1 00:00:05,300 --> 00:00:08,470 So, it's now time to talk about for loops. 2 00:00:08,470 --> 00:00:10,720 So, one of the reasons that computers are so useful, 3 00:00:10,720 --> 00:00:15,100 is that they can actually repeat an operations multiple times, very quickly. 4 00:00:15,100 --> 00:00:19,510 And this of course, removes the need for a human to perform a repetitive task. 5 00:00:19,510 --> 00:00:23,260 Now we've seen the For loop in earlier examples, but now it's time to look at 6 00:00:23,260 --> 00:00:25,820 them in more detail and actually explain what was happening. 7 00:00:25,820 --> 00:00:29,000 So, what a For loop does it actually takes a range of values and 8 00:00:29,000 --> 00:00:31,670 assigns them one by one to a variable. 9 00:00:31,670 --> 00:00:34,510 It then executes a block of code once for each value. 10 00:00:34,510 --> 00:00:38,190 For a simple example, we're gonna have a go printing at the values from 1 to 20. 11 00:00:38,190 --> 00:00:42,950 So the format is type in the for, 12 00:00:42,950 --> 00:00:45,900 keyword for, i in range. 13 00:00:45,900 --> 00:00:48,750 1 comma 20 in brackets, then we indent the code. 14 00:00:48,750 --> 00:00:52,490 This is the block that we want to have executed once for each value. 15 00:00:52,490 --> 00:00:57,570 So print i is now, replacement field, 16 00:00:57,570 --> 00:01:05,760 this should be double quotes at the start. dot format i, that's the simplest example. 17 00:01:05,760 --> 00:01:09,510 Again, what we're trying to do is go to get the values from 1 to 20. 18 00:01:09,510 --> 00:01:11,990 And then when we run that, you can see what happens is for 19 00:01:11,990 --> 00:01:16,520 each value of i, we've actually got the value i is now one, two, three, four five, 20 00:01:16,520 --> 00:01:19,740 right through to 19, so it didn't go through the 20. 21 00:01:19,740 --> 00:01:23,600 If you think back to this, remember how our string slices that we used 22 00:01:23,600 --> 00:01:26,580 return the characters up to, but not including, the N number? 23 00:01:26,580 --> 00:01:27,920 Well, ranges work in the same way. 24 00:01:27,920 --> 00:01:30,410 The last value specified is not included. 25 00:01:30,410 --> 00:01:34,670 So effectively, the way to rate is for i in range (1-20). 26 00:01:34,670 --> 00:01:36,720 Would be to actually say to yourself, 27 00:01:36,720 --> 00:01:39,590 I'm going to actually loop through the values 1 to 19. 28 00:01:39,590 --> 00:01:44,150 So the computer will actually repeat this code in here in the block, 29 00:01:44,150 --> 00:01:48,260 in this case 19 times, and incrementing the value of i each time. 30 00:01:48,260 --> 00:01:51,170 So the first time it runs, you can see i was set to 1, 31 00:01:51,170 --> 00:01:55,570 i is set to 2, i is set to 3, and so on and so forth, right through into 19. 32 00:01:55,570 --> 00:01:58,700 Now we're gonna talk more about naming variables in a later video. 33 00:01:58,700 --> 00:02:01,580 Up until now we've just used simple words that describe the value 34 00:02:01,580 --> 00:02:02,660 that we've been dealing with. 35 00:02:02,660 --> 00:02:05,110 And normally, you should avoid using single character names for 36 00:02:05,110 --> 00:02:06,450 variables like we've got here, like I. 37 00:02:06,450 --> 00:02:10,759 Because I don't really give any indication of what the variable is for, 38 00:02:10,759 --> 00:02:12,140 but there is an exception to that and 39 00:02:12,140 --> 00:02:16,780 loop control variables are one of those few places where I and J are acceptable. 