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Loops

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Notes

The loop blocks you've used in Scratch can all be recreated in C!

Loops allow for the repetition of code until a condition is met.

There are three types of loops commonly used in C:

  • for
  • while
  • do while

Here's how a for loop works:

Variables are initialized.

The condition is evaluated.

  • If the condition is true, the code in the loop's body runs, variables are updated, and the condition is reevaluated.
  • If the condition is false, the loop terminates.

This for loop in C is analogous in functionality to the scratch blocks above it.

  • i is initialized to 0.
  • "This is CS50!" prints.
  • i increments.
  • The loop terminates when i is equal to 10, and "This is CS50!" will be printed a total of 10 times.

This for loop iterates through the length of a string, capitalizing all alphabetic characters.

Note how we've initialized two variables, i and j, through the use of a comma.

Here's how a while loop works:

The condition is evaluated.

  • If the condition is true, the code in the loop's body runs, and the condition is reevaluated.
  • If the condition is false, the loop terminates.

This while loop in C is analogous in functionality to the scratch blocks on the left.

  • The condition is evaluated.
  • count will be printed and decremented.
  • The loop terminates when count < 0, and the numbers 10 through 0 have been printed out in descending order.

This while loop iterates through a string, counting each character until the null terminator (\0) is reached.

Here's how a do while loop works:

The code in the loop's body runs once.

The condition is evaluated.

  • If the condition is true, the code in the loop's body runs again, and the condition is reevaluated.
  • If the condition is false, the loop terminates.

Do while loops are ideal for vetting user input.

  • The user is prompted to enter a positive number.
  • If the number is less than 1, the user will be reprompted to enter a positive number.

Slides ( / )

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Tips and Tricks (/)

If there’s just a single line of code in a loop’s body, curly braces are unnecessary.

So, both of these are valid:

for (int i = 0; i < 3; i++)
    for (int j = 3; j > 0; j--)
        printf("i = %d, j = %d \n", i, j);
for (int i = 0; i < 3; i++)
{
    for (int j = 3; j > 0; j--)
    {
        printf("i = %d, j = %d \n", i, j);
    }
}

The for loop’s initialization, condition, and update are separated by semicolons — not commas!

Do:

for (int i = 0; i < 10; i++)
    printf("This is CS50!\n");

Don't:

for (int i = 0, i < 10, i++)
    printf("This is CS50!\n");

Keep computationally-intensive processes like strlen() out of the loop’s condition. Here, we place strlen() in the initialization instead, so it’s only executed once rather than on every iteration.

Do:

for (int i = 0, j = strlen(name); i < j; i++)
    name[i] = toupper(name[i]);

Don't:

for (int i = 0; i < strlen(name); i++)
    name[i] = toupper(name[i]);

A common bug is to add a semicolon after the loop’s declaration. This may cause you to wonder why the code in your loop isn’t executing or only executes once. The reason is that the loop is working, but it’s performing a no-op (empty statement) each time through the loop, assuming that the semi-colon is the statement you want to perform on each iteration.

Do:

for (int i = 0; i < 10; i++)
    printf("This is CS50!\n");

Don't:

for (int i = 0; i < 10; i++);
    printf("This is CS50!\n");
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While Loop Conversion

Rewrite the following while loop as a do while loop:

int n = 0;
while (n <= 0)
{
    printf("Please enter a positive integer: ");
    n = GetInt();
}

Try out some pseudocode here! 
#include <cs50.h>
#include <stdio.h>

int main(int argc, string argv[])
{
    // TODO
}

Sample Pseudocode

Download Pseudocode 
#include <cs50.h>
#include <stdio.h>

int main(int argc, string argv[])
{
    // do

        // print instructions for user
        // get int from user

    // while n is not positive

}

Sample Code

Download Solution 
#include <cs50.h>
#include <stdio.h>

int main(int argc, string argv[])
{
    int n;

    // do
    do
    {
        // print instructions for user
        printf("Please enter a positive integer: ");
        // get int from user
        n = GetInt();
    }
    // while n is not positive
    while (n < 1);
}

Square

Take as input a non-negative integer, n, no larger than 23. Use nested for loops to print an n-by-n square using hashes (#) at the command line.

jharvard@run.cs50.net (~): ./a.out
Give me a number between 1 and 23: 3
###
###
###

