#include <stdio.h>
char s[] = "`1234567890-=QWERTYUIOP[]\\ASDFGHJKL;'ZXCVBNM,./";
int main ()
{
int i, c;
while ((c = getchar()) != EOF) {
for (i = 1; s[i] && s[i] != c; i++);
if (s[i]) putchar(s[i-1]);
else putchar(c);
/* code */
}
return 0;
}
Whats does the condition s[i] && s[i] != c; inside the for loop mean?
I haven't seen it before.
Thanks!
First of all the variable s is a null terminated C-string, which is an array of chararcters with the last character being '\0'. You access that array with the index i in your code.
With that for loop you loop through the s string until a null terminator '\0' or the char c is found.
The null terminator '\0' is 0 decimal which means false in boolean logic because everthing else than 0 is true.
If you write for example:
char a = 'A'; /* same as char a = 65; */
if (a) { ... }; /* same as if (a != 0) */
that means: if a is true which is: if a is not false and better: if a is not equal to 0. This statement will evaluate to true because 'A' is 65 decimal (ASCII code) which is not equal to 0.
The for loop you've asked for can be rewritten as:
for (i = 1; s[i] != '\0' && s[i] != c; i++);
I would recommend to use explicit statements like s[i] != '\0' because it is easier to read.
s[i] in if is going to check whether an element exists on s[i] . if element exists on s[i] then it's truthy if not then it's falsy .
c is being intialized with user input in while loop
c = getchar()
s[i] != c // it means that my inputted value can not be equal to s[i].
Related
The following is getline implementation in K&R on page 29 ,
# define MAXLINE 1000
int getLine(char s[], int lim)
{
int c ,i ;
for(i = 0; i < lim - 1 && (c = getchar()) != EOF && c!= '\n'; ++i)
s[i] = c;
if(c == '\n'){
s[i] = c;
++i;
}
s[i] = '\0';
return i;
}
I don't understand why we need to do "i < lim - 1" here in the for loop. For correct indexing, why doesn't " i < lim " suffice ?
Any help would be much appreciated...
The function is going to build a string (stored in the array specified by the parameter char s[]) that is a sequence of characters terminated with the zero terminating character '\0'.
So one element of the array is reserved for the terminating zero character that is appended to the character array before exiting the function
s[i] = '\0';
For example if the variable lim is equal to 5 then you may enter in the array no more than 4 characters in the range of indices [0, 3]. In this case (if all lim-1 characters will be filled) in the last element of the array there will be written the terminating zero character '\0' at position 4.
** I understood how the function getline is working, it simply assigning value in each s[] array address which gets stored into the char line[] array because function argument has local scope there is no conflict due to the use of different array names unless it shares the same data type, But my concern is that why checker function has no effect on an actual string. correct me if the above understanding is wrong and why the checker function not working.**
the expected result was, getting string without trailing blanks and tabs like (hello World) but instead of that actual input that I typed in is printed which is ('\s'hello World'\t').
#define MAXLINE 50
int getline(char line[], int max);
int checker(char line[]);
int main(){
char line[MAXLINE];
while( getline(line, MAXLINE) > 0 )
if( checker(line) > 0 )
printf("%s",line);
return 0;
}
int getline(char s[],int lim){
int c,i,j;
j=0;
for(i=0; (c=getchar()) != EOF && c != '\n';i++){
if(i < lim-1){
s[j]=c;
++j;
}
}
if(c == '\n'){
s[j] = c;
++j;
++i;
}
s[j] = '\0';
return i;
}
int checker(char s[]){
int i;
i=0;
while(s[i] != '\n' )
++i;
--i;
while(i >= 0 && (s[i] == ' ' || s[i] == '\t') )
i++;
if( i >= 0){
++i;
s[i] = '\n';
++i;
s[i] = '\0';
}
return i;
}
If you are trying to trim trailing blanks and tabs from your string, try changing the contents of the second while loop in checker() to contain i-- rather than i++.
