How does this C for-loop print text-art pyramids?

Question

This is my first time posting in here, hopefully I am doing it right.

Basically I need help trying to figure out some code that I wrote for class using C. The purpose o

Answer 1

for ( int tall = 0; tall < user_i; tall++ ) { ... }

I think of this in natural language like this:

1. int tall = 0; // Start with an initial value of tall = 0
2. tall < user_i; // While tall is less than user_i, do the stuff between the curly braces
3. tall++; // At the end of each loop, increment tall and then recheck against the condition in step 2 before doing another loop

And with nice indentation in your code now it's much easier to see how the for loops are nested. The inner loops are going to be run for every iteration of the outer loop.

Answer 2

The simplest answer to why that is happening is because one loop prints spaces like this represented by -.

----------
---------
--------
-------
------
-----

and so on. The hashes are printed like this

------#
-----##
----###
---####
--#####
-######

since hash printing loop at second stage needs to print equal no of hashes more to complete the pyramid it can be solved by two methods one by copying and pasting hash loop twice or by making the loop run twice next time by following modification

for ( int hash = 0; hash <= tall*2; hash++ )
        {
            printf ( "#" );
        }

The logic to build such loops is simple the outermost loop prints one line feed to seperate loops,the inner loops are responsible for each lines contents. The space loop puts spaces and hash loop appends hash at end of spaces. [My answer can be redundant because i did not read other answers so much carefully,they were long] Result:

       #
      ###
     #####
    #######
   #########
  ###########

Answer 3

This type of small programs are the one which kept fascinating me in my earlier days. I think they play a vital role in building logics and understand how, when, where and in which situation which loop will be perfect one.
The best way one can understand what happening is to manually debug each and every statement.
But to understand better lets understand how to build the logic for that, I am making some minor changes so that we can understand it better,

• n is no of lines to print in pyramid n =5
• Replacing empty spaces ' ' with '-' (dash symbol)

Now pyramid will look something like,

                            ----#
                            ---##
                            --###
                            -####
                            #####

Now steps to design the loop,

• First of all we have to print n lines i.e. 5 lines so first loop will be running 5 times.
  for (int rowNo = 0; rowNo < n; rowNo++ ) the Row number rowNo is similar to tall in your loop
• In each line we have to print 5 characters but such that we get our desired figure, if we closely look at what logic is there,
  rowNo=0 (Which is our first row) we have 4 dashes and 1 hash
  rowNo=1 we have 3 dashes and 2 hash
  rowNo=2 we have 2 dashes and 3 hash
  rowNo=3 we have 1 dashes and 4 hash
  rowNo=4 we have 0 dashes and 5 hash
• Little inspection can reveal that for each row denoted by rowNo we have to print n - rowNo - 1 dashes - and rowNo + 1 hashes #,
  Therefore inside our first for loop we have to have two loops, one to print dashes and one to print hashes.
  Dash loop will be for (int dashes= 0; dashes < n - rowNo - 1; dashes ++ ), here dashes is similar to space in your original program
  Hash loop will be for (int hash = 0; hash < rowNo + 1; dashes ++ ),
• The last step after every line we have to print a line break so that we can move to next line.

Hope, the above explanation provides a clear understanding how the for loops that you have written will be formulated.

In your program there are only minor changes then my explanation, in my loops I have used less then < operator and you have used <= operator so it makes difference of one iteration.

Answer 4

I will explain the process of how I would come to understand this code to the point where I would be comfortable using it myself. I will pretend that I did not read you description, so I am starting from scratch. The process is divided into stages which I will number as I go. My goal will be to give some general techniques which will make programs easier to read.

Stage 1: Understand coarse structure

The first step will be to understand in general terms what the program does without getting bogged down in details. Let's start reading the body of the main function.

int main(void) {
    int user_i;
    printf("Hello there and welcome to the pyramid creator program\n");
    printf("Please enter a non negative INTEGER from 0 to 23\n");
    scanf("%d", &user_i);

So far, we have declared in integer, and told the user to enter a number, and then used the scanf function to set the integer equal to what the user entered. Unfortunately it is unclear from the prompt or the code what the purpose of the integer is. Let's keep reading.

    while (user_i < 0 || user_i > 23) {
        scanf("%d", &user_i);
    }

Here we possibly ask the user to enter additional integers. Judging by the prompt, it seems like a good guess that the purpose of this statement is to make sure that our integer is in the appropriate range and this is easy to check by examining the code. Let's look at the next line

     for (int tall = 0; tall < user_i; tall++) {

This is the outer for loop. Our enigmatic integer user_i appears again, and we have another integer tall which is going between 0 and user_i. Let's look at some more code.

        for (int space = 0; space <= user_i - tall; space++) {
            printf(" ");
        }

This is the first inner for loop. Let's not get bogged down in the details of what is going on with this new integer space or why we have user_i - tall appearing, but let's just note that the body of the foor loop just prints a space. So this for loop just has the effect of printing a bunch of spaces. Let's look at the next inner for loop.

        for (int hash = 0; hash <= tall; hash++) {
            printf("#");
        }

This one looks similar. It just prints a bunch of hashes. Next we have

        printf("\n");

This prints a new line. Next is

    }

    return 0;
}

This means that the outer for loop ends, and after the outer for loop ends, the program ends.

