maximum value by 0 (★★☆)
`hint: argmax`
#### 46. Create a structured array with `x` and `y` coordinates covering the [0,1]x[0,1] area (★★☆)
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40 changes: 20 additions & 20 deletions 100_Numpy_exercises_with_hints_with_solutions.md
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Expand Up @@ -19,7 +19,7 @@ File automatically generated. See the documentation to update questions/answers/
import numpy as np
```
#### 2. Print the numpy version and the configuration (★☆☆)
`hint: np.__version__, np.show_config)`
`hint: np.__version__, np.show_config`

```python
print(np.__version__)
Expand Down Expand Up @@ -72,8 +72,8 @@ print(Z)
`hint: reshape`

```python
nz = np.nonzero([1,2,0,0,4,0])
print(nz)
Z = np.arange(9).reshape((3, 3))
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Why this change ?

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The current solution corresponds to question 10.

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My bad... Thanks for fixing it!

print(Z)
```
#### 10. Find indices of non-zero elements from [1,2,0,0,4,0] (★☆☆)
`hint: np.nonzero`
Expand Down Expand Up @@ -151,7 +151,7 @@ print(0.3 == 3 * 0.1)
`hint: np.diag`

```python
Z = np.diag(1+np.arange(4),k=-1)
Z = np.diag(np.arange(1,5),k=-1)
print(Z)
```
#### 19. Create a 8x8 matrix and fill it with a checkerboard pattern (★☆☆)
Expand Down Expand Up @@ -292,8 +292,9 @@ Z = np.ones(1) / 0
_ = np.seterr(**defaults)

# Equivalently with a context manager
nz = np.nonzero([1,2,0,0,4,0])
print(nz)
with np.errstate(all='ignore'):
Z = np.arange(3) / 0
print(Z)
```
#### 32. Is the following expressions true? (★☆☆)
```python
Expand Down Expand Up @@ -323,23 +324,22 @@ print(Z)
`hint: np.add(out=), np.negative(out=), np.multiply(out=), np.divide(out=)`

```python
A = np.ones(3)*1
B = np.ones(3)*2
C = np.ones(3)*3
A = np.ones(3)
B = np.full(3, 2)
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I prefer the version with ones (in order to keep it simple)

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Okay, I will rebase and drop commit. Should the C matrix stay as well?

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ok


np.add(A,B,out=B)
np.divide(A,2,out=A)
np.negative(A,out=A)
np.multiply(A,B,out=A)
```
#### 36. Extract the integer part of a random array using 5 different methods (★★☆)
`hint: %, np.floor, np.ceil, astype, np.trunc`
#### 36. Extract the integer part of a random array of positive numbers using 4 different methods (★★☆)
`hint: %, np.floor, astype, np.trunc`

```python
Z = np.random.uniform(0,10,10)

print (Z - Z%1)
print (np.floor(Z))
print (np.ceil(Z)-1)
print (Z.astype(int))
print (np.trunc(Z))
```
Expand Down Expand Up @@ -409,12 +409,12 @@ Z.flags.writeable = False
Z[0] = 1
```
#### 44. Consider a random 10x2 matrix representing cartesian coordinates, convert them to polar coordinates (★★☆)
`hint: np.sqrt, np.arctan2`
`hint: np.hypot, np.arctan2`

```python
Z = np.random.random((10,2))
X,Y = Z[:,0], Z[:,1]
R = np.sqrt(X**2+Y**2)
R = np.hypot(Y,X)
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Y,X -> X,Y (for consistency)

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The order is such so that it's consistent with the order of arguments in the arctan2 invocation. It's my personal mnemonic, but I will change it if you insist.

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I would prefer yes.

T = np.arctan2(Y,X)
print(R)
print(T)
Expand Down Expand Up @@ -493,7 +493,7 @@ print(Z)
```python
Z = np.random.random((10,2))
X,Y = np.atleast_2d(Z[:,0], Z[:,1])
D = np.sqrt( (X-X.T)**2 + (Y-Y.T)**2)
D = np.hypot(X-X.T, Y-Y.T)
print(D)

# Much faster with scipy
Expand Down Expand Up @@ -552,7 +552,7 @@ for index in np.ndindex(Z.shape):

```python
X, Y = np.meshgrid(np.linspace(-1,1,10), np.linspace(-1,1,10))
D = np.sqrt(X*X+Y*Y)
D = np.hypot(X, Y)
sigma, mu = 1.0, 0.0
G = np.exp(-( (D-mu)**2 / ( 2.0 * sigma**2 ) ) )
print(G)
Expand Down Expand Up @@ -673,10 +673,10 @@ print(F)
# Author: Nadav Horesh

w,h = 16,16
I = np.random.randint(0,2,(h,w,3)).astype(np.ubyte)
I = np.random.randint(0,2,(h,w,3))
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Why did you remove the cast to ubyte ?

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In a sufficiently long array, the expected number of different "colors" should be 2**3 == 8. For the solution to work, F must be a matrix of at least 24-bit integers, but this is not what happens on my machine: I[..., 1] * 256 has type np.uint16, and the same happens with I[..., 0] * 256 * 256, leading to overflow. Writing (I[..., 0] * 256) * 256 doesn't solve the problem. A cast to a sufficient wide type has to happen at some point, or the initial cast has to go.

