forked from UWPCE-PythonCert/ProgrammingInPython
-
Notifications
You must be signed in to change notification settings - Fork 1
Expand file tree
/
Copy pathIteration.html
More file actions
509 lines (379 loc) · 32.5 KB
/
Copy pathIteration.html
File metadata and controls
509 lines (379 loc) · 32.5 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
<!DOCTYPE html>
<html class="writer-html5" lang="en" >
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Iteration — Programming in Python 7.0 documentation</title>
<link rel="stylesheet" href="../_static/css/theme.css" type="text/css" />
<link rel="stylesheet" href="../_static/pygments.css" type="text/css" />
<!--[if lt IE 9]>
<script src="../_static/js/html5shiv.min.js"></script>
<![endif]-->
<script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script>
<script src="../_static/jquery.js"></script>
<script src="../_static/underscore.js"></script>
<script src="../_static/doctools.js"></script>
<script src="../_static/language_data.js"></script>
<script async="async" src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.7/latest.js?config=TeX-AMS-MML_HTMLorMML"></script>
<script type="text/javascript" src="../_static/js/theme.js"></script>
<link rel="index" title="Index" href="../genindex.html" />
<link rel="search" title="Search" href="../search.html" />
<link rel="next" title="Slicing Lab" href="../exercises/slicing.html" />
<link rel="prev" title="Python Sequences" href="Sequences.html" />
</head>
<body class="wy-body-for-nav">
<div class="wy-grid-for-nav">
<nav data-toggle="wy-nav-shift" class="wy-nav-side">
<div class="wy-side-scroll">
<div class="wy-side-nav-search" style="background: #4b2e83" >
<a href="../index.html">
<img src="../_static/UWPCE_logo_full.png" class="logo" alt="Logo"/>
</a>
<div role="search">
<form id="rtd-search-form" class="wy-form" action="../search.html" method="get">
<input type="text" name="q" placeholder="Search docs" />
<input type="hidden" name="check_keywords" value="yes" />
<input type="hidden" name="area" value="default" />
</form>
</div>
</div>
<div class="wy-menu wy-menu-vertical" data-spy="affix" role="navigation" aria-label="main navigation">
<p class="caption"><span class="caption-text">Topics in the Program</span></p>
<ul class="current">
<li class="toctree-l1"><a class="reference internal" href="../topics/01-setting_up/index.html">1. Setting up your Environment</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/02-basic_python/index.html">2. Basic Python</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/03-recursion_booleans/index.html">3. Booleans and Recursion</a></li>
<li class="toctree-l1 current"><a class="reference internal" href="../topics/04-sequences_iteration/index.html">4. Sequences and Iteration</a><ul class="current">
<li class="toctree-l2"><a class="reference internal" href="Sequences.html">Python Sequences</a></li>
<li class="toctree-l2 current"><a class="current reference internal" href="#">Iteration</a></li>
<li class="toctree-l2"><a class="reference internal" href="../exercises/slicing.html">Slicing Lab</a></li>
<li class="toctree-l2"><a class="reference internal" href="../exercises/list_lab.html">List Lab</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="../topics/05-text_handling/index.html">5. Basic Text Handling</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/06-exceptions/index.html">6. Exception Handling</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/07-unit_testing/index.html">7. Unit Testing</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/08-dicts_sets/index.html">8. Dictionaries and Sets</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/09-files/index.html">9. File Handling</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/10-modules_packages/index.html">10. Modules and Packages</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/11-argument_passing/index.html">11. Advanced Argument Passing</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/12-comprehensions/index.html">12. Comprehensions</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/13-intro_oo/index.html">13. Intro to Object Oriented Programing</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/14-magic_methods/index.html">14. Properties and Magic Methods</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/15-subclassing/index.html">15. Subclassing and Inheritance</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/16-multiple_inheritance/index.html">16. Multiple Inheritance</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/17-functional_programming/index.html">17. Introduction to Functional Programming</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/18-advanced_testing/index.html">18. Advanced Testing</a></li>
<li class="toctree-l1"><a class="reference internal" href="../topics/99-extras/index.html">19. Extra Topics</a></li>
</ul>
</div>
</div>
</nav>
<section data-toggle="wy-nav-shift" class="wy-nav-content-wrap">
<nav class="wy-nav-top" aria-label="top navigation">
<i data-toggle="wy-nav-top" class="fa fa-bars"></i>
<a href="../index.html">Programming in Python</a>
</nav>
