reversing only certain words of a string

问题

I have a string with the name "Mustang Sally Bob" After i run my code i want the string output to be like this: gnatsuM yllaS boB My approach is to count the words until the space and save the index of where the space is located in the string. then Then I want to print the characters starting from the space backwards.

#include <stdio.h>

int main()
{
    char* test="Mustang Sally Bob";

    int length; //string length
    int x;
    for(length=0;test[length] !=0&&test[length];length++); //get string length
    int counter;
    int words = 0;
    int space_index =0;
    for(counter=0;counter<length;counter++) {
        words++;
        if(test[counter]==' ') {
            space_index=counter;
            for(x=space_index-1;x>=words;x--) {
                printf("%c",test[x]);
            }
            words=0;    
            space_index = 0;
        }
    }

    return 0;
}

but when I execute this code the output I get is yllaS g does anyone know why i cant get the full string?

回答1:

In general the approach is incorrect.

For example an arbitrary string can start with blanks. In this case the leading blanks will not be outputted.

The last word is ignored if after it there is no blank.

The variable words does not keep the position where a word starts.

Calculating the length of a string with this loop

for(length=0;test[length] !=0&&test[length];length++);

that can be written simpler like

for ( length = 0; test[length] != '\0' ; length++ );

is redundant. You always can rely on the fact that strings are terminated by the zero-terminating character '\0'.

I can suggest the following solution

#include <stdio.h>

int main( void )
{
    const char *test = "Mustang Sally Bob";

    for ( size_t i = 0; test[i] != '\0'; )
    {
        while ( test[i] == ' ' ) putchar( test[i++] );

        size_t j = i;

        while ( test[i] != '\0' && test[i] != ' ' ) i++;

        for ( size_t k = i; k != j; ) putchar( test[--k] );
    }

    return 0;
}

The program output is

gnatsuM yllaS boB

You could append the program with a check of the tab character '\t' if you like. In C there is the standard C function isblank that performs such a check.

Here is a demonstrative program that uses the function isblank. I also changed the original string literal.

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

int main( void )
{
    const char *test = " Mustang\tSally\tBob ";

    puts( test );

    for ( size_t i = 0; test[i] != '\0'; )
    {
        while ( isblank( ( unsigned char )test[i] ) ) putchar( test[i++] );

        size_t j = i;

        while ( test[i] != '\0' && !isblank( ( unsigned char)test[i] ) ) i++;

        for ( size_t k = i; k != j; ) putchar( test[--k] );
    }

    putchar( '\n' );

    return 0;
}

The program output is

 Mustang    Sally   Bob 
 gnatsuM    yllaS   boB

来源：https://stackoverflow.com/questions/59162499/reversing-only-certain-words-of-a-string