emacs - replace C++ comment (//) wth C comment (/* */) - emacs

Question

Is there any easy way to replace a C++ comment (//) with a C comment (/* */) in emacs?
Has someone written an elisp function to do it?
Or can some sort of clever regexp be applied?

Answer 1

use query-replace-regex or similar function with following search string: ^\(.*?\)//\(.*\)$ and following replacement string \1/* \2 */

Answer 2

This C program converts all the consecutive occurrences of '//' comments in a file sample.txt with '/* ...*/' and saves result to output.txt.
#include <stdio.h>
#include<string.h>
void trimLeading(char * string);
int main(int argc, char* argv[])
{
FILE* file = fopen("sample.txt", "r");
FILE* fp = fopen("output.txt", "w"); //to create file if it does not exist and to empty file if it exists.
fclose(fp);
if(file==NULL) {
perror("Error opening file sample.txt.");
}
if(fp==NULL) {
perror("Error opening file output.txt.");
}
/* should check the result */
long pos;
char line[256];
char buff[256];
int flag=0; //flag=1,when multi-line comment occurs in input file.
int line_length=0;
while (fgets(line, sizeof(line), file)) {
trimLeading(line);
while((strlen(line)==0)){
fgets(line,sizeof(line),file);
trimLeading(line);
}
//line_length=strlen(line);
//printf("%d",line_length);
strcpy(buff,line);
if(strstr(buff,"//")){
trimLeading(buff);
//printf("\nbuff=%s,line=%s,flag=%d\n",buff,line,flag);
if((buff[0]=='/')&&(buff[1]=='/')) {
/*******************/
if(flag==0){
/*look at next line*/
pos = ftell(file);
fgets(line,sizeof(line),file);
trimLeading(line);
//line_length=strlen(line);
//printf("%d",line_length);
while((strlen(line)==0)){
fgets(line,sizeof(line),file);
trimLeading(line);
}
fseek( file,pos, SEEK_SET );
//printf("line==%s,flag=%d",line,flag);
if((line[0]=='/')&&(line[1]=='/') ){
flag=1;
buff[0]='/';
buff[1]='*';
/*write output to file output.txt*/
fp = fopen("output.txt", "a");
fseek( fp,0, SEEK_END);
fprintf(fp, "%s",buff);
fclose(fp);
/************/
printf("%s", buff);
} else{
/*write output to file output.txt*/
fp = fopen("output.txt", "a");
fseek( fp,0, SEEK_END);
fprintf(fp, "%s",buff);
fclose(fp);
/************/
printf("%s",buff);
}
}else if(flag==1){
buff[0]='*';
buff[1]='*';
/*look at next line*/
pos = ftell(file);
fgets(line,sizeof(line),file);
trimLeading(line);
//line_length=strlen(line);
//printf("%d",line_length);
while((strlen(line)==0)){
fgets(line,sizeof(line),file);
trimLeading(line);
}
fseek( file,pos, SEEK_SET );
//printf("line==%s,flag=%d",line,flag);
//printf("line[0]=%c,line[1]=%c",line[0],line[1]);
if(!((line[0]=='/')&&(line[1]=='/') )){
strcat(buff,"*/ \n");
flag=0;
//printf("\nFLAG1==%d\n",flag);
}
/*write output to file output.txt*/
fp = fopen("output.txt", "a");
fseek( fp,0, SEEK_END);
fprintf(fp, "%s",buff);
fclose(fp);
/************/
printf("%s", buff);
}
} else{
/*write output to file output.txt*/
fp = fopen("output.txt", "a");
fseek( fp,0, SEEK_END);
fprintf(fp, "%s",buff);
fclose(fp);
/************/
printf("%s",buff);
flag=0;
//printf("\nFLAG2==%d\n",flag);
}
} else {
pos = ftell(file);
trimLeading(line);
line_length=strlen(line);
//printf("%d",line_length);
while((strlen(line)==0)){
fgets(line,sizeof(line),file);
trimLeading(line);
}
fseek( file,pos, SEEK_SET );
//printf("line==%s,flag=%d",line,flag);
/*write output to file output.txt*/
fp = fopen("output.txt", "a");
fseek( fp,0, SEEK_END);
fprintf(fp, "%s",buff);
fclose(fp);
/************/
printf("%s",buff);
flag=0;
//printf("\nFLAG3==%d\n",flag);
}
}
fclose(file);
return 0;
}
void trimLeading(char * string)
{
int lastSpaceIndex, i, j;
lastSpaceIndex = 0;
/* Finds the last index of whitespace character */
while(string[lastSpaceIndex] == ' ' || string[lastSpaceIndex] == '\t' || string[lastSpaceIndex] == '\n')
{
lastSpaceIndex++;
}
/* Shifts all trailing characters to its left */
i = 0;
while(string[i + lastSpaceIndex] != '\0')
{
string[i] = string[i + lastSpaceIndex];
i++;
}
string[i] = '\0'; //Make sure that string is NULL terminated
}
Sample.txt:
#include<stdio.h>
int main()
{
int a=10,b=20,c=0;
//samplea
//sampleb
//samplec
///////sampled
c=a+b;
//samplee
return 0;
}