<div class="wy-nav-content">
<div class="rst-content style-external-links">
<div role="navigation" aria-label="breadcrumbs navigation">
<ul class="wy-breadcrumbs">
<li><a href="../index.html" class="icon icon-home"></a> »</li>
<li><a href="../topics/04-sequences_iteration/index.html"><span class="section-number">4. </span>Sequences and Iteration</a> »</li>
<li>Iteration</li>
<li class="wy-breadcrumbs-aside">
<a href="../_sources/modules/Iteration.rst.txt" rel="nofollow"> View page source</a>
</li>
</ul>
<div class="rst-breadcrumbs-buttons" role="navigation" aria-label="breadcrumb navigation">
<a href="../exercises/slicing.html" class="btn btn-neutral float-right" title="Slicing Lab" accesskey="n">Next <span class="fa fa-arrow-circle-right"></span></a>
<a href="Sequences.html" class="btn btn-neutral float-left" title="Python Sequences" accesskey="p"><span class="fa fa-arrow-circle-left"></span> Previous</a>
</div>
<hr/>
</div>
<div role="main" class="document" itemscope="itemscope" itemtype="http://schema.org/Article">
<div itemprop="articleBody">
<div class="section" id="iteration">
<span id="id1"></span><h1>Iteration<a class="headerlink" href="#iteration" title="Permalink to this headline">¶</a></h1>
<p>Repetition, Repetition, Repetition, Repe…</p>
<div class="section" id="for-loops">
<h2>For Loops<a class="headerlink" href="#for-loops" title="Permalink to this headline">¶</a></h2>
<p>We’ve seen simple iteration over a sequence with <code class="docutils literal notranslate"><span class="pre">for</span> <span class="pre">...</span> <span class="pre">in</span></code>:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [170]: </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="s2">"a string"</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="go">a</span>
<span class="go">s</span>
<span class="go">t</span>
<span class="go">r</span>
<span class="go">i</span>
<span class="go">n</span>
<span class="go">g</span>
</pre></div>
</div>
<div class="section" id="no-indexing-required">
<h3>No Indexing Required<a class="headerlink" href="#no-indexing-required" title="Permalink to this headline">¶</a></h3>
<p>Contrast this with other languages, where you must build and use an <code class="docutils literal notranslate"><span class="pre">index</span></code> to iterate through an array.</p>
<div class="highlight-javascript notranslate"><div class="highlight"><pre><span></span><span class="k">for</span><span class="p">(</span><span class="kd">var</span> <span class="nx">i</span> <span class="o">=</span> <span class="mf">0</span><span class="p">;</span> <span class="nx">i</span> <span class="o"><</span> <span class="nx">arr</span><span class="p">.</span><span class="nx">length</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span><span class="p">)</span> <span class="p">{</span>
<span class="kd">var</span> <span class="nx">value</span> <span class="o">=</span> <span class="nx">arr</span><span class="p">[</span><span class="nx">i</span><span class="p">];</span>
<span class="nx">alert</span><span class="p">(</span><span class="nx">i</span> <span class="o">+</span> <span class="s2">") "</span> <span class="o">+</span> <span class="nx">value</span><span class="p">);</span>
</pre></div>
</div>
<p>If you <em>do</em> need an index, you can use <code class="docutils literal notranslate"><span class="pre">enumerate</span></code>:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [140]: </span><span class="k">for</span> <span class="n">idx</span><span class="p">,</span> <span class="n">letter</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="s1">'Python'</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">idx</span><span class="p">,</span> <span class="n">letter</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="go">0 P 1 y 2 t 3 h 4 o 5 n</span>
</pre></div>
</div>
</div>
<div class="section" id="range-and-for-loops">
<h3><code class="docutils literal notranslate"><span class="pre">range</span></code> and <code class="docutils literal notranslate"><span class="pre">for</span></code> Loops<a class="headerlink" href="#range-and-for-loops" title="Permalink to this headline">¶</a></h3>
<p>The <code class="docutils literal notranslate"><span class="pre">range</span></code> builtin is useful for looping a known number of times:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [171]: </span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="go">0</span>
<span class="go">1</span>
<span class="go">2</span>
<span class="go">3</span>
<span class="go">4</span>
</pre></div>
</div>
<p>But you don’t really need to do anything at all with <code class="docutils literal notranslate"><span class="pre">i</span></code></p>
<p>In fact, it’s a common convention to make this clear with a “nothing” name:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [21]: </span><span class="k">for</span> <span class="n">__</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"*"</span><span class="p">)</span>
<span class="gp"> ....:</span>
<span class="go">*</span>
<span class="go">*</span>
<span class="go">*</span>
<span class="go">*</span>
<span class="go">*</span>
</pre></div>