40 00:02:16,780 --> 00:02:18,690 I is taken to be short for index and 41 00:02:18,690 --> 00:02:22,160 the convention is that it's acceptable name in a For loop. 42 00:02:22,160 --> 00:02:26,620 So when you are creating a For loop like this, using I is quite okay and 43 00:02:26,620 --> 00:02:29,180 you find that there is a lot of code out there that actually does that. 44 00:02:29,180 --> 00:02:30,160 So another example of this. 45 00:02:30,160 --> 00:02:34,320 We can actually see the variable uses an index in the next example which I'm about 46 00:02:34,320 --> 00:02:38,190 to type out, where we print each character in a string once at a time. 47 00:02:38,190 --> 00:02:40,470 So I'm going to actually just put that on the next line. 48 00:02:40,470 --> 00:02:43,240 Being sure to go to the start of the block and not to indent it. 49 00:02:43,240 --> 00:02:45,300 So we're gonna type in number. 50 00:02:45,300 --> 00:02:46,470 number= 9,223,372,036,854,775,807 51 00:02:46,470 --> 00:02:52,860 And what we're gonna do is gonna put for 52 00:02:52,860 --> 00:03:00,420 I in range 0 len, which stands for length, number. 53 00:03:01,900 --> 00:03:05,900 Colon again, and 54 00:03:05,900 --> 00:03:11,330 print number, i in square brackets. 55 00:03:11,330 --> 00:03:14,220 There's a couple of things here, we've introduced a new function. 56 00:03:14,220 --> 00:03:17,580 This function here is called len which is an abbreviation for length and 57 00:03:17,580 --> 00:03:20,270 it actually returns the length of a string. 58 00:03:20,270 --> 00:03:23,090 So, how many characters actually exist in that string. 59 00:03:23,090 --> 00:03:26,430 So, in this case it should actually return 25 if I'm counting correctly. 60 00:03:26,430 --> 00:03:30,980 But with that said, the way that we actually access individual characters 61 00:03:30,980 --> 00:03:33,990 in a string, they actually start from zero in 62 00:03:33,990 --> 00:03:38,420 this case to whatever the length is minus one, so 24 in this case. 63 00:03:38,420 --> 00:03:41,330 And because we established earlier that when we put a range in, 64 00:03:41,330 --> 00:03:44,390 we only go up into one less than the final number, 65 00:03:44,390 --> 00:03:47,140 we know that in the case the len(number) is gonna be correct. 66 00:03:47,140 --> 00:03:48,130 Cuz it's only gonna go through and 67 00:03:48,130 --> 00:03:51,640 process up to one less than that final number in the For loop. 68 00:03:51,640 --> 00:03:54,320 So that's actually one of the reasons why the range runs up to but 69 00:03:54,320 --> 00:03:56,200 not including the final value. 70 00:03:56,200 --> 00:03:59,150 So in many other programming languages, the loop would have to be specified 71 00:03:59,150 --> 00:04:01,970 as from zero to, and you might have to do something like this. 72 00:04:01,970 --> 00:04:04,910 Length, number, take one, because you'd have to code it for 73 00:04:04,910 --> 00:04:08,010 the fact you're starting from zero, and not starting from one, 74 00:04:08,010 --> 00:04:09,270 as you can see in the example up here. 75 00:04:09,270 --> 00:04:13,130 But in this case, with Python, you don't actually have to do that, 76 00:04:13,130 --> 00:04:16,360 because of course we established that when you use a range, 77 00:04:16,360 --> 00:04:19,440 it actually finishes at one less than the number you specify anyway. 78 00:04:19,440 --> 00:04:22,770 Basically, it works more how you'd expect and your free from having to consider 79 00:04:22,770 --> 00:04:25,960 whether you need to put this minus one and other such trivia which is really cool. 80 00:04:25,960 --> 00:04:29,490 So if we actually run this now, you can see what's happened here. 81 00:04:29,490 --> 00:04:30,960 Each time we've gone through the loop. 82 00:04:30,960 --> 00:04:34,890 It's actually printed out one element of that string. 83 00:04:34,890 --> 00:04:36,530 And by using the square bracket, 84 00:04:36,530 --> 00:04:40,570 what that is saying is print the character at position i. 85 00:04:40,570 --> 00:04:45,370 So we started on the first time position i, i had the value of of zero, so 86 00:04:45,370 --> 00:04:47,730 position zero is nine. 87 00:04:47,730 --> 00:04:50,900 Next time I'm gonna go through and position one is the comma, and so 88 00:04:50,900 --> 00:04:53,470 right through to the end and we should have an 807 on the end there. 