Try out some pseudocode here! 
#include <cs50.h>
#include <stdio.h>

int main(int argc, string argv[])
{
    // TODO
}

Sample Pseudocode

Download Pseudocode 
#include <cs50.h>
#include <stdio.h>

int main(int argc, string argv[])
{
    // get dimension from user

    // iterate across height

    // iterate across width

    // print #

    // move on to next line
}

Sample Code

Download Solution 
#include <cs50.h>
#include <stdio.h>

int main(int argc, string argv[])
{
    int n;

    // get dimension from user
    do
    {
        printf("Give me a number between 1 and 23: ");
        n = GetInt();
    }
    while (n < 1 || n > 23);

    // iterate across height
    for (int i = 0; i < n; i++)
    {
        // iterate across width
        for (int i = 0; i < n; i++)

            // print #
            printf("#");

        // move on to the next line
        printf("\n");
    }
}

Multiples

Prerequisites:

Take as input a number between 1 and 100. Use a for loop to print every multiple of that number between 1 and 100 (inclusive).

jharvard@run.cs50.net (~): ./a.out
Give me a number between 1 and 100: 5
5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100

Try out some pseudocode here! 
#include <cs50.h>
#include <stdio.h>

int main(int argc, string argv[])
{
    // TODO
}

Sample Pseudocode

Download Pseudocode 
#include <cs50.h>
#include <stdio.h>

int main(int argc, string argv[]) {
    // get multiple from user

    // iterate from multiple through 100

    // if not first, print comma and space

    // print number

}

Sample Code

Download Solution 
#include <cs50.h>
#include <stdio.h>

int main(int argc, string argv[])
{
    int multiple;

    // get multiple from user
    do
    {
        printf("Give me a number between 1 and 100: ");
        multiple = GetInt();
    }
    while (multiple <= 0 || multiple > 100);


    // iterate from multiple through 100
    for (int i = multiple; i <= 100; i = i + multiple)
    {
        // if not first, print comma and space
        if (i != multiple)
            printf(", ");

        // print number
        printf("%i", i);
    }

    printf("\n");
}

Myspace

Prerequisites:

Write a program that prompts the user for a message, and then outputs the message with its first letter capitalized, with all letters in alternating case, as per the sample output below. For simplicity, you may assume that the user will only input lowercase letters and spaces.

jharvard@run.cs50.net (~): ./a.out
thanks for the add
ThAnKs FoR tHe AdD

Try out some pseudocode here! 
#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>

int main (int argc, string argv[])
{

	// TODO

}

Sample Pseudocode

Download Pseudocode 
#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>

int main(int argc, string argv[])
{
    // get string from user

    // for each character of string

        // if character is a letter

            // if letter should be capitalized

                // print capitalized letter

            // else

                // print lowercase letter

        // if character is a space

            // print space

}

Sample Code

Download Solution 
#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>

int main(int argc, string argv[])
{
    // get string from user
    string input = GetString();

    bool capitalize = true;

    // for each character of string
    for (int i = 0, j = strlen(input); i < j; i++)
    {
        // if character is a letter
        if (isalpha(input[i]))
        {
            // if letter should be capitalized
            if (capitalize == true)

                // print capitalized letter
                printf("%c", toupper(input[i]));

            else

                // print lowercase letter
                printf("%c", input[i]);

            // flip bool
            capitalize = !capitalize;
        }

        // if character is a space
        else

            // print space
            printf("%c", input[i]);
    }

}

For Loop Conversion

Prerequisites:

Rewrite the following double for loop as a single for loop. Make sure it prints exactly the same output, even though you may not have variables i and j around.

for (int i = 0; i < 5; i++)
    for (int j = 0; j < 5; j++)
        printf("i = %d, j = %d\n", i, j);

Try out some pseudocode here! 
#include <cs50.h>
#include <stdio.h>

int main(int argc, string argv[])
{
    // TODO
}

Sample Pseudocode

Download Pseudocode 
#include <cs50.h>
#include <stdio.h>

int main(int argc, string argv[])
{
    // iterate from 0 through 25

        // use division and mod to print combos of numbers
}

Sample Code

Download Solution 
#include <cs50.h>
#include <stdio.h>

int main(int argc, string argv[])
{
    // iterate from 0 through 25
    for (int i = 0; i < 25; i++)

        // use division and mod to print combos of numbers
        printf("i = %i, j = %i\n", i / 5, i % 5);
}

Videos

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Monday, Week 1

An introduction to loops
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Tommy's Loops Short

Tommy gives a general overview