</div>
</div>
<div class="section" id="no-namespace">
<h3>No Namespace<a class="headerlink" href="#no-namespace" title="Permalink to this headline">¶</a></h3>
<p>Be alert that a loop does not create a local namespace:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [172]: </span><span class="n">x</span> <span class="o">=</span> <span class="mi">10</span>
<span class="gp">In [173]: </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="k">pass</span>
<span class="gp"> .....:</span>
<span class="gp">In [174]: </span><span class="n">x</span>
<span class="gh">Out[174]: </span><span class="go">2</span>
</pre></div>
</div>
</div>
<div class="section" id="loop-control">
<h3>Loop Control<a class="headerlink" href="#loop-control" title="Permalink to this headline">¶</a></h3>
<p>Sometimes you want to interrupt or alter the flow of control through a loop.</p>
<p>Loops can be controlled in two ways, with <code class="docutils literal notranslate"><span class="pre">break</span></code> and <code class="docutils literal notranslate"><span class="pre">continue</span></code>.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">break</span></code> keyword will cause a loop to immediately terminate:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [141]: </span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">101</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">i</span> <span class="o">></span> <span class="mi">50</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="k">break</span>
<span class="gp"> .....:</span>
<span class="go">0 1 2 3 4 5... 46 47 48 49 50 51</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">continue</span></code> keyword will skip later statements in the loop block, but
allow iteration to continue:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [143]: </span><span class="k">for</span> <span class="ow">in</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">101</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">i</span> <span class="o">></span> <span class="mi">50</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="k">break</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">i</span> <span class="o"><</span> <span class="mi">25</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="k">continue</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="go"> 25 26 27 28 29 ... 41 42 43 44 45 46 47 48 49 50</span>
</pre></div>
</div>
<p>Take some time to look at these examples carefully, and make sure you understand them. It’s probably a good idea to write a bit of code to experiment as well.</p>
</div>
<div class="section" id="else">
<h3>else<a class="headerlink" href="#else" title="Permalink to this headline">¶</a></h3>
<p>For loops can also take an optional <code class="docutils literal notranslate"><span class="pre">else</span></code> block.</p>
<p>This is <strong>not</strong> a feature of most languages, but it can be handy.</p>
<p>Executed only when the loop exits normally (not via break):</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [147]: </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">11</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="k">break</span>
<span class="gp"> .....: </span><span class="k">else</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'finished'</span><span class="p">)</span>
<span class="go">finished</span>
<span class="gp">In [148]: </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">5</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="gp"> .....: </span> <span class="k">break</span>
<span class="gp"> .....: </span><span class="k">else</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'finished'</span><span class="p">)</span>
<span class="go">5</span>
</pre></div>
</div>
<p>This is a really nice, unique Python feature!</p>
<p>If Python didn’t have <code class="docutils literal notranslate"><span class="pre">else</span></code> on loops, you’d need to set a flag, something like:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">it_did_break</span> <span class="o">=</span> <span class="kc">False</span>
<span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="k">if</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">11</span><span class="p">:</span>
<span class="n">it_did_break</span> <span class="o">=</span> <span class="kc">True</span>
<span class="k">break</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">it_did_break</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'finished'</span><span class="p">)</span>
</pre></div>
</div>
<p>That’s klunkier, no?</p>
<p>Make sure to try this a bit yourself too, to make sure you get it.</p>
</div>
</div>
<div class="section" id="while-loops">
<h2>While Loops<a class="headerlink" href="#while-loops" title="Permalink to this headline">¶</a></h2>
<p>While loops are different – they are not for iterating over a collection, but rather for repeating something an unknown number of times – and maybe even forever – or until the program terminates.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">while</span></code> keyword is for when you don’t know how many loops you need.</p>
