CS102B - Lesson 4

While Loops

• Continues to run until the expression inside the while is terminated
• Commonly used with some form of counter

  expr1;
  while (expr2)
  {
    statement
    expr3;
  }

While Loops Example

#include <stdio.h>

/* K&R pg. 11
 * print Fahrenheit-Celsius table 
 * for fahr = 0, 20, ..., 300 */

int main()
{
    float fahr, celsius;
    int lower, upper, step;

    lower = 0; /* lower limit of temperature table */
    upper = 300; /* upper limit */
    step = 20; /* step size*/

    fahr = lower;
    while (fahr <= upper)
    {
        celsius = 5.0 * (fahr-32.0) / 9.0;
        printf("%3.0f %6.1f\n", fahr, celsius);
        fahr = fahr + step;
    }
    return 0;
}

• Note that fahr increases by 20 per iteration.

Infinite Loop

#include <stdio.h>

int main()
{
    int i = 0;
    while (i >= 0)
    {
        printf("This is an infinite loop! (or is it?) i=%d\n", i);
        i++;
    }
    return 0;
}

• How will you make the above loop truly infinite?

For Loops

• Similar to while, except in one line

for (expr1; expr2; expr3)
    statement

• A typical expr3 is i++

For Loops Example

#include <stdio.h>

// K&R pg. 13
// Print Fahrenheit-Celsius table
int main()
{
    int fahr;
    for (fahr = 0; fahr <= 300; fahr += 20)
    {
        printf("%3d %6.1f\n", fahr, (5.0/9.0)*(fahr-32));
    }

    return 0;
}

Declaring and Accessing Arrays

#include <stdio.h>

int main()
{
    int intArr[5];
    intArr[0] = 0;
    intArr[1] = 1;
    intArr[2] = 2;
    intArr[3] = 3;
    intArr[4] = 4;
    printf("%d %d %d %d %d\n",
           intArr[0], intArr[1], intArr[2],
           intArr[3], intArr[4]);

    char charArr[3] = {'A', 'B', 'C'};
    for (int ii=0; ii<3; ii++)
    {
        printf("%c ", charArr[ii]);
    }
    printf("\n");
    return 0;
}

Count Digits, White Space, and Others

#include <stdio.h>

// K&R pg. 22
// count digits, white space, others

int main()
{
    int c, i, nwhite, nother;
    int ndigit[10];

    nwhite = nother = 0;
    for (i = 0; i < 10; ++i)
    {
        ndigit[i] = 0;
    }

    while ((c = getchar()) != EOF)
    {
        if (c >= '0' && c <= '9')
            ++ndigit[c-'0'];
        else if (c == ' ' || c == '\n' || c == '\t')
            ++nwhite;
        else
            ++nother;
    }

    printf("digits =");
    for (i = 0; i < 10; ++i)
        printf(" %d", ndigit[i]);
    printf(", white space = %d, other = %d\n",
        nwhite, nother);

    return 0;
}

• Run the program against itself with ./a.exe < count.c
• How would you convert this to using a switch statement?

atoi

#include <stdio.h>
#include <ctype.h>

// K&R pg. 61
// atoi: convert s to integer; version 1
int main()
{
    char s[] = "-10";

    int i, n, sign;

    for (i = 0; isspace(s[i]); i++) // skip white space
        ;
    sign = (s[i] == '-') ? -1 : 1;
    if (s[i] == '+' || s[i] == '-') // skip sign
        i++;
    for (n = 0; isdigit(s[i]); i++)
        n = 10 * n + (s[i] - '0');

    int result = sign * n;
    printf("%d", result);
    return 0;
}

• Note that the functionality is living in main.
• Note that a lot of curly braces are not necessary.
• How would you modify this to use user input using getchar()?

atoi with getchar()