Notice we have found two major pieces to the code. The first piece is where a value to user_i is obtained, and the second piece, the outer for loop, must be where this value is used to draw the pyramid. Next let's try to figure out what user_i means.

Stage 2: Discover the meaning of user_i

Now since a new line is printed for every iteration of the outer loop, and the enigmatic user_i controls how many iterations of the outer loop there are, and therefore how many new lines are printed, it would seem that user_i controls the height of the pyramid that is created. To get the exact relationship, lets assume that user_i had the value 3, then tall would take on the values 0,1, and 2, and so the loop would execute three times and the height of the pyramid would be three. Notice also that if user_i would increase by one, then the loop would execute once more and the pyramid would be higher by one. This means that user_i must be the height of the pyramid. Let's rename the variable to pyramidHeight before we forget. Our main function now looks like this:

int main(void) {
    int pyramidHeight;
    printf("Hello there and welcome to the pyramid creator program\n");
    printf("Please enter a non negative INTEGER from 0 to 23\n");
    scanf("%d", &pyramidHeight);
    while (pyramidHeight < 0 || pyramidHeight > 23) {
        scanf("%d", &pyramidHeight);
    }

    for (int tall = 0; tall < pyramidHeight; tall++) {
        for (int space = 0; space <= pyramidHeight - tall; space++) {
            printf(" ");
        }
        for (int hash = 0; hash <= tall; hash++) {
            printf("#");
        }
        printf("\n");
    }

    return 0;
}

Stage 3: Make a function that gets the pyramid height

Since we understand the first part of the code, we can move it into a function and not think about it anymore. This will make the code easier to look at. Since this part of the code is responsible for obtaining a valid height, let's call the funcion getValidHeight. After doing this, notice the pyramid height will not change in the main method, so we can declare it as a const int. Our code now looks like this:

#include <stdio.h>

const int getValidHeight() {
    int pyramidHeight;
    printf("Hello there and welcome to the pyramid creator program\n");
    printf("Please enter a non negative INTEGER from 0 to 23\n");
    scanf("%d", &pyramidHeight);
    while (pyramidHeight < 0 || pyramidHeight > 23) {
        scanf("%d", &pyramidHeight);
    }
    return pyramidHeight;
}

int main(void) {
    const int pyramidHeight = getValidHeight();
    for (int tall = 0; tall < pyramidHeight; tall++) {
        for (int space = 0; space <= pyramidHeight - tall; space++) {
            printf(" ");
        }
        for (int hash = 0; hash <= tall; hash++) {
            printf("#");
        }
        printf("\n");
    }

    return 0;
}

Stage 4: understand the inner for loops.

We know the inner for loops print a character repeatedly, but how many times? Let's consider the first inner for loop. How many spaces are printed? You might think that by analogy with the outer for loop there are pyramidHeight - tall spaces, but here we have space <= pyramidHeight - tall where the truly analogous situation would be space < pyramidHeight - tall. Since we have <= instead of <, we get an extra iteration where space equals pyramidHeight - tall. Thus we see that in fact pyramidHeight - tall + 1 spaces are printed. Similarly tall + 1 hashes are printed.

Stage 5: Move printing of multiple characters into their own functions.

Since printing a character multiple times is easy to understand, we can move this code into their own functions. Let's see what our code looks like now.

#include <stdio.h>

const int getValidHeight() {
    int pyramidHeight;
    printf("Hello there and welcome to the pyramid creator program\n");
    printf("Please enter a non negative INTEGER from 0 to 23\n");
    scanf("%d", &pyramidHeight);
    while (pyramidHeight < 0 || pyramidHeight > 23) {
        scanf("%d", &pyramidHeight);
    }
    return pyramidHeight;
}

void printSpaces(const int numSpaces) {
    for (int i = 0; i < numSpaces; i++) {
        printf(" ");
    }
}

void printHashes(const int numHashes) {
    for (int i = 0; i < numHashes; i++) {
        printf("#");
    }
}

int main(void) {
    const int pyramidHeight = getValidHeight();
    for (int tall = 0; tall < pyramidHeight; tall++) {
        printSpaces(pyramidHeight - tall + 1);
        printHashes(tall + 1);
        printf("\n");
    }

    return 0;
}

Now when I look at the main function, I don't have to worry about the details of how printSpaces actually prints the spaces. I have already forgotten if it uses a for loop or a while loop. This frees my brain up to think about other things.

