Understanding Substr

Question

I have a block of javascript here. Simple script that does as it is supposed to here:

var char, num, i, line, j;

 char = input[0];
 num = input[1];
 l = "";
 var Lines = new Array();

 for ( i=0; i <= num; i++ ) {
       l = l+char+char+char;
             for ( j=0; j <= num; j++ ) {
                   Lines[i] = l.substr(0, 1*i);
        }
           print(i +" " + Lines[i]);
    }

My issue is I do not understand this line: Lines[i] = l.substr(0, 1*i);

I am trying to learn here okay and this is one of the few sites I see that have been helpful so please refrain from assinine remarks. This is not homework. It is me trying to make sense of things since the teacher's way of teaching is not helping with my understanding. In fact the whole class is having an issue.

the desired output is to simply print a character over and over a given number of times while creating a new line so if say M is entered (char) and 6 is entered as the input (num)... the 1st line prints one M the 2nd line prints 2 Ms, 3rd three Ms, and so forth for the given number of times, in this case is 6 making 6 lines and 6 Ms (right triangle I guess you can say. This way can be used to make a diamond too.)

I just do not get where "*i" comes into it. I know substr(0,1) is just the 1st character. I do not see how the current number i is affects the substr result.

Explanation: char is the character entered, num is the number entered. In class we use an HTML file to run the javascript where the print statement is used instead of say document.write and having to save .js files. The teacher does not even get into all that stuff. In case you are wondering the class is Programming Logic 105. I haven't looked at programming since high school during the PASCAL, BASIC and C days. I am not used to the dynamic nature of Javascript. I guess the school deems Javascript as a good open door into programming.

the 2nd argument is a length. it is just multiplying `i` by 1. — Daniel A. White, Nov 19 '14 at 00:26
Tip: Throw away that code and start over. You should only need a single loop. And you don't need to multiply your `i` variable with `1`, it is already a number. — Bergi, Nov 19 '14 at 00:29
throw it away? why? I know there are better ways to do it, I'm sure just due to the extra reading I have done but using say a repeat is not what was taught in the class. — ninja_unmatched, Nov 19 '14 at 00:33
Okay so using a single loop I would then have to change the code in the curly braces of the for loop because the result is not correct. Leaving the code as is but placing it in 1 loop you get 6 lines but the consecutive Ms stops at 3 MMM. — ninja_unmatched, Nov 19 '14 at 00:42

score 0 · Answer 1 · answered Nov 19 '14 at 00:31

0

Using this Lines[i] = l.substr(0, 1+i); instead might do the trick.

answered Nov 19 '14 at 00:31

Anubhav

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This gets the same result when using one loop instead of placing a loop inside a loop. It outputs: M, MM, MMM, MMM, MMM, MMM (each on new lines of course. Just typing it that way cause the site isn't displaying it right. – ninja_unmatched Nov 19 '14 at 00:49
The original code uses while loops. I prefer for loop so I converted it `code`var sp=" "; var c= input[0].charAt(0).toUpperCase(); var i, n, l; l="" n=input[1]; i=0 while (i=0)) {print(Lines[i]); i=i-1;} – ninja_unmatched Nov 19 '14 at 00:58

score 0 · Answer 2 · answered Mar 27 '15 at 16:03

You have three options in Javascript:

//slice
//syntax: string.slice(start [, stop])
"Good news, everyone!".slice(5,9); // extracts 'news'

//substring 
//syntax: string.substring(start [, stop])
"Good news, everyone!".substring(5,9); // extracts 'news'

//substr
//syntax: string.substr(start [, length])
"Good news, everyone!".substr(5,4); // extracts 'news'

Understanding Substr

2 Answers2