<p>It continues to execute the body until the associated condition does not evaluate to True:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">while</span> <span class="n">a_condition</span><span class="p">:</span>
<span class="n">some_code</span>
<span class="n">in_the_body</span>
</pre></div>
</div>
<div class="section" id="while-vs-for">
<h3><code class="docutils literal notranslate"><span class="pre">while</span></code> vs. <code class="docutils literal notranslate"><span class="pre">for</span></code><a class="headerlink" href="#while-vs-for" title="Permalink to this headline">¶</a></h3>
<p><code class="docutils literal notranslate"><span class="pre">while</span></code> is more general than <code class="docutils literal notranslate"><span class="pre">for</span></code></p>
<p>– you can always express <code class="docutils literal notranslate"><span class="pre">for</span></code> as <code class="docutils literal notranslate"><span class="pre">while</span></code>, but not always vice-versa.</p>
<p><code class="docutils literal notranslate"><span class="pre">while</span></code> is more error-prone – requires some care to terminate.</p>
<p>The loop body must make progress, so the associated condition can become <code class="docutils literal notranslate"><span class="pre">False</span></code>.</p>
<p>Care must be taken to avoid an unintended error – infinite loops:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
<span class="k">while</span> <span class="n">i</span> <span class="o"><</span> <span class="mi">5</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="terminating-a-while-loop">
<h3>Terminating a while Loop<a class="headerlink" href="#terminating-a-while-loop" title="Permalink to this headline">¶</a></h3>
<p>Use <code class="docutils literal notranslate"><span class="pre">break</span></code>:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [150]: </span><span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="n">i</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">i</span> <span class="o">></span> <span class="mi">10</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="k">break</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="go">1 2 3 4 5 6 7 8 9 10</span>
</pre></div>
</div>
<p>Set a flag:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [156]: </span><span class="kn">import</span> <span class="nn">random</span>
<span class="gp">In [157]: </span><span class="n">keep_going</span> <span class="o">=</span> <span class="kc">True</span>
<span class="gp">In [158]: </span><span class="k">while</span> <span class="n">keep_going</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="n">num</span> <span class="o">=</span> <span class="n">random</span><span class="o">.</span><span class="n">choice</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">))</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">num</span><span class="p">)</span>
<span class="gp"> .....: </span> <span class="k">if</span> <span class="n">num</span> <span class="o">==</span> <span class="mi">3</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="n">keep_going</span> <span class="o">=</span> <span class="kc">False</span>
<span class="gp"> .....:</span>
<span class="go">3</span>
</pre></div>
</div>
<p>Use a condition:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [161]: </span><span class="k">while</span> <span class="n">i</span> <span class="o"><</span> <span class="mi">10</span><span class="p">:</span>
<span class="gp"> .....: </span> <span class="n">i</span> <span class="o">+=</span> <span class="n">random</span><span class="o">.</span><span class="n">choice</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">))</span>
<span class="gp"> .....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="gp"> .....:</span>
<span class="go">0 0 2 3 4 6 8 8 8 9 12</span>
</pre></div>
</div>
</div>
<div class="section" id="similarities">
<h3>Similarities<a class="headerlink" href="#similarities" title="Permalink to this headline">¶</a></h3>
<p>Both <code class="docutils literal notranslate"><span class="pre">for</span></code> and <code class="docutils literal notranslate"><span class="pre">while</span></code> loops can use <code class="docutils literal notranslate"><span class="pre">break</span></code> and <code class="docutils literal notranslate"><span class="pre">continue</span></code> for
internal flow control.</p>
<p>Both <code class="docutils literal notranslate"><span class="pre">for</span></code> and <code class="docutils literal notranslate"><span class="pre">while</span></code> loops can have an optional <code class="docutils literal notranslate"><span class="pre">else</span></code> block.</p>
<p>In both loops, the statements in the <code class="docutils literal notranslate"><span class="pre">else</span></code> block are only executed if the
loop terminates normally (no <code class="docutils literal notranslate"><span class="pre">break</span></code>).</p>
</div>
</div>
<div class="section" id="pythonic-iteration">
<h2>Pythonic Iteration<a class="headerlink" href="#pythonic-iteration" title="Permalink to this headline">¶</a></h2>