89 00:04:53,470 --> 00:04:56,650 The last three digits there, so, you can see this is working quite nicely. 90 00:04:56,650 --> 00:04:58,170 And it didn't crash or anything like that and 91 00:04:58,170 --> 00:04:59,650 actually come up with the right number. 92 00:04:59,650 --> 00:05:02,310 That's one example but let's take this all to the next level. 93 00:05:02,310 --> 00:05:06,300 And let's just imagine that your processing data at someone elses exported 94 00:05:06,300 --> 00:05:07,210 from a spread sheet program. 95 00:05:07,210 --> 00:05:11,310 And let's just say that the spreadsheet was set to display numbers with commas or 96 00:05:11,310 --> 00:05:14,130 some other separators like in Europe for example it could be a full stop or 97 00:05:14,130 --> 00:05:15,020 a space. 98 00:05:15,020 --> 00:05:16,690 Normally, before we could process these numbers we 99 00:05:16,690 --> 00:05:19,620 would have to strip out the separators, but there's other ways to do this. 100 00:05:19,620 --> 00:05:21,970 And one way could be to check each character and 101 00:05:21,970 --> 00:05:24,440 only print it out if it's one of the digits from zero to nine. 102 00:05:24,440 --> 00:05:26,790 So what we could do is actually take this code again. 103 00:05:26,790 --> 00:05:27,710 I'm just gonna copy that. 104 00:05:27,710 --> 00:05:30,420 And paste it down here. 105 00:05:30,420 --> 00:05:33,900 We leave the same for range in here, but we add another test here. 106 00:05:33,900 --> 00:05:41,050 And the test we're gonna add is if number i, in square brackets, 107 00:05:41,050 --> 00:05:46,260 in a string, one, two, three, four, five, six, seven, eight, nine. 108 00:05:46,260 --> 00:05:48,980 That's the case, and then we come back onto the print line, and then we actually 109 00:05:48,980 --> 00:05:53,260 indent that because we only want that executed, if this actually returns true. 110 00:05:53,260 --> 00:05:57,350 So, if you remember the in function, it returns true if whatever we're testing on 111 00:05:57,350 --> 00:06:00,570 the left hand side is actually in whatever is in the right hand side. 112 00:06:00,570 --> 00:06:03,880 So, in this case we're looking for the element in this case nine. 113 00:06:03,880 --> 00:06:07,150 And as you get that nine, yes, it's going to actually print that out. 114 00:06:07,150 --> 00:06:09,900 So, what this should do is actually go through and print all the numbers and 115 00:06:09,900 --> 00:06:11,660 it should actually skip the commas. 116 00:06:11,660 --> 00:06:13,730 Cuz of course, a comma is not actually in this string. 117 00:06:13,730 --> 00:06:15,250 So, let's just run that and see if it works. 118 00:06:17,170 --> 00:06:22,810 And what we should probably do is just to make sure, 119 00:06:22,810 --> 00:06:26,070 we'll just comment that out for now, and run that again. 120 00:06:26,070 --> 00:06:29,220 And you can see that it's now successfully 9223372036854775807. 121 00:06:29,220 --> 00:06:34,290 So it successfully skipped anything that wasn't a digit from 0 to 9. 122 00:06:34,290 --> 00:06:37,760 Now one of the disadvantages with that is you notice that when using the print, 123 00:06:37,760 --> 00:06:41,000 automatically Python's actually adding the end of line character. 124 00:06:41,000 --> 00:06:43,060 So it's actually starting on a new line. 125 00:06:43,060 --> 00:06:46,090 So we can actually stop that if we want though, because it actually syntax. 126 00:06:46,090 --> 00:06:48,510 So you may actually seen this when you actually 127 00:06:48,510 --> 00:06:50,950 typing a comma after number, so I'll go back and enter a comma there. 