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04/12/23 Course material created by D. Woit 1

CPS 393Introduction to Unix and C

START OF WEEK 8 (C-2)


Program Control Statements-similar to Java

<<If Statement>>

• if (a < 7) printf("yes");

• if (a < 7) {• printf("yes ");• printf("it is\n");• }

• if (a < 7 && b != 5) • a=a+1;• else {• a=a+b;• printf("%d\n",a);• }• See sample1


Operators• Relational Operators: > >= < <= == !=• Each operator requires 2 operands .• Result evaluates to integer 0 when false; int 1 when true.

• Note1: in C, any non-0 (int) value is "true“ 0 is "false" • Logical Operators:• && and• || or• ! not.• Note: !5 is 0 (false)• !!5 is 1 (true) •


Operators• Can nest ifs• if (i != 0)• if (i != 1)• if (i != 2)• if (i != 3)• if (i != 4)• if (i != 5)• if (i != 6)• if (i != 7)• if (i != 8)• if (i != 9)• printf("%d is not a digit\

n",i);

• else binds with nearest "else-less" if above

• if (i != 0)• if (i > 0) printf("\a");• else printf("\a\a\a");

• The else binds to which if?


If then else• No "elseif" but can do "else if"• if (c >= '0' && c <= '9') printf("digit");• else if (c >= 'a' && c <= 'z') printf("lower case letter");• else if (c >= 'A' && c <= 'Z') printf("upper case letter");• else printf("not alphanumeric");


Fact.c• /*Source: fact.c

• Purpose: function fact(long n) computes n!

• Input argument: n an integer

• Output: n! an integer

• Note: a program using fact.c must include fact.h

• */

• long fact(long n)

• {

• if (n == 0)

• return(1);

• else

• return (n*fact(n-1));

• }

• /*Source: fact.h• Purpose: header file for

fact.c• */• long fact(long n);

• Ifs good for checking for *valid input*

• e.g., what should fact check about n?

• See example usefact.c


• /*Source: dimcheck.c• Input: int length, width (length &

width of room)• Output: the square footage

(integer) of room• Exit: 0 if good dimensions; 1

otherwise• */ • int GetDim(void);• int main(void) {• int dim1, dim2 /*dimension of

room */• Int ExitFlag; /*code to exit with

0=good*/• /*initialize ExitFlag to good value*/• ExitFlag=0;

• /*read in dimensions*/• dim1 = GetDim();• dim2 = GetDim();• /*check for valid input*/• if ((dim1 <= 0) || (dim2 <= 0) )

{ /*invalid dims*/• fprintf(stderr,"Input error.

Please re-run program\n");• ExitFlag=1;• }• else• printf("Floor Space is %d\

n",dim1*dim2);• exit(ExitFlag);• }

Example program to compute dimensions of the room


Example program to compute dimensions of the room

• int GetDim(void) {• int dim; /*dimension */• int sret; //scanf return value• printf("Enter a dimension (pos. int.): ");• sret=scanf ("%d",&dim);• if (sret != 1) return(-1);• else return (dim);• }• Note: echo $? in shell displays a pgms's exit status


HMWK• here is a file called math2.c which contains functions for factorial and

fibonacci• /*Source: math2.c• Purpose: source code for factorial and fibonacci functions• */• long fact(long n) {• if (n == 0) return(1);• else return (n*fact(n-1));• }• long fibonacci(long n) {• if (n == 0) return (0);• else if (n == 1) return (1);• else return (fibonacci(n-1) + fibonacci(n-2));• }


HMWK continued

• Write file math2.h, a header file for math2.c• Write a program evaluate.c which prompts the user to• enter a 0 or a 1. If 0 entered, calculate fact(n) where• n is prompted for and read in. If 1 entered, calculate• fibonacci(n), where n is prompted for and read in.• Make sure your program does error checking on all input.• Your program should exit with 0 if successful; with 1 if• user didn't enter a "0 or 1"; and with 2 if the user • entered an invalid (including negative) value for "n".


