CS50’s 2nd Problem Set – Readability

blur book stack books bookshelves

I managed to solve CS50’s “readability” problem some weeks ago, but I couldn’t find the time to explain how I worked it out until today. Anyway, a happy outcome is worth waiting for 🙂

This time, we were challenged to develop an algorithm that computes the grade level needed to comprehend some text (that must be entered by users) with the Coleman-Liau index. Higher reading levels correlate with longer sentences and words.

The index reads as follows:

index = 0.0588 * average number of letters per 100 words - 0.296 * average number of sentences per 100 words  - 15.8

You can find an example of a similar algorithm (maybe this one uses another index) in the Hemingway app: http://www.hemingwayapp.com

Screen Shot 2020-08-04 at 08.37.26

As always, I first wrote some pseudocode to define the steps needed:

  1. Prompt users for the text.
  2. Count the number of letters. Letters can be any uppercase or lowercase alphabetic characters, but shouldn’t include any punctuation, digits, or other symbols.
  3. Print out the letter number count.
  4. Count the number of words in a sentence. Assume that a sentence will not start or end with a space, and will not have multiple spaces in a row.
  5. Print out the word number count.
  6. Count the number of sentences. A sentence is any sequence of characters that ends with “.” or “!” or “?“.
  7. Print out the sentence number count.
  8. Modify the program to output the grade level according to the Coleman-Liau index . Round result to nearest whole number. If the resulting index number is 16 or higher, print out "Grade 16+". If the index number is less than 1, print out "Before Grade 1".

Then, I started to write the code by adding the required libraries:

#include <stdio.h>
#include <cs50.h>
#include <string.h>
#include <ctype.h>
#include <math.h>

Next I prompted the user for the text and counted the number of letters, words and sentences as follows. I always add a lot of comments to the code, I know…it helps me organise the information and quickly identify the steps. Maybe it is not the “cleanest” approach, but it works for me.

I have used three new functions to solve this:

  • strlen (string.h library) -> counts the length of a string (the number of letters in this case)
  • isspace (ctype.h library) -> checks if a character is a white space or not.
  • isalpha (type.h library) -> checks if the character is an alphabet (a-z both upper and lowercase) or not.
int main(void)

//Prompt user for a string of text
string userinput = get_string("Text: ");

// Calculate number of letters with strlen
int lettertotal = strlen(userinput);

int lettercount = 0;

for (int i = 0; (i < lettertotal); i++)
if (isalpha(userinput[i]))
lettercount = lettercount + 1;
//printf ("Number of letters: %i\n", lettercount);
// Calculate number of words with strlen
int wordtotal = strlen(userinput);
// Attention! wordcount must start with 1, not with 0!
int wordcount = 1;

for (int i = 1; (i < wordtotal); i++)
if ((isspace(userinput[i])) && (isalpha(userinput[i + 1])))
wordcount = wordcount + 1;
//printf ("Number of words: %i\n", wordcount);

int sentencetotal = strlen(userinput);

int sentencecount = 0;
for (int i = 0; (i < sentencetotal); i++)
if (userinput[i] == '.' || userinput[i] == '?' || userinput[i] == '!' || userinput[i] == ':') //|| userinput[i] == ';')
sentencecount = sentencecount + 1;
//printf ("Number of sentences: %i\n", sentencecount);

Next I calculated the average numbers of letters and sentences per 100 words (I used the float function for that):

// Calculate average number of letters per 100 words. Float needed.
float averagenumberoflettersper100words = ((float)lettercount * 100) / (float)wordcount;
//printf ("Average number of letters per 100 words: %i\n", (int)averagenumberoflettersper100words);

// Calculate average number of sentences per 100 words
float averagenumberofsentencesper100words = ((float)sentencecount * 100) / (float)wordcount;
//printf ("Average number of sentences per 100 words: %i\n", (int)averagenumberofsentencesper100words);

And finally the grade:

// Calculate grade
float grade = 0.0588 * averagenumberoflettersper100words - 0.296 * averagenumberofsentencesper100words - 15.8;
//float finalgrade = round(grade);

if (grade >= 1 && grade <= 16)

printf("Grade %i\n", (int) round(grade));



if (grade < 1)
printf("Before Grade 1\n");

if (grade > 16)
printf("Grade 16+\n");

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