Since checker() is intended to change the string, perhaps a different name would be better. The word check does not usually imply modification. A well chosen name is a great help to the next person who encounters your code.
The bug seems to be here:
while(i >= 0 && (s[i] == ' ' || s[i] == '\t') )
i++;
^^^^
This shall probably be i--;
That said... Your function isn't secure. It lacks some checks to prevent access outside the char array.
For instance:
What happens if the input string has no '\n' ?
What happens if the input string is a space followed by '\n' ?
Also the getline function has a problem. If the input is longer than lim, the code will do s[lim] = '\0'; which is out of bounds.
i could not understand what mgetline does in this code.
anyone can help me?
int mgetline(char s[],int lim)
{
int c, i;
for(i = 0; i < lim - 1 && (c = getchar()) != EOF && c != '\n'; ++i)
s[i] = c;
if(c == '\n')
{
s[i] = c;
++i;
}
s[i] = '\0';
return i;
}
The function basically reads characters one-by-one from the the standard input stream stdin until you enter a \n (newline) or the array limit of s, lim, is reached. The characters are stored in the char s[] and the length of what was read is returned.
It's hard to answer with more detail since it's a little unclear what it is you don't understand, but I've tried to annotate the code to make it somewhat clearer.
This is the same code, only reformatted to fit my comments.
int mgetline(char s[], int lim) {
int c, i;
for(i = 0; // init-statement, start with `i` at zero
i < lim - 1 // condition, `i` must be less than `lim - 1`
&& // condition, logical AND
(c = getchar()) !=EOF // (function call, assignment) condition, `c` must not be EOF
&& // condition, logical AND
c != '\n'; // condition, `c` must not be `\n` (newline)
++i) // iteration_expression, increase i by one
{
s[i] = c; // store the value of `c` in `s[i]`
}
if(c == '\n') { // if a newline was the last character read
s[i] = c; // store it
++i; // and increase i by one
}
s[i] = '\0'; // store a null terminator last
return i; // return the length of the string stored in `s`
}
In the condition part of the for loop you have 3 conditions that must all be true for the loop to enter the statement for(...;...;...) statement. I've made that statement into a code block to make it easier to see the scope. EOF is a special value that is returned by getchar() if the input stream (stdin) is closed.
Note: If you pass an array of one char (lim == 1) this function will cause undefined behavior. Any program reading uninitialized variables has undefined behavior - and that's a bad thing. In this case, if lim == 1, you will read c after the loop and c will then still be uninitialized.
Either initialize it:
int mgetline(char s[], int lim) {
int c = 0, i;
or bail out of the function:
int mgetline(char s[], int lim) {
if(lim < 2) {
if(lim == 1) s[0] = '\0';
return 0;
}
int c, i;
I need to write a program that reads from the input a sentence terminated by a new line. The program should write an encryption of the sentence in a way that the letter 'a' is swapped with 'z', 'b' with 'y' and so on. I am not supposed to use arrays, that is why I can't find a solution for this problem.
This is my code:
#include<stdio.h>
int main() {
char alphabet, temp;
while( scanf("%c", &alphabet) != '\n') {
for(char i ='a', j='z'; i <= 'z', j>='a'; i++, j--) {
if(alphabet == i) {
temp = j;
}
}
printf("%c", temp);
}
return 0;
}
When I input: abc as an output I get zyxxxxxxxxxxxxxxxxxxxxxxxxx(an infinite number of x).
So to sum up:
Input:abc. Output: zyxxxxx(infinite number of x).
You messed up the return value of scanf:
while( scanf("%c", &alphabet) != '\n'){
You want to stop the loop when the entered character (in alphabet) is a line break.
But you test the return value of scanf which will never be '\n' as it returns the number of converted fields.
