求大神解释 left operand must be l-value，要怎么修改

[摆渡604]

void savef()
{
FILE *fp;
int j;

if((fp=fopen("f.txt","w+"))=NULL) 这里出错left operand must be l-value
{
printf("cannot open file!\n");
exit(0);
}
for(j=0;j<1024;j++)
{
fprintf(fp,"%d,%d,%.2f",wij[j].i,wij[j].j,wij[j].w);
fprintf(fp,"%.2f",ff32[j]);
if((j+1)%32==0)fprintf(fp,"\n");
}
fclose(fp);
}
为什么下面的程序没错
float f[1024]={0};
void load()
{
FILE *fp;
int i,j;
if((fp=fopen("f32.txt","r+"))==NULL) 这么没错
{
printf("cannot open file!");
exit(0);
}
for(i=0;i<32;i++)
for(j=0;j<32;j++)
{
fscanf(fp,"%f",&f[i*32+j]);
}
fclose(fp);

}

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谢谢！ 看了好久没看出来

[竞天问]

if((fp=fopen("f.txt","w+"))==NULL)