1. Introduction to Computer Organization & Systems
John Barr
Topics:
Intro to C for C++ programmers
Types in C: int and floating point
C I/O

2. Output: printf
printf(“score = %d\n”,score);

3. Input: scanf
scanf(“%d %d”,&exam1, &exam2);

4. A program to process a character variable
What if you replace %c with %d?
#include <stdio.h>
char ch;
main (){
scanf(“%c”,&ch);
printf(“Original character: %c\n”, ch);
ch++;
printf(“Following character: %c\n”, ch); }

5. A program to process a character variable (Cont’d)
Using %c
Using %d
bash-2.03$ a.out s
Original character: 115
Following character: 116
The ASCII code
for a “s”

6. ASCII We’ll talk more about this next week!
These assignments also work in UNICODE!

7. Error Checking What if the user enters the wrong type?
C will convert characters to int
But will (normally) only read a single character
Example
#include <stdio.h>
int main() {
int x = 0;
while (x != -1){
printf("Enter an integer: ");
scanf("%d", &x); }
return 0;
See what happens on the next slide
This example is in Student/comp210/examples/charErr.c

8. Error Checking Admins-Computer-52:~/Classes/CS210/PractSoln barr$ !g
gcc testPurge0.c
Admins-Computer-52:~/Classes/CS210/PractSoln barr$ !a
a.out
Enter an integer: 5
Enter an integer: 6
Enter an integer: a
Enter an integer:
How to stop infinite loops: ^c
^z then us ps and kill -9
Continues forever

9. Error Checking What if the user enters the wrong type?
C will convert characters to int
But will (normally) only read a single character
#include <stdio.h>
#include <stdio_ext.h>
int main() {
int x = 0;
while (x != -1){
printf("Enter an integer: ");
scanf("%d", &x);
__fpurge(stdin); }
return 0;
__fpurge works in Linux. Note the two underscores before the name fpurge
See “man __fpurge”
Note: fpurge is only available in BSD 4.4.

10. Char ^d is the EOF symbol
#include <stdio.h> int lower(int c) { if (c >= 'A' && c <= 'Z') return(c + 'a' - 'A'); else return(c); } int main() int c; printf("Enter some characters. To end input hit ^D on a new line\n"); /* read char until end of file */ while ( (c = getchar()) != EOF) putchar(lower(c));
^d is the EOF symbol
See /home/barr/Student/comp210/examples/str1.c

11. Strings as arrays Continued on next slide
#include <stdio.h>
int strequal(char x[ ], char y[ ]) {
int i=0;
if (x == y) return(1);
while (x[i] == y [i])
if (x[i] == '\0')
return (1);
i++; }
return(0);
Continued on next slide
See /home/barr/Student/comp210/examples/str2.c

12. Strings as arrays
int main() { char str1[10],str2[10]; printf("Enter a string\n"); scanf("%s ",str1); printf("\nEnter another string\n"); scanf("%s ”,str2); if (strequal(str1,str2)) printf(“The strings are equal\n”); else printf(“The strings are not equal\n”); }
Very tricky, this C: the space after %s means ignore the newline character.

13. Strings and Pointers
#include <stdio.h> #include <ctype.h> int main() { char alpha[ ] = "abcdefghijklmnopqrstuvwxyz"; char *str_ptr; str_ptr = alpha; while(*str_ptr != '\0') printf ("%c ",toupper(*str_ptr++) ); printf("\n"); }
An array name is a pointer
Strings always end with the ‘\0’ character
“toupper” is a function defined in the ctype.h library.

14. Strings: readline
#include <stdio.h> #define SIZE 100 int readline(char s[ ],int max) { int c,i=0; max--; while (i < max && (c = getchar()) != EOF && c != '\n') s[i++] =c; if (c != '\n') s[i++] = c; s[i] = '\0'; return(i); }

15. Strings: readline
int main() { char line[SIZE]; int n; while ( (n = readline(line, SIZE) ) > 0) printf("n = %d \t line = %s\n", n, line); }

16. Strings and dynamic memory
#include <string.h> /* compare two strings character by character using array syntax */ int strequal(char x[ ],char y[ ]) { int i=0; if (x == y) return(1); while (*(x +i) == *(y + i)) if (*(x + i) == '\0') return (1); i++; } return(0);

17. Strings and dynamic memory
int main() { char *str1,*str2; str1 = (char *) malloc(20 * sizeof(char)); str2 = (char *) malloc(20 * sizeof(char)); printf("Enter a string or ! to halt \n"); scanf("%s",str1); while (strcmp(str1, "!") != 0){ printf("\nEnter another string\n"); scanf("%s",str2); if (strequal(str1,str2)) printf("The strings are equal\n"); else printf("The strings are not equal\n"); }
Strcmp is a library function
strequal is a user defined function

18. Strings and pointers Use #define to put in debug statements
int strequal(char *x, char *y) ; // function prototype
#define DEBUG 1
int main() {
char *str1,*str2;
str1 = (char *) malloc(10 * sizeof(char));
str2 = (char *) malloc(10 * sizeof(char));
printf("Enter a string or ! to halt \n");
scanf("%s",str1);
#ifdef DEBUG
printf("This is a debug statement. str1 is %s \n", str1);
#endif
while (strcmp(str1, "!") != 0){
printf("\nEnter another string\n");
scanf("%s",str2);
if (strequal(str1,str2))
printf("The strings are equal\n");
else
printf("The strings are not equal\n"); }
Use #define to put in debug statements
If the name DEBUG is #defined, then
#ifdef is true and the printf is included.

19. Strings and pointers *x is the contents of what x points at
#include <stdio.h> #include <string.h> int strequal(char *x, char *y) { if (x == y) return(1); while (*x++ == *y++) if (*x == '\0' && *y == '\0') return (1); } return(0);
*x is the contents of what x points at
Return true if at end of string