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Merge pull request #26 from Jingjing092/main
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slicing
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@luweizheng
luweizheng authored Nov 2, 2023
2 parents d30c700 + 536b7fb commit cdddde0
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336 changes: 336 additions & 0 deletions ch-numpy/ndarray-slicing-index.ipynb
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{
"cells": [
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# 切片与索引\n",
"\n",
"从左往右索引时起始位置从0开始，从右往左索引时起始位置从-1开始。"
]
},
{
"cell_type": "code",
"execution_count": 9,
"metadata": {},
"outputs": [],
"source": [
"import numpy as np"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"### 一维数组的情况"
]
},
{
"cell_type": "code",
"execution_count": 10,
"metadata": {},
"outputs": [
{
"data": {
"text/plain": [
"array([2, 0, 2, 3, 9, 2, 3, 4])"
]
},
"execution_count": 10,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"#生成一维数组a\n",
"a = np.array([2,0,2,3,9,2,3,4])\n",
"a"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"#### 一维数组的索引"
]
},
{
"cell_type": "code",
"execution_count": 11,
"metadata": {},
"outputs": [
{
"data": {
"text/plain": [
"9"
]
},
"execution_count": 11,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"#索引查找数组a中编号为4的元素，即查找第5个元素\n",
"#注：编号从0开始\n",
"a[4]"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"#### 一维数组的切片"
]
},
{
"cell_type": "code",
"execution_count": 12,
"metadata": {},
"outputs": [
{
"data": {
"text/plain": [
"array([2, 2, 9])"
]
},
"execution_count": 12,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"#将数组a切片[起始编号：终止编号（不包含）：步长]\n",
"a[0:6:2]"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"### 多维数组的情况"
]
},
{
"cell_type": "code",
"execution_count": 13,
"metadata": {},
"outputs": [
{
"data": {
"text/plain": [
"array([[[ 0, 1, 2],\n",
" [ 3, 4, 5],\n",
" [ 6, 7, 8],\n",
" [ 9, 10, 11]],\n",
"\n",
" [[12, 13, 14],\n",
" [15, 16, 17],\n",
" [18, 19, 20],\n",
" [21, 22, 23]],\n",
"\n",
" [[24, 25, 26],\n",
" [27, 28, 29],\n",
" [30, 31, 32],\n",
" [33, 34, 35]]])"
]
},
"execution_count": 13,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"#生成多维数组b\n",
"b = np.arange(36).reshape((3,4,3))\n",
"b"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"#### 多维数组的索引"
]
},
{
"cell_type": "code",
"execution_count": 14,
"metadata": {},
"outputs": [
{
"data": {
"text/plain": [
"11"
]
},
"execution_count": 14,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"#查找b数组中，编号为0的元素中编号为3的维度下编号为2的元素\n",
"#索引b数组中第1个矩阵中第4行第3个元素\n",
"b[0,3,2]"
]
},
{
"cell_type": "code",
"execution_count": 15,
"metadata": {},
"outputs": [
{
"data": {
"text/plain": [
"19"
]
},
"execution_count": 15,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"#查找b数组中，编号为1的元素中编号为2的维度下编号为1的元素\n",
"#索引b数组中第2个矩阵中第3行第2个元素\n",
"b[1,2,1]"
]
},
{
"cell_type": "code",
"execution_count": 16,
"metadata": {},
"outputs": [
{
"data": {
"text/plain": [
"19"
]
},
"execution_count": 16,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"#查找b数组中，编号为-2的元素中编号为-2的维度下编号为-2的元素\n",
"#索引b数组中第2个矩阵中第3行第2个元素，与上述[1,2,1]索引元素一致\n",
"b[-2,-2,-2]"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"#### 多维数组的切片"
]
},
{
"cell_type": "code",
"execution_count": 17,
"metadata": {},
"outputs": [
{
"data": {
"text/plain": [
"array([ 6, 18, 30])"
]
},
"execution_count": 17,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"#选取一个维度进行切片\n",
"b[:,2,-3] #选择每个矩阵中从上往下第3行，从右往左第3个元素"
]
},
{
"cell_type": "code",
"execution_count": 18,
"metadata": {},
"outputs": [
{
"data": {
"text/plain": [
"array([[[ 3, 4, 5],\n",
" [ 9, 10, 11]],\n",
"\n",
" [[15, 16, 17],\n",
" [21, 22, 23]],\n",
"\n",
" [[27, 28, 29],\n",
" [33, 34, 35]]])"
]
},
"execution_count": 18,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"#对维度进行切片\n",
"#每个维度切片方法与一维数组相同\n",
"b[:,1:4:2,:] #维度起始值为1，终止值为4（不包含），步长为2"
]
},
{
"cell_type": "code",
"execution_count": 19,
"metadata": {},
"outputs": [
{
"data": {
"text/plain": [
"array([[[ 0, 2],\n",
" [ 3, 5],\n",
" [ 6, 8],\n",
" [ 9, 11]],\n",
"\n",
" [[12, 14],\n",
" [15, 17],\n",
" [18, 20],\n",
" [21, 23]],\n",
"\n",
" [[24, 26],\n",
" [27, 29],\n",
" [30, 32],\n",
" [33, 35]]])"
]
},
"execution_count": 19,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"#对每个维度切片\n",
"#使用步长跳跃切片\n",
"b[:,:,::2]"
]
}
],
"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": "3.10.11"
}
},
"nbformat": 4,
"nbformat_minor": 2
}
1 change: 0 additions & 1 deletion ch-numpy/ndarray-slicing-index.md
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Empty file added ch-numpy/random.ipynb
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1 change: 0 additions & 1 deletion ch-numpy/random.md
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