For loop

• General form of for statement is:

• for (expr1; expr2; expr3);• statement

• Is equivalent to:• expr1;• while (expr2) {• statement• expr3;• }

Example:• for (i=0; i<=9; i++) {• printf("%d",i);• printf("\n");• } • Infinite loop:• For (;;) {• …• }/* exit by break or return */


Example for for statement• /* Source: rand.c• Purpose: generate MaxNum random integers• Input: integer MaxNum (max number of rand numberrs to generate)• Output: MaxNum random integers• Exit: 0 if successful; 1 if improper user input (not +ve int)

• */ • #include <stdio.h>• #include <stdlib.h> /* rand() is defined here */• #include <time.h>• int main(void) {• int MaxNum, /*number of random nos to generate */• i, /*index */• RandNo, /*a random number */

• ExitCode=0; /*value to exit program with */


• srand(time(NULL)); //seed with fairly unique number

• printf("How many rand nos to generate? ");• if (((scanf("%d",&MaxNum)) != 1) || (MaxNum <=0)) ExitCode=1;• else• for (i=1; i <= MaxNum; i++) {• RandNo=rand();• printf("Random No %d is %d\n",i,RandNo);• //RandNo=(rand()%6); //gives numbers 0 to 5• //c = (char) (rand() % 128);• }• exit(ExitCode);• }

•


More for examples• Lines below are in forexample1.c • for (i=5; i<20; i=i+5) printf("%d",i);• for (i=10; i>0; i--) printf("%d",i);• for (i=1,j=5; i<10&&j>0; i++,j--) printf("%d",i);

• #three loops below are in forexample2.c• char c;• for (c='a'; c<='z'; c++) putchar(c);• for (c=getchar(); c!='X'; c=getchar());• printf("Thanks for finally inputting an X!!");• for ( ; ;) ; /*infinite loop*/• for (c='A'; c<='Z'; ) {• putchar(c);• c=c+2;• }


HMWK

• write a program that prints out the extended ASCII character set. Use a for loop from 0 to 255, printing the index as a char in each loop.

• • . • •


While loop • while (expression) /* if expr is non-zero, statement is executed */

• statement• Example1 first loop in program thanksX.c:• char c;• c=getchar();• while (c != 'X') • c=getchar();• printf("Thanks for finally inputting an X!!");• Example2 second loop in program thanksX• //source thanksX.c• char c;• while ( (c=getchar()) != 'X'); /* more elegant than the first one */• printf("Thanks for finally inputting an X!!"); •


Switch Statement • Use to choose between several

alternatives• can only switch on int or char

• switch(c) {• case 'a':• printf("a vowel");• break;• case 'e':• printf("a vowel");• break; • case 'i':• printf("a vowel");• break;•

• case 'o':• printf("a vowel");• break;• case 'u':• printf("a vowel");• break;• default:• printf("non-vowel");• break; //can be omitted• }

• Default is executed if none of other cases is satisfied

• See example switch1.c


Switch Statement• Example 2 (previous one

improved), see switch2.c

• switch(c) { /*falls thru with no break*/

• case 'a':

• case 'e':

• case 'i':

• case 'o':

• case 'u':

• printf("vowel");

• break;• default:• printf("non-vowel");

• } •

• Example3, see switch3.c• switch(i) {• case 1: putchar('H');• case 2: putchar('E);• case 3: putchar('L');• case 4: putchar('L');• case 5: putchar('O');• default: putchar('\a');• }• This switch does not have

breaks so it falls through• If i is 1? • If i is 3?


HMWK• (1) Write a program which reads in a character, and prints• whether the character is a vowel or non-vowel. It must work• for any case, e.g., 'a' and 'A' are both vowels. You should• not have separate cases for upper and lower case vowels. Do• this by converting the character to lower case before the• switch. Use function "tolower" which is in ctype.h (for usage• see tolower man page.) For non-letter (a-z,A-Z) characters,• print an appropriate error message. Remember exit codes.• (2) Modify your program to read in any number of characters• and report, after each read, whether it is a vowel. You should• stop processing characters when a '%' is input. You should• always remind the user that a '%' will stop the program.