Stage 6: Introducing variables and choosing good names for variables

Our main function is now easy to read. We are ready to start thinking about what it actually does. Each iteration of the for loop prints a certain number of spaces followed by a certain number of hashes followed by a new line. Since the spaces are printed first, they will all be on the left, which is what we want to get the picture it gives us.

Since new lines are printed below old lines on the terminal, a value of zero for tall corresponds to the top row of the pyramid.

With these things in mind, let's introduce two new variables, numSpaces and numHashes for the number of spaces and hashes to be printed in an iteration. Since the value of these variables does not change in a single iteration, we can make them constants. Also let's change the name of tall (which is an adjective and hence a bad name for an integer) to distanceFromTop. Our new main method looks like this

int main(void) {
    const int pyramidHeight = getValidHeight();

    for (int distanceFromTop = 0; distanceFromTop < pyramidHeight; distanceFromTop++) {
        const int numSpaces = pyramidHeight - distanceFromTop + 1;
        const int numHashes = distanceFromTop + 1;
        printSpaces(numSpaces);
        printHashes(numHashes);
        printf("\n");
    }

    return 0;
}

Stage 7: Why are numSpaces and numHashes what they are?

Everything is coming together now. The only thing left to figure out is the formulas that give numSpaces and numHashes.

Let's start with numHashes because it is easier to understand. We want numHashes to be one when the distance from the top is zero, and we want numHashes to increase by one whenever the distance from the top does, so the correct formula is numHashes = distanceFromTop + 1.

Now for numSpaces. We know that every time the distance from the top increases, a space turns into a hash, and so there is one less space. Thus the expression for numSpaces should have a -distanceFromTop in it. But how many spaces should the top row have? Since the top row already has a hash, there are pyramidHeight - 1 hashes that need to be made, so there must be at least pyramidHeight - 1 spaces so they can be turned into hashes. In the code we have chosen pyramidHeight + 1 spaces in the top row, which is two more than pyramidHeight - 1, and so has the effect of moving the whole picture right by two spaces.

Conclusion

You were only asking how the two inner loops worked, but I gave a very long answer. This is because I thought the real problem was not that you didn't understand how for loops work sufficiently well, but rather that your code was difficult to read and so it was hard to tell what anything was doing. So I showed you how I would have written the program, hoping that you would think it is easier to read, so you would be able to see what is going on more clearly, and hopefully so you could learn to write clearer code yourself.

How did I change the code? I changed the names of the variables so it was clear what the role of each variable was; I introduced new variables and tried to give them good names as well; and I moved some of the lower level code involving input and output and the logic of printing characters a certain number of times into their own methods. This last change greatly decreased the number of lines in the main function , got rid of the nesting of for loops in the main function, and made the key logic of the program easy to see.

Answer 5

A few things:

• Consider "bench checking" this. That is, trace through the loops yourself and draw out the spaces and hash marks. Consider using graph paper for this. I've been doing this for 15 years and I still trace things out on paper from time to time when they're hairy.

• The magic in this is the value "user_i - tall" and "hash <= tall". Those are the conditions on the two inner for loops as the middle values in the parenthesis. Note what they are doing:

• Because tall is "going up" from the outer-most loop, by subtracting it from user_i, the loop that prints the spaces is "going down". That is, printing fewer and fewer spaces as it goes.

• Because tall is "going up", and because the hash loop is just basically using it as-is, it's going up too. That is, printing more hash marks as you go.

So really ignore most of this code. It's generic: the outer loop just counts up, and most of the inner loops just do basic initialization (i.e. space = 0 and hash = 0), or basic incrementation (space++ and hash++).

It is only the center parts of the inner loops that matter, and they use the movement of tall from the outermost loop to increment themselves down and up respectively, as noted above, thus making the combination of spaces and hash marks needed to make a half-pyramid. Hope this helps!

Answer 6

Analyzing your loops:
The first loop

for ( int tall = 0; tall < user_i; tall++ ){...}

is controlling the row. The second loop

for ( int space = 0; space <= user_i - tall; space++ ){...}  

for the column to be filled by spaces.
For each row , it will fill all the user_i - tall columns with spaces.
Now the remaining columns are filled by # by the loop

for ( int hash = 0; hash <= tall; hash++ ){...}