為什么 scanf function 在列表中看起來較低，但首先評估它？

Question

當我運行這段代碼時，我在它的末尾按 1，而不是打印出消息一，它直接進入 scanf，只有在我輸入輸入后，它才會打印出消息一。

#include <stdio.h>

void main() {
    int year;
    int decision;
    printf( "\nThis program tells the user what the GDP per capita of post-independence South Sudan was in a given year. The availability of a figure for a certain year is subject to the limitations of the data that was provided by the World Bank. " );
point_one :
    printf( "Enter a year below to get a figure for that year.\n\n" ); // message one
    scanf( "%d" , &year );
    if(2011 > year || year > 2015 )
        printf( "\nThe figure for that year is unavailable.\n" );
    else if( year == 2011 )
        printf( "\n1,516.40 USD\n" );
    else if( year == 2012 )
        printf( "\n1,179.40 USD\n" );
    else if( year == 2013 )
        printf( "\n1,779.47 USD\n" );
    else if( year == 2014 )
        printf( "\n1,322.82 USD\n" );
    else if( year == 2015 )
        printf( "\n1,119.65 USD\n" );
    printf( "\nDo you want to try again or leave?\n\n" );
    scanf ( "%d\n\n" , &decision );
    if ( decision == 1 )
        goto point_one;
    else return;
}

Answer 1

1. void main無效。
2. 不要使用goto 。 在某些情況下使用goto是合理的，但 IMO 初學者根本不應該使用它。
3. 檢查 scanf 的返回值。
4. 使用正確的格式（“\n\n”）也不正確
5. 在這種情況下，我會使用swich而不是if s

int main(void) {
    int year;
    int decision;
    printf( "\nThis program tells the user what the GDP per capita of post-independence South Sudan was in a given year. The availability of a figure for a certain year is subject to the limitations of the data that was provided by the World Bank. " );
    do{
        printf( "Enter a year below to get a figure for that year.\n\n" ); // message one
        if(scanf( "%d" , &year ) != 1) {/* error handling */ exit(2);}
        switch(year)
        {
        case 2011:
            printf( "\n1,516.40 USD\n" );
            break;
        case 2012:
            printf( "\n1,179.40 USD\n" );
            break;
        case 2013:
            printf( "\n1,779.47 USD\n" );
            break;
        case 2014:
            printf( "\n1,322.82 USD\n" );
            break;
        case 2015:
            printf( "\n1,119.65 USD\n" );
            break;
        default:
            printf( "\nThe figure for that year is unavailable.\n" );
            break;
        }
        printf( "\nDo you want to try again or leave?\n\n" );
        if(scanf ( "%d" , &decision ) != 1) {/* error handling */ exit(3);}
    }while(decision == 1);
}

https://godbolt.org/z/793PdYdrh

但是，您可以使用 table 來代替 switch 和 ifs 並使您的代碼更好（例如，添加更多年會很容易）。

int main(void) {
    int year;
    int decision;

    unsigned data[] = {151640, 117940, 177947, 132282, 111965};
    printf( "\nThis program tells the user what the GDP per capita of post-independence South Sudan was in a given year. The availability of a figure for a certain year is subject to the limitations of the data that was provided by the World Bank. " );
    do{
        printf( "Enter a year below to get a figure for that year.\n\n" ); // message one
        if(scanf( "%d" , &year ) != 1) {/* error handling */ exit(2);}
        if(year < 2011 || year > 2015) printf( "\nThe figure for that year is unavailable.\n" );
        else printf( "\n%d.%d USD\n", data[year - 2011] / 100, data[year - 2011] % 100);

        printf( "\nDo you want to try again or leave?\n\n" );
        if(scanf ( "%d" , &decision ) != 1) {/* error handling */ exit(3);}
    }while(decision == 1);
}