Example• We use functions in ctype.h toupper, isdigit etc.• Note: there is built-in function atoi(s) which converts string

to an integer• See example makeint.c

• /*Function: MakeInt()• Source: MakeInt.c• Purpose: to convert a sequence of digits to its integer

value• Input: a sequence of digit characters, e.g., 125• Output: the integer 125 (one hundred and twenty five)• */• #include <stdio.h>• #include <ctype.h>


Example MakeInt()• int MakeInt() {

• int num=0, /*final integer value */• digit; /*digit that is read in*/• digit=getchar();• for( ; isdigit(digit); digit=getchar()) {• num=num*10;• num=num+ (digit - '0');• }• return (num);• }• digit - '0' converts char 0,1, ... 9 to int 0,1, ... 9• remember printf("%d %c", digit,digit); => 66 B• ASCII decimal code for '0' is 48, '1' is 49, … '9' is 57• so if digit is 5 digit-'0' evals to 53-48 = 5

Converting a string fo digits to an integer

• /* source tryatoi.c• purpose: to convert a sequence

of digits placed in a string into integer value

• input: a sequence of digits• output: the integer• */

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

• int myatoi(char *s);

• int main(void) {• int number;• char s[10];•

• printf("Insert a number : \n");• scanf("%s" , s );• number=myatoi(s);• printf("Number is %d\n",

number );• }• int myatoi(char *s) {• int i, num=0; /*final

integer value */•• for (i=0; s[i]>='0' &&

s[i]<='9'; ++i) • num=num*10 + s[i] - '0'; • return(num);• }


HMWK:• (2) create a a program to display, at top of screen:• ------------------------------------------------------• Welcome to the calculator program!• Please choose the operation you desire by entering• the appropriate number:• 1. addition• 2. subtraction• 3. multiplication• 4. division• 5. absolute value• 6. exit• Your choice:• ------------------------------------------------------


• . • • . • •

Note that you can clear screen using the C system command from stdlib.h, as in: system("clear"); //(system does any linux command).The program will re-display this until user enters correctinput. Then the program will prompt for necessary operands, calculate and display the result. The program will repeat theabove until the user enters choice 6 (exit program). Use aseparate function for each operation. Exit main with appropriatecode (3) Design and implement a program to print a table of squares, cubes and quatricsfor numbers 1 to N, where N is input by user. Use functions foreach of square, cube and quatric, and reuse square where possible.The table would look like:Number Square Cube Quatric------ ------ ----- -------1 1 1 12 4 8 163 9 27 81


TryScan.c• This program will read in ints and print them out until 0

entered. Try entering 99a and see what happens? • /*TryScan.c*/• #include <stdio.h>• int main(void) {• int j,i=1;• char c;• while (i!=0) {• scanf("%d",&i); # scanf obtains negative number if alfa char is

entered which is not stored in var. i• printf("i is %d\n",i);• }• exit(1);• }


TryScan

• We should try to fix TryScan.c. Could try:• adding line while ((c=getchar())!='\n');• after the scanf line, or• fflush(stdin); which doesn't work on all systems, or• j=scanf... and then after• if (j!=1) {fprintf(stderr,"BAD"); exit(0);}

• See TryScanfix.c and TryScanfix2.c•


• Fix fact(long n) so that it is robust (does something reasonable, like returns -1 when called with negative input). See fixfact.c

• #include <stdio.h>

• #include <stdlib.h>

• #include "math2.h"

• void test(int TestNumber, long result, long answer );

• int main(void) {

• test(1,fact(0),1); /*boundary*/



• test(4,fact(7),5040); /*interior*/

• test(5,fact(-1),-1); /*error and boundary*/

• test(6,fact(-3),-1); /*error*/

• exit(0);

• }

• void test(int TestNo, long result, long answer) {

• if (result != answer) printf("Test %d failed\n",TestNo);

• }


HMWK• (1)write test drivers for the square function and for the• fibonacci function. What input would you use to test the• program for computing floor space we did previously? • • • •


Static Functions and Variables • Remember: a static variable is known only to the function

in which it is declared, and retains its value from one call of the function to another.