You could try like this:
while( scanf("%c", &alphabet) == 1 && alphabet != '\n'){
Compile your code with warnings enabled (e.g. Wall flag)! This:
for(char i ='a', j='z'; i <= 'z', j>='a'; i++, j--)
should give:
warning: left-hand operand of comma expression has no effect [-Wunused-value]
You need to use the logical AND operator && and not a comma between the two comparissons.
Moreover, use getchar() instead, like this:
#include<stdio.h>
int main() {
char temp;
int ch;
while ((ch = getchar()) != '\n' && ch != EOF) {
for(char i ='a', j='z'; i <= 'z'&& j>='a'; i++, j--){
if(ch == i){
temp = j;
}
}
if(ch <= 'z'&& ch >= 'a') // do not print if 'ch' is a digit for example
printf("%c", temp);
}
return 0;
}
Output:
zyx
If you have to use scanf(), then try reading into alphabet, check the return value of scanf, and compare to newline, like this:
while(scanf("%c", &alphabet) != 0 && alphabet != '\n')
After succesfully running an entabulator, my detabulator won't pick up on a character comparison that should exit a while loop. After trying "0(tab)8(enter)(ctrl+D)" as input the tab is written correctly as spaces, but after rp is incremented to point to the 8, the while loop that should read the 8 won't exit and I get a seg fault. Here's the code:
#include <string.h>
#include <stdio.h>
#define MAXLINE 100
char doc[9001];
main(int argc, char *argv[])
{
int max = 0;
char *rp = doc;
char *wp = rp;
char *tf = wp;
char *lp = doc;
while ((*(rp++) = getchar()) != EOF);
*--rp = '\0';
rp = doc;
j = 0;
while ( (*rp != '\0') && (argc == 1)) {
if (*rp == '\n') {
lp = rp + 1;
*wp++ = *rp++;
}
while( (*rp != '\t') && (*rp != '\0') && (*rp != '\n') ) { /*this loops after a tab*/
*wp++ = *rp++;
}
if (*rp == '\t') {
rp++;
tf = lp + ((((wp - lp) / 8) + 1) * 8);
while ((tf - wp) != 0)
*wp++ = 's';
}
}
if (*rp == '\0')
*wp = '\0';
printf("%s\n", doc);
}
There are some as yet unexplored problems with the initial input loop.
You should never risk overflowing a buffer, even if you allocate 9001 bytes for it. That's how viruses and things break into programs. Also, you have a problem because you are comparing a character with EOF. Unfortunately, getchar() returns an int: it has to because it returns any valid character value as a positive value, and EOF as a negative value (usually -1, but nothing guarantees that value).
So, you might write that loop more safely, and clearly, as:
char *end = doc + sizeof(doc) - 1;
int c;
while (rp < end && (c = getchar()) != EOF)
*rp++ = c;
*rp = '\0';
With your loop as written, one of two undesirable things happens:
if char is an unsigned type, then you will never detect EOF.
if char is a signed type, then you will detect EOF when you read a valid character (often ΓΏ, y-umlaut, LATIN SMALL LETTER Y WITH DIAERESIS, U+00FF).
Neither is good. The code above avoids both problems without needing to know whether plain char is signed or unsigned.
Conventionally, if you have an empty loop body, you emphasize this by placing the semicolon on a line on its own. Many an infinite loop has been caused by a stray semicolon after a while condition; by placing the semicolon on the next line, you emphasize that it is intentional, not accidental.
while ((*(rp++) = getchar()) != EOF);
while ((*(rp++) = getchar()) != EOF)
;
What I feel is, the below loop is going into infinite loop.
while( (*rp != '\t') && (*rp != '\0') && (*rp != '\n') ) { /*this loops after a tab*/
*wp++ = *rp++;
This is because, you are checking for rp!= '\t' and so on, but here
if (*rp == '\t')
{
rp++;
tf = lp + ((((wp - lp) / 8) + 1) * 8);
while ((tf - wp) != 0)
*wp++ = 's';
}
you are filling the doc array with char 's' and which is over writing '\t' also, so the above loop is going to infinite.