Check for Understanding 2

Question 1 - Command-Line Arguments

Recall that in the lecture, we saw how to write programs in C that support command-line arguments. To do so, we’ve modified the programs main function to take two arguments: argc and agrv.

Question 1.1 - Think Back…

What is a program that we have used so far in CS50 that accepts one or more command-line arguments? Name the program, describe what it does (briefly), and describe what the command-line arguments are used for.

Question 1.2 - argc and argv

In your own words describe what argc is, what it stores and what type it is.

In your own words describe what argv is, what it stores and what type it is.

Question 2 - Strings

Consider the two for loops, below, both of which print the characters of a string, s, one character per line.

// Version 1
for (int i = 0; i < strlen(s); i++)
{
    printf("%c\n", s[i]);
}

// Version 2
for (int i = 0, n = strlen(s); i < n; i++)
{
    printf("%c\n", s[i]);
}

In your own words, describe what strlen does underneath the hood.

Which version out of the two is more efficient? Why?

Question 3 - More on Strings

Consider the two programs in C below.

Version 1

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

int main(void) {
    string text = "This is CS50";
    for (int i = 0, length = strlen(text); i < length; i++)
    {
        printf("%c", text[i]);
    }
    printf("\n");
}

Version 2

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

int main(void) {
    string text = "This is CS50";
    for (int i = 0, length = strlen(text); i < length; i++)
    {
        printf("%i", text[i]);
    }
    printf("\n");
}

Notice that each program “iterates” over the given variable, text, character by character. The output of Version 1 is, predictably, “This is CS50.” But the output of Version 2 is “84104105115321051153267835348.”

In your own words, what is different between the two programs? How does the output of Version 1 relate to the output of Version 2?