• A static function is known only in the file in which it is defined/declared (like being private to that file.)

• e.g.,• Function q is only available to code in file f.c. It is not

available to main or any other code. If you try to use q() from main, it gives "undefined" error

•

04/12/23Course material created by D. Woit

30

Static Functions and Variables

• //Source f.h• int f (int x);

• //Source f.c• static int q(int x) {• return(x*x);• }• int f (int x) {• return (q(x));• } •

• //Source: m.c• #include <stdio.h>• #include "f.h"• int main(void) {• int y=f(7);• printf("%d\n",y);• return(0);• }• Function q is only available to

code in file f.c. It is not available to main or any other code. If you try to use q() from main, it gives "undefined" error


Static variables within functions:• On a moon, the following program

prints something like:• 1 2 3 4 5 • 1 1 1 1 1 • //Source: accum.c• int fs(int x) {• static int count=0;• count=count+x;• return (count);• } • int fn(int x) {• int count=0; • count=count+x;• return (count);• }

• int main(void) {• int i;• for (i=0;i<5;i++) printf("%d

",fs(1));• putchar('\n');• for (i=0;i<5;i++) printf("%d

",fn(1));• putchar('\n');• }


Static functions• A function,f, that is not declared "static" will be exportable. • i.e., you can put f's definition in f.c and declaration in f.h

• Then if you include f.h in another file, M.c, and compile both M.c and f.c, f will be available to the code in M.c

• If f was static, then it would not be available to code in M.c


Arrays and Strings • Some differences from java.• <<1-Dimensional arrays>>• int A[20]; gives A[0] A[1] ... A[19]• Assign: A[14]=1005;• Reference: printf("%d", A[14]);• Initialize at declaration: • int A[5] = {1,2,7,15,3};• or later:• int A[5];• for (i=0; i<5; i++) scanf("%d",&A[i]);


Arrays and Strings• Compiler does not check array bounds :

– printf("%d",A[92]); compiles OK

• char a[10], b[10];• for (i=0; i<10; i++) b[i]=i;• If we want to copy array b into array a• Incorrect: a = b; /*compiler error:incompatible types*/• Correct: for (i=0; i<10; i++) a[i]=b[i];•

•


example• /*Source: FindLtr.c• Purpose: output occurrences of A,B,...Z found in input• Input: chars from stdin• Output: each upper case letter and its occurrences• */• #include <stdio.h>• #include <ctype.h>

•

• •


• int main(void) {• int i, /*index */• c, /*char from stdin*/• letter[26]; /*array for upper case letters*/• for (i=0; i<26; i++) letter[i]=0; /*initialize array*/• while((c=getc(stdin)) != EOF) /*^d on UNIX */• if (isupper(c)) ++letter[c - 'A']; /*increment right slot*/• for (i=0; i<26; ++i) { /*print results*/• if (i%6 == 0) printf("\n"); /*new line every 6 chrs*/• printf("%5c: %4d", 'A'+i, letter[i]);• }• printf("\n\n");• exit(0);• } • EOF is ^d UNIX, or ^z DOS/Windows• 'A' is 65, 'B' is 66, etc. Therefore,• 'A' - 'A' = 0, 'B' - 'A' = 1, etc.• This prog can be tried with the input redirected from a input file• FindLtr < direwolves.txt


Strings • C has no string type like java.• In C, string: null-terminated char array null is '\0' (= byte

00000000)• Always declare array 1 larger than string length (for \0)• compiler automatically null-terminates string constants• "Hello There"• gets() in stdio.h