<p>I’ve already said it, but it bears repeating:</p>
<p>for loops are for iterating over something (an “iterable”) – you almost never want to iterate over the indexes, and then access items with the index.</p>
<div class="section" id="nifty-for-loop-tricks">
<h3>Nifty for loop tricks<a class="headerlink" href="#nifty-for-loop-tricks" title="Permalink to this headline">¶</a></h3>
<p><strong>tuple unpacking:</strong></p>
<p>remember this?</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="o">=</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span>
</pre></div>
</div>
<p>You can do that in a for loop, also:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [4]: </span><span class="n">l</span> <span class="o">=</span> <span class="p">[(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">),</span> <span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">),</span> <span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">)]</span>
<span class="gp">In [5]: </span><span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span> <span class="ow">in</span> <span class="n">l</span><span class="p">:</span>
<span class="go"> print("i:{}, j:{}".format(i, j))</span>
<span class="go">i:1, j:2</span>
<span class="go">i:3, j:4</span>
<span class="go">i:5, j:6</span>
</pre></div>
</div>
</div>
<div class="section" id="looping-through-two-iterables-at-once">
<h3>Looping through two iterables at once:<a class="headerlink" href="#looping-through-two-iterables-at-once" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p><code class="docutils literal notranslate"><span class="pre">zip</span></code></p>
</div></blockquote>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [10]: </span><span class="n">l1</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
<span class="gp">In [11]: </span><span class="n">l2</span> <span class="o">=</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="gp">In [12]: </span><span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="n">l1</span><span class="p">,</span> <span class="n">l2</span><span class="p">):</span>
<span class="go"> print("i:{}, j:{}".format(i, j))</span>
<span class="go">i:1, j:3</span>
<span class="go">i:2, j:4</span>
<span class="go">i:3, j:5</span>
</pre></div>
</div>
<p>There can be more than two:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span><span class="p">,</span> <span class="n">k</span><span class="p">,</span> <span class="n">l</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="n">l1</span><span class="p">,</span> <span class="n">l2</span><span class="p">,</span> <span class="n">l3</span><span class="p">,</span> <span class="n">l4</span><span class="p">):</span>
</pre></div>
</div>
</div>
<div class="section" id="need-the-index-and-the-item">
<h3>Need the index and the item?<a class="headerlink" href="#need-the-index-and-the-item" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><p><code class="docutils literal notranslate"><span class="pre">enumerate</span></code></p>
</div></blockquote>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [2]: </span><span class="n">l</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'this'</span><span class="p">,</span> <span class="s1">'that'</span><span class="p">,</span> <span class="s1">'the other'</span><span class="p">]</span>
<span class="gp">In [3]: </span><span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">item</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">l</span><span class="p">):</span>
<span class="gp"> ...: </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"the </span><span class="si">{:d}</span><span class="s2">th item is: </span><span class="si">{:s}</span><span class="s2">"</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="n">item</span><span class="p">))</span>
<span class="gp"> ...:</span>
<span class="go">the 0th item is: this</span>
<span class="go">the 1th item is: that</span>
<span class="go">the 2th item is: the other</span>
</pre></div>
</div>
</div>
</div>
</div>
</div>
</div>
<footer>
<div class="rst-footer-buttons" role="navigation" aria-label="footer navigation">
<a href="../exercises/slicing.html" class="btn btn-neutral float-right" title="Slicing Lab" accesskey="n" rel="next">Next <span class="fa fa-arrow-circle-right"></span></a>
<a href="Sequences.html" class="btn btn-neutral float-left" title="Python Sequences" accesskey="p" rel="prev"><span class="fa fa-arrow-circle-left"></span> Previous</a>
</div>
<hr/>
<div role="contentinfo">
<p>
© Copyright 2020, University of Washington, Natasha Aleksandrova, Christopher Barker, Brian Dorsey, Cris Ewing, Christy Heaton, Jon Jacky, Maria McKinley, Andy Miles, Rick Riehle, Joseph Schilz, Joseph Sheedy, Hosung Song. Creative Commons Attribution-ShareAlike 4.0 license
</p>
</div>
Built with <a href="http://sphinx-doc.org/">Sphinx</a> using a
<a href="https://github.com/rtfd/sphinx_rtd_theme">theme</a>
provided by <a href="https://readthedocs.org">Read the Docs</a>.
</footer>
</div>
</div>
</section>
</div>
<script type="text/javascript">
jQuery(function () {
SphinxRtdTheme.Navigation.enable(true);
});
</script>
</body>
</html>