>>> import numpy as np
>>> w, h = 16, 16
>>> I = np.random.randint(0, 2, (h, w, 3))
>>> I2 = I.astype(np.ubyte)
>>> F = I[...,0]*256*256 + I[...,1]*256 +I[...,2]
>>> F2 = I2[...,0]*256*256 + I2[...,1]*256 +I2[...,2]
>>> print(len(np.unique(F)), len(np.unique(F2)))
8 4
>>> print(F.dtype, F2.dtype)
int64 uint16

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ok

F = I[...,0]*256*256 + I[...,1]*256 +I[...,2]
n = len(np.unique(F))
print(np.unique(I))
print(n)
```
#### 67. Considering a four dimensions array, how to get sum over the last two axis at once? (★★★)
`hint: sum(axis=(-2,-1))`
Expand Down Expand Up @@ -826,9 +826,9 @@ np.negative(Z, out=Z)
def distance(P0, P1, p):
T = P1 - P0
L = (T**2).sum(axis=1)
U = -((P0[:,0]-p[...,0])*T[:,0] + (P0[:,1]-p[...,1])*T[:,1]) / L
U = ((P0 - p)*T).sum(axis=1) / L
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Did you check this is correct ?

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?

U = U.reshape(len(U),1)
D = P0 + U*T - p
D = P0 - U*T - p
return np.sqrt((D**2).sum(axis=1))

P0 = np.random.uniform(-10,10,(10,2))
Expand Down
36 changes: 18 additions & 18 deletions 100_Numpy_exercises_with_solutions.md
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Original file line number Diff line number Diff line change
Expand Up @@ -72,8 +72,8 @@ print(Z)


```python
nz = np.nonzero([1,2,0,0,4,0])
print(nz)
Z = np.arange(9).reshape((3, 3))
print(Z)
```
#### 10. Find indices of non-zero elements from [1,2,0,0,4,0] (★☆☆)

Expand Down Expand Up @@ -151,7 +151,7 @@ print(0.3 == 3 * 0.1)


```python
Z = np.diag(1+np.arange(4),k=-1)
Z = np.diag(np.arange(1,5),k=-1)
print(Z)
```
#### 19. Create a 8x8 matrix and fill it with a checkerboard pattern (★☆☆)
Expand Down Expand Up @@ -211,7 +211,7 @@ print(Z)
# Author: Evgeni Burovski

Z = np.arange(11)
Z[(3 < Z) & (Z < 8)] *= -1
Z[(3 < Z) & (Z <= 8)] *= -1
print(Z)
```
#### 26. What is the output of the following script? (★☆☆)
Expand Down Expand Up @@ -292,8 +292,9 @@ Z = np.ones(1) / 0
_ = np.seterr(**defaults)

# Equivalently with a context manager
nz = np.nonzero([1,2,0,0,4,0])
print(nz)
with np.errstate(all='ignore'):
Z = np.arange(3) / 0
print(Z)
```
#### 32. Is the following expressions true? (★☆☆)
```python
Expand Down Expand Up @@ -323,23 +324,22 @@ print(Z)


```python
A = np.ones(3)*1
B = np.ones(3)*2
C = np.ones(3)*3
A = np.ones(3)
B = np.full(3, 2)

np.add(A,B,out=B)
np.divide(A,2,out=A)
np.negative(A,out=A)
np.multiply(A,B,out=A)
```
#### 36. Extract the integer part of a random array using 5 different methods (★★☆)
#### 36. Extract the integer part of a random array of positive numbers using 4 different methods (★★☆)


```python
Z = np.random.uniform(0,10,10)

print (Z - Z%1)
print (np.floor(Z))
print (np.ceil(Z)-1)
print (Z.astype(int))
print (np.trunc(Z))
```
Expand Down Expand Up @@ -414,7 +414,7 @@ Z[0] = 1
```python
Z = np.random.random((10,2))
X,Y = Z[:,0], Z[:,1]
R = np.sqrt(X**2+Y**2)
R = np.hypot(Y,X)
T = np.arctan2(Y,X)
print(R)
print(T)
Expand Down Expand Up @@ -493,7 +493,7 @@ print(Z)
```python
Z = np.random.random((10,2))
X,Y = np.atleast_2d(Z[:,0], Z[:,1])
D = np.sqrt( (X-X.T)**2 + (Y-Y.T)**2)
D = np.hypot(X-X.T, Y-Y.T)
print(D)

# Much faster with scipy
Expand Down Expand Up @@ -552,7 +552,7 @@ for index in np.ndindex(Z.shape):

```python
X, Y = np.meshgrid(np.linspace(-1,1,10), np.linspace(-1,1,10))
D = np.sqrt(X*X+Y*Y)
D = np.hypot(X, Y)
sigma, mu = 1.0, 0.0
G = np.exp(-( (D-mu)**2 / ( 2.0 * sigma**2 ) ) )
print(G)
Expand Down Expand Up @@ -673,10 +673,10 @@ print(F)
# Author: Nadav Horesh

w,h = 16,16
I = np.random.randint(0,2,(h,w,3)).astype(np.ubyte)
I = np.random.randint(0,2,(h,w,3))
F = I[...,0]*256*256 + I[...,1]*256 +I[...,2]
n = len(np.unique(F))
print(np.unique(I))
print(n)
```
#### 67. Considering a four dimensions array, how to get sum over the last two axis at once? (★★★)

Expand Down Expand Up @@ -826,9 +826,9 @@ np.negative(Z, out=Z)
def distance(P0, P1, p):
T = P1 - P0
L = (T**2).sum(axis=1)
U = -((P0[:,0]-p[...,0])*T[:,0] + (P0[:,1]-p[...,1])*T[:,1]) / L
U = ((P0 - p)*T).sum(axis=1) / L
U = U.reshape(len(U),1)
D = P0 + U*T - p
D = P0 - U*T - p
return np.sqrt((D**2).sum(axis=1))

P0 = np.random.uniform(-10,10,(10,2))
Expand Down
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