#include <stdio.h>
#include <ctype.h>

// K&R pg. 61
// atoi: convert s to integer; version 1
int main()
{
    char s[5]; // needs a const
    char c = getchar();
    int cidx = 0;
    while (c != '\n')
    {
        s[cidx] = c;
        c = getchar();
        cidx++;
    }


    int i, n, sign;

    for (i = 0; isspace(s[i]); i++) // skip white space
        ;
    sign = (s[i] == '-') ? -1 : 1;
    if (s[i] == '+' || s[i] == '-') // skip sign
        i++;
    for (n = 0; isdigit(s[i]); i++)
        n = 10 * n + (s[i] - '0');

    int result = sign * n;
    printf("%d", result);
    return 0;
}

Shell Sort

#include <stdio.h>

// K&R pg. 62
// shellsort: sort v[0]...v[n-1] into increasing order
int main()
{
    int v[10] = {4, 6, 2, 1, 3, 5, 7, 9, 8, 0};
    int n = 10;
    int gap, i, j, temp;
    for (gap = n/2; gap > 0; gap /= 2)
        for (i = gap; i < n; i++)
            for (j=i-gap; j>=0 && v[j]>v[j+gap]; j-=gap)
            {
                temp = v[j];
                v[j] = v[j+gap];
                v[j+gap] = temp;
            }

    printf("Sorted: ");
    for (int ii=0; ii<n; ii++)
        printf("%d ", v[ii]);

    return 0;
}

• Note the nested loops.
• You will learn more about sorts in DSA I.

Reverse

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

// K&R pg. 62
// reverse: reverse string s in place

int main()
{
    char s[] = "CS102B";

    int c, i, j;
    for (i=0, j=strlen(s)-1; i<j; i++, j--)
    {
        c = s[i];
        s[i] = s[j];
        s[j] = c;
    }
    printf("%s", s);
    return 0;
}

• Note the complex expressions in the for loop.
• Strings are just arrays of chars!

Do While

• The do will always execute first
• This loop will end when the while part is not satisfied.

do
    statement
while (expression);

Do While Example

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

// K&R pg. 64
// itoa: convert n to characters in s

int main()
{
    int n = -77;
    char s[10];

    int idx, sign;
    if ((sign = n) < 0) // record sign
        n = -n;         // make n positive
    idx = 0;
    do {
        s[idx++] = n % 10 + '0';
    } while ((n /= 10) > 0);
    if (sign < 0)
        s[idx++] = '-';
    s[idx] = '\0';

    // reverse s
    int c, i, j;
    for (i=0, j=strlen(s)-1; i<j; i++, j--)
    {
        c = s[i];
        s[i] = s[j];
        s[j] = c;
    }
    printf("%s", s);
    return 0;
}

Break & Continue

• break -- allows you to exist a for, while, or do loop early
• continue -- allows the next iteration of the loop to execute

Break Example

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

// K&R pg. 65
// trim: remove trailing blanks, tabs, newlines
int main()
{
    char s[] = "HELLO   ";
    int n;
    for (n = strlen(s)-1; n >= 0; n--)
        if (s[n] != ' ' && s[n] != '\t' && s[n] != '\n')
            break;
    s[n+1] = '\0';
    printf("%d\n", n);
    printf("%s.", s);

    return 0;
}

Continue Example

#include <stdio.h>

// skip printing a number with continue

int main()
{
    for (int ii=0; ii<=10; ii++)
    {
        if (ii==7)
            continue;
        printf("%d ", ii);
    }

    return 0;
}

goto

• DO NOT USE
• Allows you to go to other lines in your program directly

scanf

• Don't worry about what the syntax means for now

#include <stdio.h>

// K&R pg. 158
// rudimentary calculator

int main()
{
    double sum, v;
    sum = 0;
    while (scanf("%lf", &v) == 1)
        printf("\t%.2f\n", sum+=v);

    return 0;
}

Arguments & Return Values

• Functions splits up the main function to more readable chunks
• Generally, you don't want functions to be too large (<100 lines)
• Functions are in the form:

  return-type function-name(parameter declarations, if any)
  {
    declarations
    statements
    return
  }

Example of Using A Function

#include <stdio.h>

// K&R pg. 24-25
// power function

// Function prototype at the beginning
int power(int m, int n);

int main()
{
    int i;
    for (i=0; i<10; ++i)
        printf("%d %d %d\n", i, power(2,i), power(-3,i));
    return 0;
}

// power: raise base to n-th power; n >= 0
int power(int base, int n)
{
    int i, p;
    p = 1;
    for (i=1; i<=n; ++i)
        p = p * base;
    return p;
}