– reads chars up to and including carriage return or EOF– discards the '\n' (or EOF) and null-terminates string


• /*Source: str1.c */• #include <stdio.h>• int main(void) {• int i; /*index*/• char str[40]; /*string max 39 chars*/• printf("Enter a string (max 39 chars):");• /* gets(str); compiler hates this func */• fgets(str, 40, stdin) ; /* nicer and preferred by compiler*/• for (i=0; str[i]; i++) /*loops until str[i] is \0 */• printf("%c",str[i]);• /*==printf(str);• ==printf("%s",str);• */• putchar('\n');• exit(0);• }


Fgets and fputs• fgets(str, length, fp);• gets chars from "file" pointed to by fp (fp could be stdin) • and stores them in array str, until either event occurs:• 1. '\n' read (and transfered to str)• 2. (length-1) chars have been read, or• 3. EOF• terminates str with a '\0'• returns ptr to str if successful• Otherways it returns NULL ptr• e.g., fgets(str,40,stdin); would stop memory overwrite• However, it also puts \n in str (if str not full) If you• don't want the \n, you have to remove it yourself (copy• a '\0' over the '\n').


• /*Source:tryfgets.c*/• #include <stdio.h>• int main(void) {• int i; /*index*/• char str[40]; /*string max 39 chars*/• printf("Enter a string (max 38 chars):"); //leave slots for

newline and null• fgets(str,40,stdin);• fputs(str,stdout);• exit(0);• }• Note: String only 38 chars now (\n and \0)• enter <=38 chars, it includes the newline in str• enter >38 chars, there is no \n, but 40th slot (str[39]) gets \0


Functions on strings see strops.c• char a[40], b[40];• strcpy(b,"Hello");• strcpy(a,b);• printf("%s %s",a,b);

•

• strcpy(a,"Hi");• strcat(a," There");

• printf(a);

• strcmp(a,b) returns:• 0 if same• <0 if a < b in dictionary order

(str length irrel)• >0 if a > b

• strcpy(b, "Hello");• strcpy(a, "Hellp");• printf("b has length

%d",strlen(b));• printf("%d", strcmp(a,b)); //prints

1• printf("%d",

strcmp("Hello",b)); //prints 0• null string contains only \0• strcpy(a,""); /*or*/ a[0]='\0';• int i;• i=atoi("127"); /*converts digit

string to integer*/• printf("%d",i); /* prints integer

127 */


useful string functions are: sprintf, sscanf • int sprintf (char *out_string, const char *format_string, ... ); • -const is a qualifier indicating the data cannot be modified

• -works like printf but prints into string out_string instead of stdout • char S[20]; • double x=94.12345;• sprintf(S, "%.2lf", x); --> S is "94.12" the '\0' added automatically• int sscanf(char *str, const char *format_string, ...);• does a scanf from a string instead of stdin• Nice, because we can use gets to read whole line including \n• (no worry about scanf "stuck" on the \n)• remember: gets discards the read-in '\n' and null-terminates the string• char S[40];• int x,y;• gets(S);• sscanf(S, "%d %d", &x,&y);


Multi-dimensional Arrays • int A[3][4]; /*3 rows (0-2), 4 columns (0-3)*/

• int A[3][4][2]; /*array of 2-dim arrays - a prism */

• Initialization:

• int A[3] = {1,4,7};

• char B[3] = {'A','B','C'}; /*no null term*/

• char C[4] = "ABC"; /*null term added automatically so need [4] */

• int D[2][3] = {

• 2,4,7,

• 6,9,5

• }; /*D[0][0] is 2; D[0][1] is 4, etc */

•


/*Source: 3dimary.c*/ • #include <stdio.h>• int main(void){• int A[2][3][2]= {1,2,3,4,5,6,7,8,9,10,11,12};• int i,j,k;• for (i=0; i<2; i++)• for (j=0; j<3; j++)• for (k=0; k<2; k++)• printf("A[%d][%d][%d]=%d\n",i,j,k,A[i][j][k]);• exit(0);• }• Automatic (local) arrays not initialized by compiler• Global & Static are initialized to 0 by compiler (like all global & static

variables)


Unsized Arrays:• (compiler counts # of constants to find size needed) • int A[] = {1,4,2,6}; /*size 4*/• char UserMsg[] = "Enter a number: ";• int b[][2] = { 1,4,• 7,9,• 3,5• }; • Need to include one dimension-count to help compiler

• Why unsized?