128 00:06:52,050 --> 00:06:53,500 And notice how this comes up automatically and 129 00:06:53,500 --> 00:06:55,740 IntelliJ is saying there's some other options here. 130 00:06:55,740 --> 00:07:00,160 And by default, end=\n which is a new line character. 131 00:07:00,160 --> 00:07:03,320 So that's what happens automatically and that consequently each number, 132 00:07:03,320 --> 00:07:05,190 each time it's hitting a number in the loop. 133 00:07:05,190 --> 00:07:07,910 When we're doing the print, it's actually skipping to the next line. 134 00:07:07,910 --> 00:07:10,280 So we'll be providing of course to first parameter, 135 00:07:10,280 --> 00:07:13,240 which is obviously the object that we want printed. 136 00:07:13,240 --> 00:07:17,380 But as you can see in the screen, we can also specify separator and a value for end. 137 00:07:17,380 --> 00:07:20,180 So we're gonna come back to separators, and the last two options file and 138 00:07:20,180 --> 00:07:23,900 flush will be covered when we talk about file IO in a later video. 139 00:07:23,900 --> 00:07:25,900 But the end parameter is the one that's interesting here. 140 00:07:25,900 --> 00:07:29,660 Cuz at the moment you can see that it defaults again, to \n, which is new line. 141 00:07:29,660 --> 00:07:32,620 But we can actually override that, so for example, if we set that 142 00:07:32,620 --> 00:07:36,870 to an empty string, then a new line's not gonna be started after each character. 143 00:07:36,870 --> 00:07:41,590 So what we could do is we could type something like end = '', 144 00:07:41,590 --> 00:07:44,480 and that's overwriting the standard n = '\n'. 145 00:07:44,480 --> 00:07:48,150 And what we should see when we run it is the numbers 146 00:07:48,150 --> 00:07:49,240 now appearing from left to right. 147 00:07:49,240 --> 00:07:51,870 Because there's not a new line being automatically added each time 148 00:07:51,870 --> 00:07:52,590 the printout's made. 149 00:07:52,590 --> 00:07:57,100 Now if we wanted to use a number that we come up for performing calculations. 150 00:07:57,100 --> 00:08:00,610 What we could do is we can concatenate each character onto the new string and 151 00:08:00,610 --> 00:08:03,100 then convert the final string to an integer. 152 00:08:03,100 --> 00:08:06,020 So, we have to initialize a string variable to hold the digits first or 153 00:08:06,020 --> 00:08:07,130 Python will give an error. 154 00:08:07,130 --> 00:08:10,910 Because it obviously won't know if a plus should attempt to perform an addition or 155 00:08:10,910 --> 00:08:11,750 concatenation. 156 00:08:11,750 --> 00:08:16,390 So the way we do that is we come back up here and we'll type something like 157 00:08:16,390 --> 00:08:21,580 cleanedNumber = '', so that's what we're gonna start with. 158 00:08:21,580 --> 00:08:23,220 We're gonna leave the loop as it was before. 159 00:08:23,220 --> 00:08:25,300 We're gonna leave the if there as well. 160 00:08:25,300 --> 00:08:27,570 But instead of printing out at this point, 161 00:08:27,570 --> 00:08:32,750 what we're in fact going to do is we're gonna type cleanedNumber = cleanedNumber 162 00:08:32,750 --> 00:08:38,370 + number[i], I'm gonna rid of that print. 163 00:08:38,370 --> 00:08:40,970 Now back to the left, so it's gonna go through and each time it finds a number. 164 00:08:40,970 --> 00:08:44,860 It's gonna add only that digit to the cleaned number string that we've actually 165 00:08:44,860 --> 00:08:45,610 defined on line nine. 166 00:08:45,610 --> 00:08:49,110 So now what we want to do is convert that to an integer. 