Replacing a part of string with another string in C - c

Question

this is my source code. When I input a string "I am in CSE 2nd year." and replace CSE 2nd(ie loc 9 to 15) by ECE 3rd, I get some garbage vales at the end of string. Also there is a newline at the beggining of rslt2 string. There is something wrong with rslt2. Can anyone please rectify the error?
//splitting a string and replace latter part of string by another string
#include<stdio.h>
#include<string.h>
int main()
{
int i,count=0,loc2,scount=0,rcount=0,loc=0; //scount represents counter for subset and rcount for replacement and loc from where we will split the string
char str[100],sub[100],newss[100],rslt[100],rslt2[100]; //newss=new substr, rslt and rslt2=former and latter part of original string
printf("Enter a String:\n");
fgets(str,100,stdin);
printf("\nString Entered by User:\n");
fflush(stdin);
puts(str);
printf("\nLoc Char\n"); //Creates Colums 'Char' and 'Loc'
for(i=0;str[i]!='\0';i++)
{
count++; //Counts length of String
printf("%d. %c\n",count,str[i]); //Prints Characters with it its Corresponding Location
}
printf("\n\nLength of String: %d\n\n",count);
printf("Enter the locations of Characters from where subset will start and end: \n");
scanf("%d%d",&loc,&loc2); //stores indices of begining and end of substring
printf("\n\nSubset formed from Existing String:\n");
for(i=loc-1;i<loc2;i++)
{
scount++;
sub[i]=str[i]; //stores substring in "sub"
printf("%c",sub[i]);
}
printf("\n\nLength of Subset: %d\n",scount);
for(i=0;i<(loc-1);i++)
{
rslt[i]=str[i]; //Stores former part of string in resultant string
}
for(i=loc2;i<strlen(str);i++)
{
rslt2[i]=str[i]; //Stores latter part of string in resultant string2
}
printf("\n\nEnter a Replacement for Subset(Of Equal Length as that of Subset):\n");
fflush(stdin);
fgets(newss,100,stdin);
for(i=0;newss[i]!='\0';i++)
rcount++;
printf("\n\nLength of New Subset: %d\n",rcount-1); //-1 to subtract length of null char
if(rcount-1!=scount) //to check whether replacement string and substring are of same len
printf("\nSince length of both subsets is not same. \nHence Replacement is Not Possible\n");
else //Concatination of 3 substrings
{
printf("\nResultant String:\n");
for(i=0;i<(loc-1);i++)
printf("%c",rslt[i]);
printf("\n");
for(i=0;newss[i]!='\0';i++)
printf("%c",newss[i]);
for(i=loc2;rslt2[i]!='\0';i++)
printf("%c",rslt2[i]);
}
return 0;
}

Answer 1

Here's an example of how to implement that program.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXL 100
int main( void )
{
int count, lengthI, lengthR, start, end;
char initial[MAXL], replacment[MAXL], range[MAXL], result[MAXL];
// get user input
printf( "Initial string: " );
fflush( stdout );
fgets( initial, MAXL, stdin );
printf( "Replacement string: " );
fflush( stdout );
fgets( replacment, MAXL, stdin );
printf( "Start and end: ");
fflush( stdout );
fgets( range, MAXL, stdin );
count = sscanf( range, "%d%d", &start, &end ); // indices of beginning and end of range in initial string
// remove newline character from the input strings, if necessary
lengthI = strlen( initial );
if ( lengthI > 0 && initial[lengthI - 1] == '\n' )
initial[--lengthI] = '\0';
lengthR = strlen( replacment );
if ( lengthR > 0 && replacment[lengthR - 1] == '\n' )
replacment[--lengthR ] = '\0';
// range checking to verify that user inputs are valid and the resulting string will fit into the buffer
if ( count != 2 || start < 0 || start > lengthI || end < start || end > lengthI )
{
fprintf( stderr, "Invalid start and end values\n" );
exit( 1 );
}
if ( lengthI + lengthR - (end - start) + 1 > MAXL )
{
fprintf( stderr, "Resulting string would be too long\n" );
exit( 2 );
}
// create a new string with the substring replaced
if ( start > 0 ) // copy characters from the initial string up to the start index
strncpy( result, initial, start ); // note: this step may leave the result string unterminated
strcpy( &result[start], replacment ); // append the repacement string
// guarantees the result string is terminated
if ( end < lengthI ) // append characters from the initial that are after the end index
strcat( result, &initial[end] ); // terminates the result string (provided that strcat is called)
// print the result
printf( "%s\n", result );
}
Comments:
Don't mix fgets and scanf. Even if you're aware of the issues involved with doing so, it's still easy to get it wrong. Best to just read lines with fgets, and then parse with sscanf as necessary.
fflush(stdin) is non-standard. fflush is only guaranteed to work on stdout. On some systems, fpurge can be used to erase unread input.
When dealing with strings in C (aka arrays of characters), range checking is essential. Buffer overruns are the #1 cause of crashes, unexpected behavior, and security holes in C programs. Always range check user inputs, and always verify that a newly created string will fit into the buffer provided.
Always make sure that a newly created string ends with a null character (aka '\0'), and be sure to include that null character when calculating buffer sizes.
Note that in the sample code, strncpy may leave the string unterminated. The subsequent call to strcpy will terminate the string. The strcat function will also terminate the string. However, the call to strcat is conditional based on user input. Hence, without the call to 'strcpy', we'd have extra work to do to guarantee the the string gets its mandatory null terminator.

Answer 2

use negated scanf ie scanf(" %[^\n]",str_name); instead of gets();