• easy modification, especially if use sentinel when• looping (no change of declaration size and no• change of loop index max)•


Arrays of Strings:• char colors[5][9];• /*5 strings, each string max 9 chars incl null*/• char colors[][7] = {• "blue",• "red",• "pink",• "purple" };• printf("%s",colors[2]); //prints pink• printf("%c",colors[2][3]); //prints k

• see example strary.c•


Arrays of Strings:• char names[3][40];• gets(names[0]);• gets(names[1]);• gets(names[2]); /*fills array*/• for (i=0;<3; i++) printf(names[i]);•

• •


/*Source: printword.c */

• /* Purpose: print the english word equivalent of a digit

• Input: a digit, e.g., 9• Output: the word equivalent, e.g., nine• Note: should exit with bad code if invalid input• */• #include <stdio.h> • int main(void) {• /*set up array of english words for digits*/• char digits [10][6]= {• "zero", "one", "two", "three", "four", "five",• "six", "seven", "eight", "nine" };• char num; /*digit to read in*/


• printf("Enter a digit: ");

• num = getchar();• num = num - '0';• if (num >= 0 && num < 10)• printf("\n%s\n", digits[num]);• else printf("\nThat wasn't a digit\n");• return 0;• } • • ASCII for '0' is 48, for '1' is 49 etc• so if enter 2: num = '2' - '0' = 50 - 48 = 2• so digit[2] is printed•

•


Passing arrays to Functions:• /*Source: trypas.c*/

• #include <stdio.h>

• #include <stdlib.h>

• void f1(int num[5],int loc), f2(int num[],int loc);

• int main(void) {

• int i,loc=0;

• int count[5] = {0,0,0,0,0};

• f1(count,loc);

• f2(count,loc);

• printf("count=");

• for (i=0; i<5; i++) printf("%d ", count[i]);

• printf("\nloc=%d\n",loc);

• exit(0);

• }

• void f1(int num[5], int loc) {• num[0] = 99;• loc=99;• }• void f2(int num[], int loc) {• num[1] = 22;• loc=22;• }


Comments on trypas.c• It prints:

– count=99 22 0 0 0 – loc=0

• Why? loc makes sense, but why was count changed?• (Remember, normally call-by-value, e.g., a COPY• of the value is passed down, so what's happening?)• *Address* of arrays are passed down (only for arrays, not

regular variables)• Therefore, like arrays passed call by value (but they're not

really)• Will be more clear after pointers


/*Source: encode.c */• /* Purpose: encode a string by

alternately taking chars off each end (starting with left side) and

• stopping when middle of string reached e.g., "Hi there" would become "Heir eth"

• */ • #include <stdio.h>• #include <string.h>

• int main(void) {• char str[80];• int i,j; •

• printf("Enter string to be encoded: ");

• gets(str);• /*encode it*/• i=0;• j=strlen(str)-1;• while (i <=j) {• if (i<j) printf("%c

%c",str[i],str[j]);• else printf("%c", str[i]); /*i==j*/ • i++; j--;• }• printf("\n");• return 0;• }


/*Source: encode.c */• Try tracing with "My Dogs":• i j output• - - ------• 0 6 Ms• 1 5 yg• 2 4 _o• 3 3 D• Giving: Msyg oD

• HMWK:• 1. Consider all the numbers from 1 to 4321 written out.

How many times would the• digit '5' appear in total? Write a C program to figure this

out.

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