167 00:08:49,110 --> 00:08:54,030 So we type newNumber = int(cleanedNumber) 168 00:08:55,540 --> 00:08:56,700 And I could type something like print, 169 00:08:56,700 --> 00:09:00,330 because as far as Python's concerned this is now a number. print( 170 00:09:00,330 --> 00:09:06,320 "The number is".format(newNumber)) 171 00:09:06,320 --> 00:09:07,190 So if we run this. 172 00:09:08,280 --> 00:09:11,300 We get the number is 92 blah, blah, blah, blah, blah 807. 173 00:09:11,300 --> 00:09:14,110 So as far as Python'ss concerned that is now a proper number. 174 00:09:14,110 --> 00:09:17,120 So that's how you'd actually go about converting from a string and 175 00:09:17,120 --> 00:09:20,110 then actually building up a number that's actually in a string. 176 00:09:20,110 --> 00:09:22,460 And then converting that string back to an integer. 177 00:09:22,460 --> 00:09:26,100 Thing here is that, you need to actually be very careful with your indentation, 178 00:09:26,100 --> 00:09:29,000 because we're nesting blocks within blocks and the For loop contains an 179 00:09:29,000 --> 00:09:32,460 if block, which contains the block that performs the concatenation. 180 00:09:32,460 --> 00:09:37,620 So, if line 15 is actually indented by 4 or 8 spaces, then the code's still gonna work, 181 00:09:37,620 --> 00:09:39,950 but newNumber's gonna be assigned a value each time through the loop. 182 00:09:39,950 --> 00:09:43,690 So, in other words if I actually tab that, like so, 183 00:09:44,970 --> 00:09:47,960 actually run this and we still go the same result. 184 00:09:47,960 --> 00:09:51,830 But what's actually happened here is each time in this loop new 185 00:09:51,830 --> 00:09:54,020 Number's been assigned that value. 186 00:09:54,020 --> 00:09:55,020 And it doesn't make, it's not 187 00:09:55,020 --> 00:09:57,660 really, it's not a really problem here when you've only got a few numbers to deal with. 188 00:09:57,660 --> 00:10:01,620 If you can imagine that if we had 10,000 numbers to actually deal with. 189 00:10:01,620 --> 00:10:04,470 You're actually repeating this assignment, of 190 00:10:04,470 --> 00:10:09,000 the value int(cleanedNumber) each time we actually run, so it's a lot processing. 191 00:10:09,000 --> 00:10:11,690 So, the end result could be significant delays you're introducing, 192 00:10:11,690 --> 00:10:13,840 even if the final answer, as you saw, was still correct. 193 00:10:13,840 --> 00:10:17,660 So, in this case, it's far better to actually have that calculation 194 00:10:17,660 --> 00:10:21,550 do occur only once, and that's after you've been through the entire for loop. 195 00:10:21,550 --> 00:10:25,570 and we built up the entire cleanedNumber string performed only once 196 00:10:25,570 --> 00:10:28,160 as opposed to actually performing it each time through the loop. 197 00:10:28,160 --> 00:10:31,110 Now, note this is a specific example. 198 00:10:31,110 --> 00:10:33,250 Where our orignal number came from a spreadsheet program. 199 00:10:33,250 --> 00:10:35,400 We knew that it was a valid number. 200 00:10:35,400 --> 00:10:38,700 Just, you know they happen to contain some extraneous formatting. 201 00:10:38,700 --> 00:10:41,200 So this code that we've actually written, wouldn't be suitable for 202 00:10:41,200 --> 00:10:44,440 validating numbers that had been typed on the keyboard, for example. 203 00:10:44,440 --> 00:10:47,710 And also it wouldn't be suitable if the spreadsheet contains scientific data, 204 00:10:47,710 --> 00:10:51,040 where the numbers could be in an exponential or scientific format, 205 00:10:51,040 --> 00:10:53,110 such as 1.353e+18. 206 00:10:53,110 --> 00:10:58,200 So we're gonna be looking at catering both of those or catering for 207 00:10:58,200 --> 00:11:00,420 both those situations in future videos. 208 00:11:00,420 --> 00:11:01,520 So I'm gonna end this video here. 209 00:11:01,520 --> 00:11:04,530 In the next video, we're gonna continue with our discussion on four loops. 210 00:11:04,530 --> 00:11:05,870 So I'll see you in that next video.